Historical Identification: Per Dreyer, NGC 4000 (4th Lord Rosse, 1860 RA 11 50 44, NPD 64 04.7) is "very faint, very small, a little extended, 8th magnitude star 2 arcmin to east, 5597 is to the southeast," (GC) 5597 being NGC 4005. The position precesses to RA 11 57 56.5, Dec +25 08 33, nearly dead center on the galaxy listed above, the description fits and a comparison of the sketch shown below to the sky makes the identification certain.
Discovery Notes: In Dreyer's chart of the region (shown below) NGC 4000 is labeled as ν.

 Above, a sketch drawn by John Dreyer of the region near JH 1033 (= NGC 3997), as shown on page 105 of Lawrence Parsons' (the 4th Lord Rosse) 1881 publication of observations done with his father's 72-inch "Leviathan" by him, his father (William Parsons, the 3rd Lord Rosse), and their assistants (in this case, R. J. Mitchell and John Dreyer. NGC 3987, 3989, 3993 and 4005 were observed or discovered by R. J. Mitchell in 1854, NGC 3999 and 4000 were discovered by the 4th Lord Rosse in 1878, and NGC 4009, 4011, 4015, 4018, 4021, 4022 and 4023 were discovered by John Dreyer in 1878. Dreyer drew the arrow pointing westward, all the positions of stars and nebulae, and assigned the Greek letters for the nebulae; I added the NGC designations, the Northward arrow and the labels for Northward and Westward.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 4865 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4000 is about 225 to 230 million light years away, in reasonable agreement with a single redshift-independent distance estimate of about 205 to 210 million light years. Given that and its apparent size of about 1.0 by 0.25 arcmin (from the images below), the galaxy is about 65 thousand light years across.

Historical Identification: Per Dreyer, NGC 4001 (= GC 2642, 3rd Lord Rosse, 1860 RA 11 50 45, NPD 41 54.2) is "small, round, 7 arcmin northwest of h 1040", (JH 1040) being NGC 4010. The position precesses to RA 11 58 00.3, Dec +47 19 03, about 1.5 arcmin southwest of the galaxy listed above, the description fits (including the position relative to NGC 4010) and there is nothing else nearby, so the identification is certain.
Discovery Notes: Although Dreyer credits the discovery to William Parsons, 3rd Earl of Rosse, he notes that many of Rosse's nebular discoveries were actually made by one of his assistants, in this case Bindon Stoney.
NED Note: NED lists this as being in a pair with NGC 4010, but that is a nearby galaxy, whereas NGC 4001 is a very distant one, so they in no way related to each other. In addition, NED lists this as a dwarf elliptical (which might seem correct if it really was as close as NGC 4010, and if viewed only in older low-resolution images), but it is neither a dwarf nor an elliptical, as shown in its description line, and in the discussion of its physical information.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 14300 km/sec (and H₀ = 70 km/sec/Mpc), a straightforward calculation indicates that NGC 4601 is about 665 to 670 million light years away. However, for objects at such distances we should take into account the expansion of the Universe during the time it took their light to reach us. Doing that shows that the galaxy was about 630 million light years away at the time the light by which we see it was emitted, about 645 million years ago (the difference between the two numbers being due to the expansion of the intervening space during the light-travel time). Given that and its apparent size of about 0.55 by 0.35 arcmin, the galaxy is about 100 thousand light years across.

Historical Identification: Per Dreyer, NGC 4002 (= GC 2643 = JH 1034 = WH III 344, 1860 RA 11 50 46, NPD 66 00.7) is "very faint, very small, round, northern of 2", the other being NGC 4003. The position precesses to RA 11 57 58.3, Dec +23 12 33, only 0.5 arcmin north northwest of the galaxy listed above, the description fits and there is nothing else nearby save for the galaxy (which is, as stated in the NGC entry) to its south, so the identification is certain.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 6895 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4002 is about 320 million light years away. Given that and its apparent size of about 1.1 by 0.5 arcmin (from the images below), the galaxy is about 100 to 105 thousand light years across.
Note About The Galaxy Type(s): Many references list NGC 4002 as a spiral galaxy and NGC 4003 as a lenticular galaxy, but as shown by the images of the galaxies (and the descriptions on this page), those types are backward. NGC 4003 is not only a spiral galaxy, but an unusual one, which may be due to a past interaction between it and NGC 4002, as they have nearly identical recessional velocities and may well be a physical pair.

Historical Identification: Per Dreyer, NGC 4003 (= GC 2644 = JH 1035 = WH III 345, 1860 RA 11 50 46, NPD 66 05.0) is "very faint, very small, round, southern of 2", the other being NGC 4002. The position precesses to RA 11 57 58.3, Dec +23 08 15, only 0.8 arcmin north northwest of the galaxy listed above (and almost within its northwestern outline), the description fits and there is nothing else nearby save the galaxy (which is, as stated in the NGC) to its north, so the identification is certain.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 6825 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4003 is about 315 to 320 million light years away, in fair agreement with redshift-independent distance estimates of about 335 to 380 million light years. Given that and its apparent size of about 1.7 by 1.1 arcmin (from the images below), the galaxy is about 155 to 160 thousand light years across.
Note About The Galaxy Type(s): Many references list NGC 4002 as a spiral galaxy and NGC 4003 as a lenticular galaxy, but as shown by the images of the galaxies (and the descriptions on this page), those types are backward. The complex galaxy type posted above refers to the triple-ringed structure of NGC 4003, which has a nucleus with a ringlike structure (with some signs of spiral dust lanes inside it), a strong bar surrounded by a second ring which evolves into broad spiral arms, and thanks to the way that the arms wrap around the galaxy, a nearly complete outer ring. There is a good chance that this galaxy and NGC 4002 are a physical pair, and if so, a past interaction between the two may be responsible for the peculiar structure of NGC 4003.

Historical Identification: Per Dreyer, NGC 4004 (= GC 2645 = JH 1036 = WH III 354, 1860 RA 11 50 53, NPD 61 20.4) is "faint, very small, round, 12th magnitude star near". The position precesses to RA 11 58 05.7, Dec +27 52 51, right on the northern portion of the galaxy listed above, the description fits (insofar as early 19th-century visual observers would have been able to tell) and there is nothing comparable nearby, so the identification is certain.
Classification Note: NGC 4004 is almost certainly the result of a spiral galaxy colliding with another galaxy, but it is currently (meaning at the time that the light by which we see it left it) in such a chaotic state that it is usually classified as either a barred irregular galaxy with an active nucleus, or even as just "peculiar". However, in a 2008 study of peculiar galaxies Petrosian et al. classified it as type SBdm pec, which seems reasonable to me, so I have adopted their classification.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 3665 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4004 is about 170 million light years away. Given that and an apparent size of about 2.2 by 0.7 arcmin for the main galaxy and its brighter extensions and at least 3.35 arcmin for outlying fragments (from the images below), the main galaxy and its brighter extensions span about 110 thousand light years, and the outer fragments are scattered across about 175 thousand light years.

Warning About Non-Standard Designation: The use of letters attached to NGC/IC objects is always a terrible idea, as there is no standard method of doing that, and a given galaxy may be given different designations, and different galaxies the same designation, leading to data referring to one object being attached to a completely different one. This is an especially egregious error when the object already has a perfectly good NGC/IC designation, as in this case, but should always be avoided in every case for the reasons already stated. As a result, the only purpose of this entry is to warn against such usage.
Physical Information: Since this galaxy already has a perfectly good IC designation, see IC 2982 for anything else.

Historical Identification: Per Dreyer, NGC 4005 (= GC 5597, Otto Struve, (Mitchell), 1860 RA 11 50 56, NPD 64 06.0) is "pretty faint, very small, much brighter middle, 7th magnitude star 2 arcmin to northwest". The position precesses to RA 11 58 08.4, Dec +25 07 15, on the western rim of the galaxy listed above, the description fits and a comparison of the sketch shown below to the sky makes the identification certain.
Discovery Note: Dreyer's 1912 summary of corrections to the NGC (based on his 1912 study of William Herschel's Scientific Papers) states "4005 = WH III 325" and "4007 to be struck out (= 4005, (N)PD 64 06)," so the duplicate entry shown in the discovery credits has been known for more than a century.
Discovery Note: Mitchell's observation was one of four nebulae discovered by him while examing the region near JH 1033 (which became NGC 3997). In Dreyer's chart of the region (shown below) NGC 4005 is labeled as α, but though Mitchell made the original discovery, Dreyer had already added the object to his Supplement to John Herschel's General Catalogue, giving credit to Struve when he did it, and repeating that in the NGC; so I have added Mitchell's name to the NGC entry in parentheses.

 Above, a sketch drawn by John Dreyer of the region near JH 1033 (= NGC 3997), as shown on page 105 of Lawrence Parsons' (the 4th Lord Rosse) 1881 publication of observations done with his father's 72-inch "Leviathan" by him, his father (William Parsons, the 3rd Lord Rosse), and their assistants (in this case, R. J. Mitchell and John Dreyer. NGC 3987, 3989, 3993 and 4005 were observed or discovered by R. J. Mitchell in 1854, NGC 3999 and 4000 were discovered by the 4th Lord Rosse in 1878, and NGC 4009, 4011, 4015, 4018, 4021, 4022 and 4023 were discovered by John Dreyer in 1878. Dreyer drew the arrow pointing westward, all the positions of stars and nebulae, and assigned the Greek letters for the nebulae; I added the NGC designations, the Northward arrow and the labels for Northward and Westward.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 4775 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4005 is about 220 to 225 million light years away, in reasonable agreement with redshift-independent distance estimates of about 225 to 265 million light years. Given that and its apparent size of about 1.05 by 0.55 arcmin (from the images below), the galaxy is about 65 to 70 thousand light years across.

Historical Identification: Per Dreyer, NGC 4006 (= GC 2647 = JH 1037, 1860 RA 11 50 56, NPD 91 21.1) is "faint, small, round, brighter middle, 11th magnitude star to northeast". The position precesses to RA 11 58 06.1, Dec -02 07 52, only 0.6 arcmin south of the center of the galaxy listed above and barely outside its southern outline, the description fits and there is nothing else nearby, so the identification is certain.
Misidentification As IC 2983: Some references misidentify IC 2983 as a duplicate observation of NGC 4006. This is an error. For more about that, see IC 2983.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 6210 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4006 is about 290 million light years away. Given that and its apparent size of about 2.1 by 1.5 arcmin (from the images below), counting its extensive outer halo, it is about 175 to 180 thousand light years across.

Historical Identification: Per Dreyer, NGC 4007 (= GC 2648 = WH III 325, 1860 RA 11 50 58, NPD 66 06.0) is "extremely faint, very small". The position precesses to RA 11 58 10.2, Dec +23 07 15, about 2.6 arcmin east of NGC 4003, so it woud be reasonable to suppose that NGC 4007 is a duplicate observation of that galaxy. However, in his notes for Herschel's first Catalogue of 1000 nebulae and clusters in the 1912 publication of The Scientific Papers of Sir William Herschel, Dreyer noticed that there was a two degree error in the NPD of GC 2648 and since the NGC was primarily an update of the GC, in the NGC as well. He did not state his reasoning, but I feel certain that it relies on the fact that in the original publication of William Herschel's first Catalogue (which is of course one of the papers republished in the 1912 work), WH III 325 is listed as being only a minute of time to the east and 5 arcmin to the south of WH III 323 and III 324, which are among the "nebulae" shown in Dreyer's sketch of observations made with the 72-inch "Leviathan" at Birr Castle near (J)h 1033 (= NGC 3997), and as a result, it could not possibly be two degrees to the south, near NGC 2003. His notes in the 1912 review of Herschel's papers therefore stated that the NPD should be 64° instead of 66°, making the 1860 position RA 11 50 58, NPD 64 06, which precesses to RA 11 58 10.4, Dec +25 07 15. That position lies almost exactly on top of α in Dreyer's sketch of the region, or NGC 4005. So in his 1912 summary of corrections to the NGC Dreyer writes "4005 = WH III 325" and "4007 to be struck out (= 4005, (N)PD 64 06)." In other words, the fact that NGC 4007 is not a duplicate observation of NGC 4003, but of NGC 4005 (as shown in the title for this entry), has been known for more than a century.
Discovery Note: Caroline Herschel's fair copy of her brother's observation of what became NGC 4007 yields an 1860 NPD of 64 07, so the two degree error in the GC's NPD must have been a typographical or copying error by John Herschel.
Physical Information: Given the duplicate entry, see NGC 4005 for anything else.

Historical Identification: Per Dreyer, NGC 4008 (= GC 2649 = JH 1038 = WH II 368, 1860 RA 11 51 03, NPD 61 01.6) is "pretty bright, pretty small, extended, pretty suddenly brighter middle, star involved on north". The position precesses to RA 11 58 15.7, Dec +28 11 39, on the western rim of the galaxy listed above, the description fits and there is nothing comparable nearby, so the identification is certain.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 3915 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4008 is about 180 to 185 million light years away, in good agreement with redshift-independent distance estimates of about 130 to 240 million light years. Given that and its apparent size of about 2.4 by 1.4 arcmin (from the images below), the galaxy is about 125 to 130 thousand light years across.

Historical Identification: Per Dreyer, NGC 4009 (Dreyer (ι), using Lord Rosse's 72-inch "Leviathan", 1860 RA 11 51 03, NPD 64 02.1) is "very faint, extremely small". The position precesses to RA 11 58 15.4, Dec +25 11 09, only 0.2 arcmin south southeast of the star listed above, the description is reasonable for a stellar object mistaken for a nebula, and a comparison of the sketch shown below to the sky makes the identification certain. Unfortunately, despite the fact that Dreyer's measurements and sketch of the region leave no doubt as to the position of the object, the galaxy about 3.5 arcmin to the northeast is often misidentified as NGC 4009, so that object is discussed in the following entry.
Note About PGC Designation: For purposes of completeness, LEDA assigns a PGC designation to almost every NGC/IC object, even if not a galaxy. Usually, a search of the database for a designation created for a stellar object returns no result, but in this case it does, so the designation is not shown in quotes. However, a search of NED for the PGC designation returns no result (though a search for NGC 4009 does).
Discovery Note: The Greek letters in Dreyer's observations of Apr 26, 1878 refer to objects shown on a chart of the region (shown below).

 Above, a sketch drawn by John Dreyer of the region near JH 1033 (= NGC 3997), as shown on page 105 of Lawrence Parsons' (the 4th Lord Rosse) 1881 publication of observations done with his father's 72-inch "Leviathan" by him, his father (William Parsons, the 3rd Lord Rosse), and their assistants (in this case, R. J. Mitchell and John Dreyer. NGC 3987, 3989, 3993 and 4005 were observed or discovered by R. J. Mitchell in 1854, NGC 3999 and 4000 were discovered by the 4th Lord Rosse in 1878, and NGC 4009, 4011, 4015, 4018, 4021, 4022 and 4023 were discovered by John Dreyer in 1878. Dreyer drew the arrow pointing westward, all the positions of stars and nebulae, and assigned the Greek letters for the nebulae; I added the NGC designations, the Northward arrow and the labels for Northward and Westward.
Physical Information: The star's magnitude is based on the GAIA database.

Historical Misidentification: As discussed in the entry for NGC 4009, Dreyer's position for what became that entry and his sketch of the region make it absolutely certain that it is the star shown in that entry; however, the desire of mid to late 20th-century astronomers to assign NGC designations to the nearest galaxy to a given position, whether that was reasonable or not, caused a misidentification of PGC 37677 as NGC 4009. This entry serves as a warning about that mistake.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 17850 km/sec (and H₀ = 70 km/sec/Mpc), a straightforward calculation indicates that PGC 37677 is about 830 million light years away. However, for objects at such distances we should take into account the expansion of the Universe during the time it took their light to reach us. Doing that shows that the galaxy was about 775 million light years away at the time the light by which we see it was emitted, about 795 to 800 million years ago (the difference between the two numbers being due to the expansion of the intervening space during the light-travel time). Given that and its apparent size of about 0.4 by 0.25 arcmin (from the images below), the galaxy is about 90 thousand light years across.

Historical Identification: Per Dreyer, NGC 4010 (= GC 2651 = JH 1040, 1860 RA 11 51 14, NPD 41 59.2) is "faint, pretty large, much extended, very gradually a little brighter middle". The position precesses to RA 11 58 28.9, Dec +47 14 03, about 2 arcmin southwest of the center of the galaxy listed above but only about 0.7 arcmin south of its western extension, the description fits and there is nothing comparable nearby so the identification is certain.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 1115 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4010 is about 50 to 55 million light years away, in good agreement with redshift-independent distance estimates of about 40 to 75 million light years. Given that and its apparent size of about 3.55 by 0.6 arcmin (from the images below), the galaxy is about 50 to 55 thousand light years across. NGC 4010 is used by the de Vaucouleurs Atlas of Galaxy Types as an example of type Sc sp. Another reference classifies it as type SB(s)cd, but since it is seen edge-on, whether it has a bar or not is essentially impossible to tell, so I have chosen to use the de Vaucouleurs classification.

Historical Identification: Per Dreyer, NGC 4011 (Dreyer (ε), using Lord Rosse's 72-inch "Leviathan", 1860 RA 11 51 14, NPD 64 07.5) is "very faint, very small, 12th magnitude star to northwest". The position precesses to RA 11 58 26.3, Dec +25 05 45, only 0.2 arcmin east southeast of the galaxy listed above, the description fits and a comparison of the sketch below to the sky makes the identification certain.
Discovery Note: The Greek letters in Dreyer's observations of Apr 26, 1878 refer to objects shown on a chart of the region (shown below).

 Above, a sketch drawn by John Dreyer of the region near JH 1033 (= NGC 3997), as shown on page 105 of Lawrence Parsons' (the 4th Lord Rosse) 1881 publication of observations done with his father's 72-inch "Leviathan" by him, his father (William Parsons, the 3rd Lord Rosse), and their assistants (in this case, R. J. Mitchell and John Dreyer. NGC 3987, 3989, 3993 and 4005 were observed or discovered by R. J. Mitchell in 1854, NGC 3999 and 4000 were discovered by the 4th Lord Rosse in 1878, and NGC 4009, 4011, 4015, 4018, 4021, 4022 and 4023 were discovered by John Dreyer in 1878. Dreyer drew the arrow pointing westward, all the positions of stars and nebulae, and assigned the Greek letters for the nebulae; I added the NGC designations, the Northward arrow and the labels for Northward and Westward.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 4440 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4011 is about 205 to 210 million light years away. Given that and its apparent size of about 0.45 by 0.15 arcmin (from the images below), the galaxy is about 25 to 30 thousand light years across.

Historical Identification: Per Dreyer, NGC 4012 (= GC 5598, Marth #225, (Safford #108), 1860 RA 11 51 17, NPD 79 12) is "very faint, small, a little extended". The position precesses to RA 11 58 28.0, Dec +10 01 14, well within the eastern outline of the galaxy listed above, the description fits (taking into account the difference between early visual observations and modern photographs) and there is nothing else nearby, so the identification is certain.
Discovery Note: Although Safford observed this object early on, his work was not published until shortly before Dreyer finished compiling the NGC, and as a result, only his "novae" were published in an Appendix to the NGC (and in the first IC), and since this object already had a prior observer Safford's observation is not mentioned in the NGC/IC at all, and only appears in the Dearborn Observatory report published in 1887, hence his observation's addition to Dreyer's entry in parentheses.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 4530 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4012 is about 210 million light years away, in good agreement with redshift-independent distance estimates of about 185 to 220 million light years. Given that and its apparent size of about 2.0 by 0.55 arcmin (from the images below), the galaxy is about 120 to 125 thousand light years across.

Historical Identification: Per Dreyer, NGC 4013 (= GC 2652 = JH 1041 = WH II 733, 1860 RA 11 51 19, NPD 45 17.2) is "bright, considerably large, much extended 62°, very suddenly very much brighter middle equivalent to a 10th magnitude star." The position precesses to RA 11 58 33.3, Dec +43 56 03, just over 0.8 arcmin southeast of the center of the galaxy listed above, the description fits and there is nothing else nearby, so the identification is certain.
Discovery Note (1): A note at the end of the NGC states "(J)h 1041 = (WH) II 733. According to (W)H the position of extension is "near the meridian". (J)h has a measure 62.3° and an estimation 65° in another obs. (Note by JH)".
Discovery Note (2): The first IC adds "Position angle of extension 60 to 70 degrees (Armagh, 1891). No change". Although that note does not mention who made the observation at Armagh, funds for the Armagh observatory had been cut to nearly nothing long before 1891, so Dreyer had no assistant for most of his time as director of the observatory, and odds are that any observations made during that time were made by Dreyer himself.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 1060 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4013 is about 50 million light years away, in good agreement with redshift-independent distance estimates of about 45 to 80 million light years. Given that and its apparent size of about 5.15 by 0.75 arcmin (from the images below), the galaxy is about 75 thousand light years across. With its remarkable tidal stream (seen in the image provided by R. Jay GaBany) the galaxy spans about 10 by 4.1 arcmin (from the Cosmotography image), or about 140 to 145 thousand light years.
NGC 4013's Tidal Stream: The Cosmotography image below shows an extensive tidal stream. The stream is significantly redder than the galaxy itself, and is therefore believed to be the result of an interaction with (and destruction of) a dwarf elliptical companion. Similar tidal streams exist near our own galaxy as a result of its "cannibalism" of much smaller neighboring galaxies, and the study of such streams in other galaxies is expected to increase our understanding of how such interactions work.
Classification Note: NGC 4013 is used by the de Vaucouleurs Atlas of Galaxy Types as an example of type Sb sp. Another reference lists it as type SABb, but its edge-on orientation makes it difficult to tell whether it has any sign of a bar, so I have chosen to use the de Vaucouleurs classification, but added "peculiar" to the classification because of its tidal stream, which is not visible in the de Vaucouleurs Atlas image. An infrared image of the galaxy shows a possible ring of hot, bright new stars at its center, but given the edge-on orientation, that is also unlikely to alter the classification.

Historical Identification: Per Dreyer, NGC 4014 (= GC 2653 = JH 1042, 1860 RA 11 51 25, NPD 73 02.5) is "pretty bright, pretty small, round, pretty suddenly brighter middle". The position precesses to RA 11 58 36.5, Dec +16 10 44, on the northeastern rim of the galaxy listed above, the description fits and there is nothing else nearby, so the identification is certain.
Discovery Note: A note at the end of the NGC states "(J)h 1042. This cannot be (WH) III 3, as (Caroline Herschel) has reduced two obs. of this latter well agreeing, and giving a RA 2 minutes of time exceeding that of h 1042, which also rests on two obs. by (J)h. (note per JH)." However, it turns out that that note was probably wrong, as NGC 4014 (= JH 1042) and NGC 4028 (= WH III 3) are now recognized as almost certainly being the same object.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 4095 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4014 is about 190 million light years away. Given that and its apparent size of about 1.7 by 0.95 arcmin (from the images below), the galaxy is about 95 thousand light years across.
Classification Note: Both NED and LEDA list this as a lenticular galaxy of type S0/a. However, as the images below show, it is clearly a spiral galaxy, as stated in the description line for this entry.

As noted in the entries for NGC 4015 and PGC 37703, misidentification of the two galaxies by NED (and/or other errors in earlier references) and a blunder in LEDA's heliocentric recessional velocity for PGC 37703 have caused considerable confusion about the relative distances and possible interaction of the apparent pair. After many hours of labor I have (hopefully) arrived at what I believe to be a correct analysis of the situation, but completing the two entries will require a little more time, so although close to being complete, the entries for NGC 4015 and its (apparent?) companion will not be complete until this paragraph is removed. NOTE TO SELF: CHECK GOTTLIEB'S OBSERVATIONS.
 NOTE TO SELF: Per Corwin, Tifft gives galactocentric Vr of 4321 km/sec for PGC 37702 (the elliptical/lenticular) and 4711 km/sec for PGC 37703 (that is for the spiral). On the other hand, SDSS gives much closer (heliocentric) values... Should probably mention both, though pointing out that LEDA still has the middle digits reversed compared to the SDSS value for the spiral.

Historical Identification: Per Dreyer, NGC 4015 (= WH III 323?, Dreyer (β), using Lord Rosse's 72-inch "Leviathan", 1860 RA 11 51 31, NPD 64 11.1) is "faint, very small, extended, much brighter middle". The position precesses to RA 11 58 43.2, Dec +25 02 08, almost on the nucleus of the galaxy listed above and a comparison of the sketch below to the sky makes the identification certain.
Discovery Note (1): A note at the end of the NGC states "GC 2646 and 2650. No nebulae noticed at Birr Castle in the places given in GC for III 323-24. H's description, "Two, southwestern very faint, a little extended, northeastern 5 or 6 arcmin distant," seems to agree with beta (β) and delta (δ) of the Birr diagram, and in the (NGC) Cat. these have been assumed to be III 323-24." This explains "= WH III 323?" in the NGC entry for NGC 4015.
Note About The PGC Designation(s): Given the above, the main question is whether NGC 4015 includes both the galaxy listed above and its fainter companion, or only the brighter one. Dreyer's observational note mentions "a tail north of the nucleus", which seems likely to refer to the spiral galaxy, in which case his "β" would be both galaxies; but neither his sketch nor his NGC description refers to that (even the elongation of the lenticular galaxy would correspond to "extended"), so it appears that although he probably saw a faint hint of the spiral it does not necessarily mean that the spiral should be included in the NGC listing. As a result, some references (e.g., Steinicke) treat both galaxies as part of NGC 4015, and others (e.g., Corwin) treat only the much brighter southern galaxy as NGC 4015.
Discovery Note (2): The Greek letters in Dreyer's observations of Apr 26, 1878 refer to objects shown on a chart of the region (shown below).

 Above, a sketch drawn by John Dreyer of the region near JH 1033 (= NGC 3997), as shown on page 105 of Lawrence Parsons' (the 4th Lord Rosse) 1881 publication of observations done with his father's 72-inch "Leviathan" by him, his father (William Parsons, the 3rd Lord Rosse), and their assistants (in this case, R. J. Mitchell and John Dreyer. NGC 3987, 3989, 3993 and 4005 were observed or discovered by R. J. Mitchell in 1854, NGC 3999 and 4000 were discovered by the 4th Lord Rosse in 1878, and NGC 4009, 4011, 4015, 4018, 4021, 4022 and 4023 were discovered by John Dreyer in 1878. Dreyer drew the arrow pointing westward, all the positions of stars and nebulae, and assigned the Greek letters for the nebulae; I added the NGC designations, the Northward arrow and the labels for Northward and Westward.
Note About NED Mis-Designations: NED misidentifies the two galaxies involved here, labeling the spiral (which is PGC 37703) as PGC 37702, and the elliptical (which is PGC 37702) as PGC 37703. Odds are that many mistakes have been made as a result of misidentifying the two galaxies, as some references state that they have the same recessional velocity, while others (to a certain extent mistakenly) state that the spiral galaxy (which obviously lies in front of the elliptical galaxy) has a recessional velocity several hundred km/sec larger, which should mean that it is further away (and is certainly wrong). Only detailed future studies (hopefully aware of the correct identifications) can resolve the mess created by such errors, hence my emphasis on this problem.
Usage By The Arp Atlas: With PGC 37703, NGC 4015 is used by the Arp Atlas as an example of material emanating from elliptical galaxies, with the note "Absorption leads directly into elliptical galaxy," indicating that in the Arp Atlas image, the spiral galaxy's dust lanes appear to pass in front of its apparent companion. They also appear to lie in front of the elliptical galaxy in more modern images, so there is no doubt that the spiral galaxy lies in front of the elliptical, though how far in front cannot be known at the present time.
Physical Information: The two galaxies have different sources for their recessional velocities, and the NED source is relatively old, so it may either be wrong, or given the fact that NED reverses the designations, both NED values may be for the same galaxy. There is also a problem in that the LEDA value for the spiral galaxy is in error, as the middle digits of the heliocentric radial velocity are reversed, so the recessional velocity relative to the Cosmic Background in LEDA is 180 km/sec too large. At any rate, the following information is presumably correct for NGC 4015 itself:
 Based on a recessional velocity relative to the Cosmic Microwave Background radiation of about 4650 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4015 is about 215 to 220 million light years away. Given that and its apparent size of about 1.1 by 0.7 arcmin (from the images below), the galaxy is about 70 thousand light years across. The galaxy has a somewhat distorted appearance in the images below, which may mean that it and its apparent companion are close enough to be gravitationally interacting; and although the spiral galaxy's edge-on appearance makes it difficult to tell, it appears to be slightly distorted as well, so they are perhaps more likely to be a physical pair rather than an optical double. NGC 4015 was listed as having an Active Galaxy Nucleus by Carl Seyfert, but it is not listed as a Seyfert galaxy, despite its very bright nucleus.
 One thing that must be noted is that although the estimated distance for PGC 37703 is slightly greater than that for NGC 4015, the spiral galaxy must lie in front of the elliptical one, thanks to the superimposition of its southwestern dust lane on the image of the elliptical galaxy. However, the estimated distances only differ by about 5 million light years, and peculiar velocities (that is, random motions of the galaxies relative to each other) can lead to errors in their Hubble Flow distances of as much as 5 to 10 million light years, so there is no reason why the two cannot be at nearly the same distance (somewhere in the range of 215 to 225 million light years), and every reason to suppose (as is absolutely certain) that the spiral can be closer to us than the elliptical. The only question is how much closer it is, and whether that makes the pair merely an optical double, or whether (as seems slightly more likely) they are a physical pair.

"Discovery" Note: Dreyer's notes in Lord Rosse's 1861 compendium indicate that he could see "a tail north of the nucleus" of his "β", suggesting that he was able to (barely) see the spiral companion of NGC 4015; but he did not note that in the NGC entry, so as noted in the entry for NGC 4015, PGC 37703 may or may not be part of NGC 4015, and is treated merely as a companion and part of Arp 138 here.
Note About NED Mis-Designations: NED misidentifies the two galaxies involved here, labeling the spiral (which is PGC 37703) as PGC 37702, and the elliptical (which is PGC 37702) as PGC 37703. Odds are that many mistakes have been made as a result of misidentifying the two galaxies, as some references state that they have the same recessional velocity, while others (to a certain extent mistakenly) state that the spiral galaxy (which obviously lies in front of the elliptical galaxy) has a recessional velocity several hundred km/sec larger, which should mean that it is further away (and is certainly wrong). Only detailed future studies (hopefully aware of the correct identifications) can resolve the mess created by such errors, hence my emphasis on this problem.
Usage By The Arp Atlas: With NGC 4015, PGC 37703 is used by the Arp Atlas as an example of material emanating from elliptical galaxies, with the note "Absorption leads directly into elliptical galaxy," indicating that in the Arp Atlas image, the spiral galaxy's dust lanes appear to pass in front of its apparent companion. They also appear to lie in front of the elliptical galaxy in more modern images, so there is no doubt that the spiral galaxy lies in front of the elliptical, though how far in front cannot be known at the present time.
Physical Information: Note: LEDA's heliocentric radial velocity has a typographical error (the middle digits are reversed, compared to the original source), and as a result the recessional velocity relative to the Cosmic Microwave Background is 180 km/sec too large. The following discussion corrects for that error:
 Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 4770 km/sec (and H₀ = 70 km/sec/Mpc), PGC 37703 is about 220 to 225 million light years away, in perfect agreement with redshift-independent distance estimates of about 220 to 225 million light years. Given that and its apparent size of about 0.85 by 0.25 arcmin (from the images shown in the entry for NGC 4015), the galaxy is about 55 thousand light years across.
 One thing that must be noted is that although the estimated distance for PGC 37703 is slightly greater than that for NGC 4015, the spiral galaxy must lie in front of the elliptical one, thanks to the superimposition of its southwestern dust lane on the image of the elliptical galaxy. However, the estimated distances only differ by about 5 million light years, and peculiar velocities (that is, random motions of the galaxies relative to each other) can lead to errors in their Hubble Flow distances of as much as 5 to 10 million light years, so there is no reason why the two cannot be at nearly the same distance (somewhere in the range of 215 to 225 million light years), and every reason to suppose (as is absolutely certain) that the spiral can be closer to us than the elliptical. The only question is how much closer it is, and whether that makes the pair merely an optical double, or whether (as seems slightly more likely) they are a physical pair.
 Since the best currently available images of the two galaxies do not justify an extreme closeup, see NGC 4015 for images of the pair.

Historical Identification: Per Dreyer, NGC 4016 (= GC 2654, 3rd Lord Rosse, 1860 RA 11 51 33, NPD 61 39.8) is "very faint". The position precesses to RA 11 58 45.4, Dec +27 33 26, about 4 arcmin northeast of the galaxy listed above, but the description (though brief) is appropriate, and there is nothing else nearby save for NGC 4017 {which can be discounted per Discovery Note (2)}, so the identification is certain.
Discovery Note (1): Although Dreyer credits the discovery to William Parsons, 3rd Earl of Rosse, he notes that many of Rosse's nebular discoveries were actually made by one of his assistants, in this case R. J. Mitchell.
Discovery Note (2): We can be certain that Mitchell's discovery was not NGC 4017 because it is listed as a "Nova" in Mitchell's discussion of that object during his observation of (J)h 1043 = (WH) II 369 (which became NGC 4017): "Another very faint nebula about 5 arcmin northwest or nearly north," said description being reasonably accurate if "northwest" is chosen instead of "nearly north", because the galaxy listed as NGC 4016 is about 6 arcmin almost exactly northwest of NGC 4017.
Usage By The Arp Atlas: With NGC 4017, NGC 4016 is used by the Arp Atlas as an example of "Unclassified Double Galaxies", with the note "Segment breaking from arm of S gal., weak filaments reach to N gal., which has figure 8 loops."
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 3735 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4016 is about 175 million light years away, in essentially perfect agreement with its companion, NGC 4017. Given that and its apparent size of about 1.75 by 0.7 arcmin (from the images below, including its north and south extensions), it is about 85 to 90 thousand light years across. Aside from its extensive 'arms' to the north and south of the main galaxy, NGC 4016 has what may be a warped polar ring (the blue figure-8 in its image), so it is truly a "peculiar" galaxy. Its gravitational interaction with its companion is presumably the cause of Arp's decision to include the pair in his Atlas.

Historical Identification: Per Dreyer, NGC 4017 (= GC 2655 = JH 1043 = WH II 369, 1860 RA 11 51 33, NPD 61 44.8) is "faint, large, extended, gradually brighter eastern middle". The position precesses to RA 11 58 45.4, Dec +27 28 26, about 1.2 arcmin north of the galaxy listed above, the description fits and there is nothing comparable nearby (even its possible companion NGC 4016), so the identification is certain.
Usage By The Arp Atlas: With NGC 4016, NGC 4017 is used by the Arp Atlas as an example of "Unclassified Double Galaxies", with the note "Segment breaking from arm of S gal., weak filaments reach to N gal., which has figure 8 loops."
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 3755 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4017 is about 175 million light years away, in good agreement with redshift-independent distance estimates of about 170 to 270 million light years, and essentially perfect agreement with its companion, NGC 4016. Given that and its apparent size of about 2.0 by 1.5 arcmin (from the images below), the main galaxy is about 100 to 105 thousand light years across, and the faint filaments reaching toward NGC 4016 may span the best part of 200 to 300 thousand light years. Its gravitational interaction with its companion is presumably the cause of Arp's decision to include the pair in his Atlas.

Historical Identification: Per Dreyer, NGC 4018 (Dreyer (κ) using Lord Rosse's 72-inch "Leviathan", 1860 RA 11 51 35, NPD 63 52.5) is "much extended northwest-southeast, 2 stars to south". The position precesses to RA 11 58 47.2, Dec +25 20 44, about 2.3 arcmin northeast of the galaxy listed above, but the description fits, there is nothing else nearby, and a comparison of the sketch below to the sky makes the identification certain.
Discovery Note: The Greek letters in Dreyer's observations of Apr 26, 1878 refer to objects shown on a chart of the region (shown below).

 Above, a sketch drawn by John Dreyer of the region near JH 1033 (= NGC 3997), as shown on page 105 of Lawrence Parsons' (the 4th Lord Rosse) 1881 publication of observations done with his father's 72-inch "Leviathan" by him, his father (William Parsons, the 3rd Lord Rosse), and their assistants (in this case, R. J. Mitchell and John Dreyer. NGC 3987, 3989, 3993 and 4005 were observed or discovered by R. J. Mitchell in 1854, NGC 3999 and 4000 were discovered by the 4th Lord Rosse in 1878, and NGC 4009, 4011, 4015, 4018, 4021, 4022 and 4023 were discovered by John Dreyer in 1878. Dreyer drew the arrow pointing westward, all the positions of stars and nebulae, and assigned the Greek letters for the nebulae; I added the NGC designations, the Northward arrow and the labels for Northward and Westward.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 4790 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4018 is about 220 to 225 million light years away, in good agreement with redshift-independent distance estimates of about 205 to 250 million light years. Given that and its apparent size of about 1.7 by 0.35 arcmin (from the images below), the galaxy is about 110 thousand light years across.

Historical Identification: Per Dreyer, NGC 4019 (= GC 2656 = JH 1044, 1860 RA 11 51 43, NPD 75 00.8) is "extremely faint, 9th magnitude star 5 arcmin to southeast". The position precesses to RA 11 58 54.3, Dec +14 12 26, but there is nothing there nor near there. However, there is a reasonable argument for what Herschel observed being the galaxy listed above, as it is the only object in the region with a suitable star (10th magnitude BD+14 2466) in the proper relative position; and as discussed in the Discovery Note, there is good reason to believe that Herschel made an error in his comparison star which would have placed his measurement much closer to that object. As a result, IC 755 is considered to be the most likely candidate for what Herschel observed (if not the only one), whence the "probably =" in the title for this entry.
Discovery Note: Corwin has done a thorough analysis of Herschel's observations in his sweep 419 (which is the one in which he observed JH 1044), and found that the only star that fits the measurements of Herschel's comparison star's position is β Leonis. Using the relative positions of that star and the observation of JH 1044, Corwin finds that the right ascension of NGC 4019 should be 2 minutes of time larger, which changes its modern position to RA 12 00 53.9, Dec +14 12 26. That is about 7.5 arcmin northwest of PGC 37912, but since Herschel himself considered the position poor, and given his statement about the star to the southeast of his object, if his original position had been that close to the galaxy no one would have doubted the identification. So although perhaps not quite as certain as might be liked, it is almost certain that IC 755 is indeed the otherwise lost NGC 4019.
Physical Information: Based on the analysis above almost every modern reference has accepted the identity of NGC 4019 as a duplicate observation of IC 755, which means that in almost all cases the galaxy listed above is called NGC 4019 instead of IC 755, despite the latter identification being absolutely certain, and the former only almost certain. However, there are cases in which the galaxy is referred to as IC 755 (for instance, in the HST press release for the image of the galaxy posted at the entry for IC 755), and given that, I have decided to place anything else involving the galaxy at the entry for IC 755, and to only discuss the duplicate entry here.

Historical Identification: Per Dreyer, NGC 4020 (= GC 2657 = JH 1045 = WH II 725, 1860 RA 11 51 44, NPD 58 48.0) is "pretty bright, pretty large, extended 19.5°, binuclear". The position precesses to RA 11 58 56.6, Dec +30 25 14, well within the northern half of the galaxy listed above, the description fits and there is nothing else nearby, so the identification is certain.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 1050 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4020 is about 45 to 50 million light years away, in good agreement with redshift-independent distance estimates of about 25 to 60 million light years. Given that and its apparent size of about 2.15 by 0.9 arcmin (from the images below), the galaxy is about 30 thousand light years across.

Historical Identification: Per Dreyer, NGC 4021 (= WH III 324?, Dreyer (δ) using Lord Rosse's 72-inch "Leviathan", 1860 RA 11 51 50, NPD 64 08.4) is "faint, small, very little extended". The position precesses to RA 11 59 02.1, Dec +25 04 50, barely outside the southwestern rim of the galaxy listed above, the description fits and a comparison of the sketch below to the sky makes the identification certain.
Discovery Note: A note at the end of the NGC states "GC 2646 and 2650. No nebulae noticed at Birr Castle in the places given in GC for III 323-24. H's description, "Two, southwestern very faint, a little extended, northeastern 5 or 6 arcmin distant," seems to agree with beta (β) and delta (δ) of the Birr diagram, and in the (NGC) Cat. these have been assumed to be III 323-24." This explains "= WH III 324?" in the NGC entry for NGC 4021. The Greek letters in Dreyer's observations of Apr 26, 1878 refer to objects shown on a chart of the region (shown below).

 Above, a sketch drawn by John Dreyer of the region near JH 1033 (= NGC 3997), as shown on page 105 of Lawrence Parsons' (the 4th Lord Rosse) 1881 publication of observations done with his father's 72-inch "Leviathan" by him, his father (William Parsons, the 3rd Lord Rosse), and their assistants (in this case, R. J. Mitchell and John Dreyer. NGC 3987, 3989, 3993 and 4005 were observed or discovered by R. J. Mitchell in 1854, NGC 3999 and 4000 were discovered by the 4th Lord Rosse in 1878, and NGC 4009, 4011, 4015, 4018, 4021, 4022 and 4023 were discovered by John Dreyer in 1878. Dreyer drew the arrow pointing westward, all the positions of stars and nebulae, and assigned the Greek letters for the nebulae; I added the NGC designations, the Northward arrow and the labels for Northward and Westward.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 10365 km/sec (and H₀ = 70 km/sec/Mpc), a straightforward calculation indicates that NGC 4021 is about 480 to 485 million light years away. However, for objects at such distances we should take into account the expansion of the Universe during the time it took their light to reach us. Doing that shows that the galaxy was about 460 to 465 million light years away at the time the light by which we see it was emitted, about 470 million years ago (the difference between the two numbers being due to the expansion of the intervening space during the light-travel time). Given that and its apparent size of about 0.55 by 0.45 arcmin (from the images below), the galaxy is about 75 thousand light years across.

Historical Identification: Per Dreyer, NGC 4022 (Dreyer (λ) using Lord Rosse's 72-inch "Leviathan", 1860 RA 11 51 51, NPD 63 57.4) is "pretty faint, very small, stellar". The position precesses to RA 11 59 03.1, Dec +25 15 50, about 2.5 arcmin nearly due north of the galaxy listed above, the description fits and there is nothing else nearby, and a comparison of the sketch below to the sky makes the identification certain.
Discovery Note: The Greek letters in Dreyer's observations of Apr 26, 1878 refer to objects shown on a chart of the region (shown below).

 Above, a sketch drawn by John Dreyer of the region near JH 1033 (= NGC 3997), as shown on page 105 of Lawrence Parsons' (the 4th Lord Rosse) 1881 publication of observations done with his father's 72-inch "Leviathan" by him, his father (William Parsons, the 3rd Lord Rosse), and their assistants (in this case, R. J. Mitchell and John Dreyer. NGC 3987, 3989, 3993 and 4005 were observed or discovered by R. J. Mitchell in 1854, NGC 3999 and 4000 were discovered by the 4th Lord Rosse in 1878, and NGC 4009, 4011, 4015, 4018, 4021, 4022 and 4023 were discovered by John Dreyer in 1878. Dreyer drew the arrow pointing westward, all the positions of stars and nebulae, and assigned the Greek letters for the nebulae; I added the NGC designations, the Northward arrow and the labels for Northward and Westward.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 4645 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4022 is about 215 to 220 million light years away. Given that and its apparent size of about 1.15 by 1.15 arcmin (from the images below), the galaxy is about 70 to 75 thousand light years across.

Historical Identification: Per Dreyer, NGC 4023 (Dreyer (γ) using Lord Rosse's 72-inch "Leviathan", 1860 RA 11 51 54, NPD 64 13.9) is "pretty faint, pretty large, diffuse." The position precesses to RA 11 59 06.0, Dec +24 59 20, within the northeastern outline of the galaxy listed above, the description fits and a comparison of the sketch below to the sky makes the identification certain.
Discovery Note: The Greek letters in Dreyer's observations of Apr 26, 1878 refer to objects shown on a chart of the region (shown below).

 Above, a sketch drawn by John Dreyer of the region near JH 1033 (= NGC 3997), as shown on page 105 of Lawrence Parsons' (the 4th Lord Rosse) 1881 publication of observations done with his father's 72-inch "Leviathan" by him, his father (William Parsons, the 3rd Lord Rosse), and their assistants (in this case, R. J. Mitchell and John Dreyer. NGC 3987, 3989, 3993 and 4005 were observed or discovered by R. J. Mitchell in 1854, NGC 3999 and 4000 were discovered by the 4th Lord Rosse in 1878, and NGC 4009, 4011, 4015, 4018, 4021, 4022 and 4023 were discovered by John Dreyer in 1878. Dreyer drew the arrow pointing westward, all the positions of stars and nebulae, and assigned the Greek letters for the nebulae; I added the NGC designations, the Northward arrow and the labels for Northward and Westward.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 4715 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4023 is about 220 million light years away. Given that and its apparent size of about 0.9 by 0.7 arcmin (from the images below), the galaxy is about 55 to 60 thousand light years across.

Historical Identification: Per Dreyer, NGC 4024 (= GC 2658 = WH II 295, 1860 RA 11 51 56, NPD 107 35.0) is "faint, very small, irregular figure, brighter middle." The second IC lists a corrected RA (per Howe) of 11 51 23. The corrected position precesses to RA 11 58 31.9, Dec -18 21 46, only an arcmin south of the galaxy listed above, the description fits and there is nothing else nearby, so the identification is certain.
Discovery Note: Although Herschel's position lies half a minute of time to the east, there is nothing else that he could have seen anywhere in the region, so although Howe's observation made the identification easy, there would have been no doubt about what Herschel observed, even without it.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 2050 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4024 is about 95 million light years away, in good agreement with redshift-independent distance estimates of about 40 to 125 million light years. Given that and its apparent size of about 1.9 by 1.7 arcmin (from the images below), the galaxy is about 50 to 55 thousand light years across.

Designation Note: Although PGC designations in the millions usually require some other designation to search databases, a search for PGC 3094106 will return a page in the LEDA database, and a search for LEDA 3094106 will return a page in the NED.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 17465 km/sec (and H₀ = 70 km/sec/Mpc), a straightforward calculation indicates that PGC 3094106 is about 810 to 815 million light years away (which means that it is not a companion of NGC 4024, but a much more distant background galaxy). However, for objects at such distances we should take into account the expansion of the Universe during the time it took their light to reach us. Doing that shows that the galaxy was about 760 million light years away at the time the light by which we see it was emitted, about 780 to 785 million years ago (the difference between the two numbers being due to the expansion of the intervening space during the light-travel time). Given that and its apparent size of about 0.45 by 0.3 arcmin, the galaxy is about 100 thousand light years across. Based on the Carnegie-Irvine image, it is possible that this galaxy is a starburst or at least active nucleus galaxy.

Historical Identification: Per Dreyer, NGC 4025 (= GC 2659 = JH 1046 = WH III 617, 1860 RA 11 51 58, NPD 51 24.7) is "extremely faint, pretty large, round." The position precesses to RA 11 59 11.1, Dec +37 48 32, about 1 arcmin north northeast of the galaxy listed above, the description is reasonable for an early visual observer and there is nothing else nearby, so the identification is certain.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 3470 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4025 is about 160 to 165 million light years away, in reasonable agreement with redshift-independent distance estimates of about 105 to 155 million light years. Given that and its apparent size of about 2.15 by 1.35 arcmin (from the images below), the galaxy is about 100 thousand light years across.

Historical Identification: Per Dreyer, NGC 4026 (= GC 2660 = JH 1047 = WH I 223, 1860 RA 11 52 11, NPD 38 15.5) is "very bright, considerably large, much extended 176°, very suddenly very much brighter middle and bright nucleus". The position precesses to RA 11 59 25.6, Dec +50 57 44, essentially dead center on the galaxy listed above, the description fits and there is nothing else nearby, so the identification is certain.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 1175 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4026 is about 55 million light years away, in good agreement with redshift-independent distance estimates of about 40 to 85 million light years. Given that and its apparent size of about 4.4 by 1.4 arcmin (from the images below), the galaxy is about 70 thousand light years across. Although the galaxy looks perfectly smooth in typical images, it has slight irregularities in its shape, and a complex distribution of dust can be observed by subtracting images taken at one wavelength from those taken at another wavelength.
Usage by the de Vaucouleurs Atlas: NGC 4026 is used by the de Vaucouleurs Atlas of Galaxy Types as an example of type S0°(s). NED essentially agrees with that, but LEDA lists it as SB0, presumably because of the small disklike structure in the center, as shown in the de Vaucouleurs Atlas image of NGC 4026.

Historical Identification: Per Dreyer, NGC 4027 (= GC 2661 = JH 3371 = WH II 296, 1860 RA 11 52 23, NPD 108 29.4) is "a globular cluster, pretty faint, pretty large, round, partially resolved (some stars seen), stars of 16th magnitude". The position precesses to RA 11 59 32.2, Dec -19 16 10, slightly southeast of the center of the galaxy listed above but well within its brightest region. The description is obviously wrong, but given the diffuse structure of the galaxy and its numerous knots of star formation, it could have been easily confused with a stellar structure by observers who believed that most nebulae were just clusters of stars too distant to resolve, and there is nothing comparable nearby, so the identification is certain.
Discovery Note: Steve Gottlieb writes that in a large telescope the unusual structure of the galaxy is obvious, but in smaller ones it appears to have a stellar center, or even to be too faint to note any details. William Herschel merely described it as "pretty bright, pretty large"; it was his son John whose mistaken description is quoted more or less verbatim in the NGC.
Usage By The Arp Atlas: NGC 4027 is used by the Arp atlas as an example of a one-armed spiral galaxy (presumably due to a collision with or near passage by another galaxy millions of years ago, the most likely candidate being PGC 37772, which is therefore considered to be part of Arp 22, even though Arp made no mention of that).
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 2025 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4027 is about 95 million light years away, in fair agreement with widely varying redshift-independent distance estimates of about 25 to 85 million light years. Given that and its apparent size of about 3.1 by 2.65 arcmin (from the images below), the galaxy is about 85 thousand light years across. NGC 4027 is used by the de Vaucouleurs Atlas of Galaxy Types as an example of type SB(s)dm. It is also part of the NGC 4038 Group, whose namesake is one of the even more spectacular Antennae Galaxies.

Warning About Non-Standard Designations: There are no rules for assigning non-standard "letter" designations to NGC/IC objects, so the same letter may be applied to two or more different galaxies, leading not only to confusion about which is which, but causing data acquired for one galaxy to be assigned to a completely different one. For that reason, such non-standard designations should never be used.
Usage By The Arp Atlas: Although Arp made no mention of PGC 37772 in his discussion of Arp 22, it is generally presumed to be a part of the Arp object because it is the most likely cause of NGC 4027's peculiar structure, and is in the field of view of the plate taken by Arp.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 2100 km/sec (and H₀ = 70 km/sec/Mpc), PGC 37772 is about 95 to 100 million light years away (and since that's about the same distance as estimated for NGC 4027, seems to confirm the possibility of a past interaction between the two galaxies). Given that and its apparent size of about 0.7 by 0.45 arcmin (from the images below), the galaxy is about 20 thousand light years across.

Historical Identification: Per Dreyer, NGC 4028 (= GC 2662 = WH III 3, 1860 RA 11 52 45, NPD 73 00.0) is "very faint, very small, very little extended, mottled but not resolved". The position precesses to RA 11 59 56.1, Dec +15 13 14, but there is nothing there nor near there. However, two minutes of time directly to the west there is a galaxy that fits the description and the note discussed in Discovery Note (2), so it appears that this is a duplicate observation of NGC 4014, as indicated in the title for this entry. That conclusion is undercut by the fact that as stated in the NGC note shown immediately below, WH made two observations of what was published as III 3, using two different stars on different nights, and both observations have the same two minute error in right ascension, which would be an unlikely coincidence; but there is nothing else that could possibly be III 3, so the identification of NGC 4028 as a duplicate observation of NGC 4014 is considered reasonably certain.
Discovery Note (1): A note at the end of the NGC states "(J)h 1042. This cannot be (WH) III 3, as (Caroline Herschel) has reduced two obs. of this latter well agreeing, and giving a RA 2 minutes of time exceeding that of h 1042, which also rests on two obs. by (J)h. (note per JH)." However, it turns out that that note was probably wrong, as NGC 4014 (= JH 1042) and NGC 4028 (= WH III 3) are now recognized as almost certainly being the same object.
Discovery Note (2): In his notes for the 1912 publication of The Scientific Papers of Sir William Herschel, Dreyer writes that Herschel's original paper states that the nebula forms an isoceles triangle with two "small" (that is, faint) stars, and adds that from a diagram of the observation, the stars are about 6 arcmin to the southwest of the nebula. This agrees with the double star about that distance to the southwest of NGC 4014, and supports the idea that NGC 4028 is a duplicate observation of NGC 4014.
Discovery Note (3): In the 1912 note by Dreyer discussed immediately above, he states that Bigourdan looked for NGC 4028 on two separate occasions, but failed to find it (which is hardly surprising, given the lack of anything near the NGC position), hence the note about Bigourdan in the discovery credits.
Physical Information: Given the fact that unless lost NGC 4028 is almost certainly a duplicate observation of NGC 4014, see that entry for anything else.

Historical Identification: Per Dreyer, NGC 4029 (= GC 5599, Marth #226, 1860 RA 11 52 50, NPD 81 02) is "very faint, very small, a little extended, stellar nucleus". The position precesses to RA 12 00 00.7, Dec +08 11 14, only about 0.7 arcmin northwest of the galaxy listed above, the description fits and there is nothing comparable nearby, so the identification is certain.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 6565 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4029 is about 305 million light years away, in good agreement with redshift-independent distance estimates of about 300 to 325 million light years. Given that and its apparent size of about 1.15 by 0.95 arcmin (from the images below), the galaxy is about 100 to 105 thousand light years across. The small galaxy to its northeast (PGC 213919) has a similar recessional velocity and may be a physical companion, in which case an interaction between the two may be responsible for the somewhat peculiar structure of NGC 4029.

NED Designation: A search of the NED requires the designation LEDA 213919.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 6625 km/sec (and H₀ = 70 km/sec/Mpc), PGC 213919 is about 305 to 310 million light years away. Given that and its apparent size of about 0.3 by 0.1 arcmin (from the images below), the galaxy is about 25 thousand light years across. Since the estimated distance is about the same as that of NGC 4029, it is possible that the two galaxies are a physical pair, which might explain the somewhat unusual structure of the larger galaxy.
Classification Note: The galaxy is too small and its image too poor to really tell what kind of galaxy it is. However, LEDA lists it as type Sb on the basis of four papers, three of which classified it as type Sc, and the fourth as type Sb, so I have chosen to list it as a presumably more or less edge-on type Sbc galaxy.

Historical Identification: Per Dreyer, NGC 4030 (= GC 2663 = JH 1048 = WH I 121, 1860 RA 11 53 14, NPD 90 19.3) is "considerably bright, large, very little extended, pretty suddenly much brighter middle, bright star near". The position precesses to RA 12 00 24.2, Dec -01 06 04, nearly dead center on the galaxy listed above, the description fits and there is nothing else nearby, so the identification is certain.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 1825 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4030 is about 85 million light years away, in good agreement with widely varying redshift-independent distance estimates of about 35 to 95 million light years (the ESO press release uses a distance of 75 million light years). Given that and its apparent size of about 3.75 by 2.65 arcmin (from the images below), the galaxy is about 90 to 95 thousand light years across. NGC 4030 is used by the de Vaucouleurs Atlas of Galaxy Types as an example of type SA(r)bc, but other references list it as SA(s)bc, so I've combined the two types and added a question mark to the result.

Historical Identification: Per Dreyer, NGC 4031 (= GC 5600, d'Arrest, 1860 RA 11 53 20, NPD 57 16.5) is "extremely faint, very small, 17th magnitude star very near to south". The position precesses to RA 12 00 31.9, Dec +31 56 44, on the southeastern rim of the galaxy listed above, the description fits and there is nothing else nearby, so the identification is certain.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 8140 km/sec (and H₀ = 70 km/sec/Mpc), a straightforward calculation indicates that NGC 4031 is about 380 million light years away. However, for objects at such distances we should take into account the expansion of the Universe during the time it took their light to reach us. Doing that shows that the galaxy was about 365 to 370 million light years away at the time the light by which we see it was emitted, about 370 to 375 million years ago (the difference between the two numbers being due to the expansion of the intervening space during the light-travel time). Given that and its apparent size of about 0.5 by 0.4 arcmin, the galaxy is about 50 to 55 thousand light years across.

Historical Identification: Per Dreyer, NGC 4032 (= GC 2664 = JH 1049 = WH II 404, 1860 RA 11 53 22, NPD 69 08.8) is "pretty faint, pretty large, round, gradually brighter middle, 12th magnitude star to northeast". The position precesses to RA 12 00 33.2, Dec +20 04 26, barely east of the center of the galaxy listed above, the description fits and there is nothing else nearby, so the identification is certain.
Note About Misidentification of NGC 4042: As discussed in the entry for NGC 4042, RC1 raised the possibility that that object was NGC 4032; so this note and the title for this entry serve as a warning that there is little or (more likely) no chance of that being true.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 1590 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4032 is about 75 million light years away. Given that and an apparent size of about 1.6 by 1.25 arcmin (from the images below), it is about 35 thousand light years across.

Historical Identification: Per Dreyer, NGC 4033 (= GC 2665 = WH II 508, Engelhardt, 1860 RA 11 53 26, NPD 107 03.8) is "pretty bright, small, a little extended, brighter middle". The position precesses to RA 12 00 35.5, Dec -17 50 34, almost dead center on the galaxy listed above, the description fits and there is nothing else nearby, so the identification is certain.
Discovery Note: The NGC description is taken from the GC, but the position is taken from Engelhardt's paper.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 2020 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4033 is about 95 million light years away, in good agreement with redshift-independent distance estimates of about 40 to 115 million light years. Given that and its apparent size of about 2.25 by 1.1 arcmin (from the images below), the galaxy is about 60 to 65 thousand light years across.

Historical Identification: Per Dreyer, NGC 4034 (= GC 2666 = WH III 903, 1860 RA 11 53 45, NPD 19 52.0) is "extremely faint, small, irregular figure, gradually a very little brighter middle". The position precesses to RA 12 01 01.1, Dec +69 21 14, about 3 arcmin northwest of the galaxy listed above, but the description fits and there is nothing else nearby, so the identification is certain.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 2450 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4034 is about 115 million light years away, in fair agreement with redshift-independent distance estimates of about 135 to 150 million light years. Given that and its apparent size of about 1.7 by 1.05 arcmin (from the images below), the galaxy is about 55 to 60 thousand light years across.

Historical Identification: Per Dreyer, NGC 4035 (= GC 2667 = JH 3372 = WH III 279, 1860 RA 11 53 51, NPD 105 10.2) is "extremely faint, pretty large, 9th magnitude star at position angle 45°±". The position precesses to RA 12 01 00.7, Dec -15 56 58, but there is nothing there. However, there is a galaxy that perfectly fits the description, including the star to its northeast, only half a minute of time to the west, and there is nothing else nearby, so the identification is certain.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 1940 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4035 is about 90 million light years away. Given that and its apparent size of about 1.0 by 0.95 arcmin (from the images below), the galaxy is about 25 to 30 thousand light years across.

Historical Identification: Per Dreyer, NGC 4036 (= GC 2668 = JH 1050 = WH I 253, 1860 RA 11 54 13, NPD 27 19.5) is "very bright, very large, extended". The position precesses to RA 12 01 26.6, Dec +61 53 44, dead center on the galaxy listed above, the description fits and there is nothing else nearby, so the identification is certain.
Discovery Notes: A note at the end of the NGC states "(J)h 1050 = (WH) I 253. d'Arrest and Schultz agree with (W)H as to description. No doubt about the identity; places agree." In his General Catalogue (GC) John Herschel gave two descriptions, one by his father ("very bright, very large, extended") and one from his own observation ("pretty bright, 25 arcsec [corresponding to small or considerably small by Herschel's definition], round"). Schultz's paper stated that the nebula is extended about 1.5 arcmin roughly west southwest to east northeast (its actual elongation is a little more east-west, but there is an obvious southwest-northeast tilt relative to the parallel of declination). d'Arrest observed the object on five nights and repeatedly confirmed William Herschel's description, giving estimated extensions of 1.4 to 3 arcmin depending on the seeing. With both Schultz and d'Arrest stating that the nebula was extended and much larger than John Herschel's estimated size, it was inevitable that Dreyer would reject the younger Herschel's description. As a result, he used d'Arrest's position and William Herschel's description for the NGC entry.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 1515 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4036 is about 70 million light years away, in good agreement with redshift-independent distance estimates of about 45 to 110 million light years. Given that and its apparent size of about 4.1 by 1.6 arcmin (from the images below), the galaxy is about 85 thousand light years across. NGC 4036 is listed as a Seyfert galaxy (type S3b).
Usage By The de Vaucouleurs Atlas: NGC 4036 is used by the de Vaucouleurs Atlas of Galaxy Types as an example of type SAB0⁺(s); however, based on the HST image, I have slightly altered this as shown in the description line for this entry.

Historical Identification: Per Dreyer, NGC 4037 (= GC 2669 = JH 1051 = WH III 77, 1860 RA 11 54 13, NPD 75 49.2) is "extremely faint, pretty large, round, mottled but not resolved". The position precesses to RA 12 01 23.7, Dec +13 24 02, essentially dead center on the galaxy listed above, the description is a perfect fit and there is nothing else nearby, so the identification is certain.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 1270 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4037 is about 55 to 60 million light years away, in reasonable agreement with redshift-independent distance estimates of about 25 to 55 million light years. Given that and its apparent size of about 2.65 by 2.6 arcmin (from the images below), the galaxy is about 45 thousand light years across.

Historical Identification: Per Dreyer, NGC 4038 (= GC 2670 = JH 1052 = WH IV 28.1, 1860 RA 11 54 44, NPD 108 05.3) is "pretty bright, considerably large, round, very gradually brighter middle". The position precesses to RA 12 01 53.8, Dec -18 52 04, nearly dead center on the galaxy listed above, the description fits and the only other galaxy in the region is accounted for by NGC 4039, so the identification is certain.
Physical Information: NGC 4038 and 4039 comprise the Antennae Galaxies, so-called because of the 550 thousand plus light year long tails of gas and stars created by their collision. The two galaxies are undergoing a hundred-million-year long collision in which individual stars rarely (if ever) collide, because they are almost infinitely small in comparison to their separation, but do get scattered into intergalactic space as a result of the other galaxy's gravity changing their original motions. Meanwhile, clouds of gas and dust, which unlike stars extend over vast distances, slam into each other, are violently compressed, and form millions of young stars, many of which are extremely hot, bright objects which heat and light up the regions in which they form. Over a very long period of time the two galaxies will pass through each other, over and over, until they merge.
 Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 1995 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4038 is about 90 to 95 million light years away (and of course at the same distance as NGC 4039), in reasonable agreement with redshift-independent distance estimates of about 45 to 90 million light years. Given that and its apparent size of about 2 by 1.25 arcmin (from the images below), the galaxy is about 55 thousand light years across and the pair, which has an apparent size of about 2.5 by 1.75 arcmin (also from the images below) spans about 65 to 70 thousand light years. Note: This size is for the main structure of the galaxy, and does not include the extended clouds and tails of stars and gas surrounding the pair of galaxies, which span at least 20 arcmin and 550 thousand light years.
Usage By The Arp Atlas: With NGC 4039, NGC 4038 is used by the Arp Atlas as an example of galaxies with the appearance of fission.
Usage By The de Vaucouleurs Atlas: NGC 4038 and 4039 are used by the de Vaucouleurs Atlas of Galaxy Types as examples of type Pec.

Historical Identification: Per Dreyer, NGC 4039 (= GC 2671 = JH 1053 = WH IV 28.2, 1860 RA 11 54 44, NPD 108 06.5) is "pretty faint, pretty large". The position precesses to RA 12 01 53.8, Dec -18 53 16, nearly dead center on the galaxy listed above, the description is reasonable and the only other galaxy in the region is accounted for by NGC 4038, so the identification is certain.
Physical Information: NGC 4038 and 4039 comprise the Antennae Galaxies, so-called because of the 550 thousand plus light year long tails of gas and stars created by their collision. See the discussion of NGC 4038 for details about the collision, and more images of the galaxies.
 Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 1995 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4039 is about 90 to 95 million light years away (and of course at the same distance as NGC 4038), in reasonable agreement with redshift-independent distance estimates of about 45 to 90 million light years. Given that and its apparent size of about 1.2 by 0.75 arcmin (from the image below), the galaxy is about 30 to 35 thousand light years across. Note: This size is for the main structure of the galaxy, and does not include the extended clouds and tails of stars and gas surrounding the pair, which span at least 20 arcmin and 550 thousand light years.
Usage By The Arp Atlas: With NGC 4038, NGC 4039 is used by the Arp Atlas as an example of galaxies with the appearance of fission.
Usage By The de Vaucouleurs Atlas: NGC 4038 and 4039 are used by the de Vaucouleurs Atlas of Galaxy Types as examples of type Pec.

Historical Identification: Per Dreyer, NGC 4040 (Swift list VI (#41), 1860 RA 11 54 48, NPD 71 23.9) is "extremely faint, pretty small, round, 3 stars near". The position precesses to RA 12 01 58.7, Dec +17 49 20, about 1.6 arcmin due west of the galaxy listed above, the description fits (the three stars lying to the south and southeast of the galaxy) and there is nothing else that Swift could have seen nearby, so the identification is certain.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 7250 km/sec (and H₀ = 70 km/sec/Mpc), a straightforward calculation indicates that NGC 4040 is about 335 to 340 million light years away, in reasonable agreement with a single redshift-independent distance estimate of about 320 million light years. However, for objects at such distances we should take into account the expansion of the Universe during the time it took their light to reach us. Doing that shows that the galaxy was about 325 to 330 million light years away at the time the light by which we see it was emitted, about 330 to 335 million years ago (the difference between the two numbers being due to the expansion of the intervening space during the light-travel time). Given that and its apparent size of about 0.55 by 0.4 arcmin (from the images below), the galaxy is about 50 to 55 thousand light years across.

Historical Identification: Per Dreyer, NGC 4041 (= GC 2672 = JH 1054 = WH I 252, 1860 RA 11 55 00, NPD 27 05.0) is "bright, considerably large, round, gradually then pretty suddenly very much brighter middle and mottled but not resolved nucleus". The position precesses to RA 12 02 12.5, Dec +62 08 14, essentially dead center on the galaxy listed above, the description fits and there is nothing else nearby, so the identification is certain.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 1360 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4041 is about 60 to 65 million light years away, in reasonable agreement with redshift-independent distance estimates of about 75 to 130 million light years. Given that and its apparent size of about 2.6 by 2.2 arcmin (from the images below), the galaxy is about 45 to 50 thousand light years across.

Historical Identification: Per Dreyer, NGC 4042 (= GC 5601, Marth #227, 1860 RA 11 55 10, NPD 69 05) is "very faint, very small". The first IC notes "Not seen by Spitaler". The position precesses to RA 12 02 20.7, Dec +20 08 14, but there is nothing there nor near there save three very faint and extremely faint objects, which are more or less equally spaced relative to Marth's position (so it is hardly surprising that Spitaler couldn't find NGC 4042). However, per Corwin, both the other two objects observed by Marth on the same night had errors of about -10 seconds of time and -1 arcmin in declination, and if Marth #227 had a similar error, its position would be about RA 12 02 30.6, Dec +20 09 14, which is still about 16 seconds of time west of the galaxy listed above, but it is the brightest of the three objects in the region and only about 3.8 arcmin from Corwin's adjusted position, so it does seem a reasonably likely albeit not certain identification.
Misidentification Note: RC1 raised the possibility that NGC 4042 might be a duplicate entry for NGC 4032, but Corwin points out that that object would require an error of more than 2 minutes of time and 5 arcmin of declination, and it is far more likely that Marth's errors for the night were more nearly consistent than not, so it is essentially certain that that suggestion is not correct, as indicated in the title for this entry. In fact, it is far more likely that NGC 4042 is lost or nonexistent than a duplicate observation of NGC 4032.
Designation Note: Neither LEDA nor NED return a result for a search for PGC 3781394, which is why that designation is in quotes in the title for this entry; but both databases return the galaxy listed above in a search for NGC 4042, so the identification, though not certain, is almost universally accepted.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 7500 km/sec (and H₀ = 70 km/sec/Mpc), a straightforward calculation indicates that PGC 3781394 is about 350 million light years away, in reasonable agreement with a single redshift-independent distance estimate of about 365 million light years. However, for objects at such distances we should take into account the expansion of the Universe during the time it took their light to reach us. Doing that shows that the galaxy was about 340 million light years away at the time the light by which we see it was emitted, almost 345 million years ago (the difference between the two numbers being due to the expansion of the intervening space during the light-travel time). Given that and its apparent size of about 0.27 by 0.18 arcmin (from the images below), the galaxy is about 25 thousand light years across.

Historical Identification: Per Dreyer, NGC 4043 (= GC 2673 = JH 1055, 1860 RA 11 55 11, NPD 84 52.6) is "pretty faint, small, round, pretty suddenly brighter middle, double star 30 seconds of time to east". The position precesses to RA 12 02 21.4, Dec +04 20 38, only about 1 arcmin north northwest of the galaxy listed above, the description fits and the double star to the east makes the identification certain.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 6815 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4043 is about 315 to 320 million light years away. Given that and its apparent size of about 0.65 by 0.5 arcmin (from the images below), the galaxy is about 60 thousand light years across.
Classification Note: The closeup image below clearly shows an inner ring, and suggests the presence of an outer one; but it shows no sign of a bar, although every reference I've seen lists this as a barred galaxy. As a result I have classified it as a transitional galaxy, giving a nod both to the usual classification and to the absence of a bar in the images shown here. There is also what appears to be a spiral arm extending from the western (right) side of the inner ring, running clockwise under the ring and ending on the eastern (left) side of the outer ring, whence my addition of an "s" to the classification.

Historical Identification: Per Dreyer, NGC 4044 (= GC 2674 = JH 1056 = WH III 491, 1860 RA 11 55 19, NPD 89 25.9) is "considerably faint, considerably small, round, brighter middle". The position precesses to RA 12 02 29.3, Dec -00 12 40, nearly dead center on the galaxy listed above, the description fits and there is nothing else nearby, so the identification is certain.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 6495 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4044 is about 300 to 305 million light years away. Given that and its apparent size of about 1.25 by 1.1 arcmin (from the images below), the galaxy is about 110 thousand light years across.

Historical Identification: Per Dreyer, NGC 4045 (= GC 2675 = JH 1057 = WH II 276, (David Todd #13a), 1860 RA 11 55 33, NPD 87 14.8) is "pretty faint, large, round, suddenly brighter middle, star to southeast". The position precesses to RA 12 02 43.3, Dec +01 58 26, well within the southeastern outline of the galaxy listed above, the description fits (including the star to the southeast) and there is nothing comparable nearby, so the identification is certain.
Discovery Notes: Todd observed this (as well as a number of other nebulae) during an unsuccessful search for a large trans-Neptunian planet (a search that proved futile for all observers until Pluto was discovered; and even that body does not fit the predicted size and mass of the object being looked for (which almost certainly does not exist, since the data that predicted its existence have been shown to be in error), so to date there has been no successful search for such an object, and probably never will be). But Todd's observations of such nebulae were published in a form that made it hard to identify them, so many were not included in the NGC, hence his name being added to Dreyer's entry in parentheses.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 2335 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4045 is about 105 to 110 million light years away, in good agreement with redshift-independent distance estimates of about 75 to 115 million light years. Given that and its apparent size of about 2.3 by 1.15 arcmin (from the images below), the galaxy is about 70 to 75 thousand light years across. Note: As pointed out in the entry for PGC 38033, that galaxy is more than twice as far from us, so the two galaxies are not companions, but merely an optical double.
Classification Note: NGC 4045 is used by the de Vaucouleurs Atlas of Galaxy Types as an example of type (R₁')SAB(rs)b.

Warning About Non-Standard Designations: There is no standard way to assign letters to NGC/IC designations, and as a result it is not unusual for the same letter to be used for more than one galaxy, or two different letters being assigned to the same galaxy. In either case this can cause confusion, and result in data for one galaxy being assigned to another. As a result, such designations should never be used.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 5550 km/sec (and H₀ = 70 km/sec/Mpc), PGC 38033 is about 255 to 260 million light years away (which, being more than twice as far as NGC 4045, means the two galaxies are not companions, but merely an optical double). Given that and its apparent size of about 0.7 by 0.35 arcmin (from the image below), the galaxy is about 50 to 55 thousand light years across.

Historical Identification: Per Dreyer, NGC 4046 (= GC 5602, d'Arrest, 1860 RA 11 55 34, NPD 87 29.2) is "faint, pretty small, in a triangle with two faint stars (Query: = h 1057)", (J)h 1057 being NGC 4045, which is indeed the same object. The "Query" is presumably based on a note at the end of the NGC which states "GC 5602. Nova d'Arrest, one observation. Perhaps = h 1057 = II 276 with an error of 15 arcmin in PD. (J)h and (W)H agree." The position precesses to RA 12 02 44.3, Dec +01 44 02, but there is nothing there nor anywhere near there. However, as discussed immediately below, there is no doubt that this is simply a mistakenly recorded obervation of NGC 4045.
Discovery Note: d'Arrest himself suggested that his "Nova" might be (WH) II 276 (which is NGC 4045), and his description of the position of the two stars in his papers is a very good fit to the position of the star to the southeast of that galaxy, and either the galaxy due south of NGC 4045 or the star near it, so d'Arrest and Dreyer's supposition that this was a duplicate observation of NGC 4045 is essentially certain.
Misidentification As PGC 38075: As noted by Corwin, a Mount Wilson observer (possibly Edwin Hubble) assigned PGC 38075 to NGC 4046, presumably based on the comment about two faint stars, but whoever made the assignment must not have read d'Arrest's paper, as the stars near that galaxy lie to the north and west, not the south and east as specified by d'Arrest. And PGC 38075 is just as from d'Arrest's position as NGC 4045, but to the north northeast instead of directly north, which requires an error in both right ascension and declination, which is more unlikely than an error in only one of the two measurements. Finally, PGC 38075 is much fainter than NGC 4045, so it is a far poorer choice for what d'Arrest saw, even in the absence of other information. However, despite all these problems, the fact that PGC 38075 was once misidentified as NGC 4046 means that the misidentification may pop up again, hence the need for this warning and the one in the entry immediately following this entry.
Physical Information: Given the presumably duplicate entry, see NGC 4045 for anything else.

Note About The Misidentification: As discussed in detail in the previous entry (which see), PGC 38075 was once misidentified as NGC 4046; so this entry serves as a warning about that error.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 6115 km/sec (and H₀ = 70 km/sec/Mpc), PGC 38075 is about 285 million light years away. Given that and its apparent size of about 0.6 by 0.4 arcmin (from the images below), the galaxy is about 50 thousand light years across.

Historical Identification: Per Dreyer, NGC 4047 (= GC 2676 = JH 1058 = WH II 741, 1860 RA 11 55 37, NPD 40 35.1) is "pretty bright, pretty small, round". The position precesses to RA 12 02 48.0, Dec +48 38 08, on the western rim of the galaxy listed above, the description fits and there is nothing else nearby, so the identification is certain.
Discovery Note: In William Herschel's 2nd Catalogue of Nebulae and Clusters, WH II 734 to II 744 are all listed as having been observed on Feb 9, 1788; however, in Caroline Herschel's "fair copy" of her brother's "sweeps" there are no entries for Feb 9, and the correct date for WH II 741 is, as shown above, Mar 9.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 3615 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4047 is about 165 to 170 million light years away, in reasonable agreement with redshift-independent distance estimates of about 175 to 245 million light years. Given that and its apparent size of about 1.15 by 0.8 arcmin for the bright central region, about 1.8 by 1.25 arcmin including the ring surrounding that region, and about 2.45 by 2.0 for the fainter outer regions (all from the images below), the bright central part of the galaxy is about 55 to 60 thousand light years across, the ring surrounding that spans about 85 to 90 thousand light years, and the entire structure spans about 120 thousand light years.

Historical Identification: Per Dreyer, NGC 4048 (= GC 2677 = JH 1059, 1860 RA 11 55 39, NPD 71 12.2) is "very faint, very small, round, pretty suddenly brighter middle". The position precesses to RA 12 02 49.5, Dec +18 01 02, on the northwestern rim of the galaxy listed above, the description fits and there is nothing else nearby, so the identification is certain.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 5140 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4048 is about 240 million light years away. Given that and its apparent size of about 0.65 by 0.45 arcmin (from the images below), the galaxy is about 45 thousand light years across. Its unusual structure, combined with the relatively low quality of currently available images, makes its classification uncertain; but it is definitely a peculiar barred galaxy.

Historical Identification: Per Dreyer, NGC 4049 (= GC 2678 = JH 1060 = WH III 390, 1860 RA 11 55 42, NPD 70 28.4) is "extremely faint, pretty small, round, gradually a little brighter middle". The position precesses to RA 12 02 52.5, Dec +18 44 50, less than 0.6 arcmin from the center of the galaxy listed above and barely off its southwestern rim, the description fits and there is nothing else nearby, so the identification is certain.
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background radiation of 1155 km/sec (and H₀ = 70 km/sec/Mpc), NGC 4049 is about 50 to 55 million light years away, on the outskirts of the Virgo cluster. Given that and its apparent size of about 1.05 by 0.6 arcmin (from the images below), the galaxy is about 15 to 18 thousand light years across. It has been suggested that the unusual structure of NGC 4064 (another galaxy on the outskirts of the Virgo Cluster), which lies as little as half a million light years from NGC 4049 (depending on how similar or different their distances from us happen to be), may be partly due to a past interaction of the two galaxies; but other possibilities make that suggestion, though not unreasonable, not entirely certain either.