Historical Identification: Per Dreyer, NGC 3100 (= GC 1997 = JH 3218, 1860 RA 09 54 28, NPD 120 59.6) is "pretty bright, pretty small, round, gradually pretty much brighter middle". The position precesses to RA 10 00 40.3, Dec -31 39 51, right on the galaxy listed above, the description fits and there is nothing else nearby so the identification is certain. The second IC adds "3103 = 3100, per Howe, confirmed by reference to Swift's original observation", so the equality of the two entries has been known for more than a century.
Physical Information: Based on a recessional velocity of 2640 km/sec, NGC 3100 is 120 to 125 million light years away. Given that and its apparent size of 3.7 by 2.1 arcmin, it is 130 to 135 thousand light years across.

Historical Identification: Per Dreyer, NGC 3101 (= GC 5519, Marth 193, 1860 RA 09 54 29, NPD 92 20) is "extremely faint". The position precesses to RA 10 01 34.8, Dec -03 00 18, just over 0.6 arcmin south southwest of the galaxy listed above and there is nothing else nearby, so the identification is certain.
Physical Information: Based on a recessional velocity of 5715 km/sec, NGC 3101 is about 265 million light years away. Given that and its apparent size of 1.2 by 0.25 arcmin, it is 90 to 95 thousand light years across.

Historical Identification: Per Dreyer, NGC 3102 (= GC 1998 = JH 662 = WH III 916, 1860 RA 09 54 37, NPD 29 13.4) is "very faint, very small, round, brighter middle, 11th magnitude star at position angle 142 ". The position precesses to RA 10 04 32.7, Dec +60 06 08, within the southeastern outline of the galaxy listed above, there is nothing else nearby and the star to the southeast makes the identification certain.
Physical Information: Based on a recessional velocity of 3065 km/sec, NGC 3102 is 140 to 145 million light years away. Given that and its apparent size of 1.05 by 1.05 arcmin, it is 40 to 45 thousand light years across.

Historical Identification: Per Dreyer, NGC 3103 (= Swift list III (#53), 1860 RA 09 55 19, NPD 121 00.7) is "extremely faint, pretty large, round". The position precesses to RA 10 01 31.6, Dec -31 41 03, nearly halfway between NGC 3100 and NGC 3108, in an otherwise stellar region. As noted in the second Index Catalog, "3103 = 3100, per Howe, confirmed by reference to Swift's original observation", which includes a note about a "coarse double star preceding (to the west)" of Swift's nova. The double star actually follows (lies to the east) of NGC 3100, but such errors in direction are relatively common in visual observations, so Howe was convinced of the equality with NGC 3100, and as a result it has been considered a certain identification of NGC 3103 for more than a century.
Discovery Notes: Despite agreeing with the above, Corwin notes that it requires only a minute of time change in Swift's right ascension for his position to exactly fall on NGC 3108, so it is only the reference to the double star that makes the equality of NGC 3100 and 3103 certain (there being no such object near NGC 3108). As a result, if Swift had not mentioned the double star it is likely that we would be (incorrectly) equating 3103 with 3108, instead of with 3100.
Physical Information: Given the duplicate entry, see NGC 3100 for anything else.

Historical Identification: Per Dreyer, NGC 3104 (= GC 1999 = JH 665 = WH IV 48, 1860 RA 09 55 19, NPD 48 35.5) is "extremely faint, pretty large, extended, very faint star involved". The position precesses to RA 10 03 51.0, Dec +40 44 02, about 1.8 arcmin southwest of the center of the galaxy listed above (though barely outside its southwestern outline), but the description is perfect and there is nothing else nearby so the identification is certain.
Physical Information: Based on a recessional velocity of 600 km/sec, NGC 3104 is 25 to 30 million light years away, in reasonable agreement with redshift-independent distance estimates of 30 to 50 million light years. Given that and its apparent size of 3.3 by 1.8 arcmin, it is 25 to 30 thousand light years across. NGC 3104 is used by the Arp Atlas as an example of a galaxy with irregular clumps.

Historical Identification: Per Dreyer, NGC 3105 (= GC 2000 = JH 3219, 1860 RA 09 55 46, NPD 144 06.3) is a "cluster, compact, a little extended, stars from 13th to 16th magnitude". The position precesses to RA 10 00 43.7, Dec -54 46 38, within the northern outline of the cluster listed above, the description fits and there is nothing similar nearby so the identification is certain.
Physical Information: NGC 3105 is between 15 to 25 thousand light years away. Its Hertzsprung-Russell Diagram includes B-type stars, indicating a probable age of 20 to 30 million years. Given its distance and its apparent size of about 2.0 by 1.5 arcmin, its few dozen brighter members are scattered across 8 to 15 light years. LEDA lists NGC 3105 as an object of unknown nature with the designation PGC 3518279, but a search of the database for that designation returns no result.

Historical Identification: Per Dreyer, NGC 3106 (= GC 2001 = JH 666 = WH II 320, 1860 RA 09 55 58, NPD 58 08.3) is "faint, small, round, suddenly brighter middle". The position precesses to RA 10 04 05.6, Dec +31 11 12, right 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 of 6205 km/sec, NGC 3106 is about 290 million light years away, in good agreement with redshift-independent distance estimates of 195 to 290 million light years. Given that and its apparent size of 1.5 by 1.5 arcmin, the brighter inner region is about 125 thousand light years across, while the 2.6 by 2.3 arcmin wide outer ring spans about 220 thousand light years.

Historical Identification: Per Dreyer, NGC 3107 (= GC 2004 = WH II 898, 1860 RA 09 56 00, NPD 75 49.8) is "pretty faint, pretty large, irregularly round, 8th magnitude star 112 arcsec distant at position angle 148 " (it is critical to note that Herschel's original description specified that the star appeared red, as stated in the Discovery Notes below). The position precesses to RA 10 03 33.3, Dec +13 29 43, at the specified offset from an 8th magnitude star, but as shown in the first image below there is nothing there, because Dreyer picked the wrong star. When the correct star is chosen, as shown in the second image below, the galaxy listed above falls at exactly the correct relative position (taking into account the precession of the Pole during the interval between Lord Rosse's measurements and the present epoch), making the identification certain.
Discovery Notes: Per Corwin, it is remarkable that Herschel's II 898 could be found at all, as his telescope was not aligned with the meridian when he noticed the nebula, so he could not make his usual measurements. Instead, he merely noted that it was "3 arcmin north of a pretty large red star", and "3/4 deg following, 1/2 deg north [of the] Georgian planet", meaning Uranus. Despite that, Caroline Herschel's reduction of the position turned out to be only 5 seconds east and 6 arcmin south of the galaxy listed above, which is considerably better than Dreyer's position thanks to Dreyer choosing the wrong "red star" in his 1877 supplement to the GC (namely HD 87176, an F5 star far too hot to appear red), causing him to calculate an incorrect position in that and all his following papers, including those of his employer (the 4th Lord Rosse) and the NGC. The correct "red star" must be HD 87300 (of spectral type K2, which would appear reddish-orange), since using Lord Rosse's offsets from that star yields a position within an arcsecond of the correct position, making the identification certain.
Physical Information: Based on a recessional velocity of 2815 km/sec, NGC 3107 is about 130 million light years away, in good agreement with a single redshift-independent distance estimate of 125 million light years. Given that and its apparent size of 0.9 by 0.65 arcmin, it is about 35 thousand light years across.

Historical Identification: Per Dreyer, NGC 3108 (= GC 2002 = JH 3220, 1860 RA 09 56 18, NPD 121 00.6) is "faint, small, round, gradually a little brighter middle". The position precesses to RA 10 02 31.0, Dec -31 41 03, within the southeastern outline of the galaxy listed above, the description fits and there is nothing else nearby so the identification is certain.
Discovery Notes: As noted in the entry for NGC 3103, if not for Swift mentioning a nearby double star, that object might have been (incorrectly) identified as a duplicate of NGC 3108.
Physical Information: Based on a recessional velocity of 2675 km/sec, NGC 3108 is about 125 million light years away, in good agreement with redshift-independent distance estimates of 115 to 135 million light years. Given that and its apparent size of 3.4 by 2.4 arcmin, it is 120 to 125 thousand light years across.

Historical Identification: Per Dreyer, NGC 3109 (= GC 2003 = JH 3221, 1860 RA 09 56 40, NPD 115 29.4) is "considerably faint, very large, very much extended 82 , a little brighter middle". The position precesses to RA 10 03 04.9, Dec -26 09 54, right on the galaxy listed above and the description fits, so the identification is certain.
Positional Note: Because of its large size and irregular structure, the position of the galaxy is hard to define. Per Corwin, Herschel's position was that of the superimposed star just west of the center of the galaxy. Corwin's position is inside the broad bar-like structure to the east of the galaxy's midpoint, and I have used a position slightly to the southwest but still within that "bar".
Physical Information: NGC 3109 is the largest member of the Sextans Group, a scattering of small galaxies only a few million light years away. Opinion is divided as to whether the Sextans Group is gravitationally bound to our Local Group of galaxies, or is the nearest separate cluster of galaxies. NGC 3109's recessional velocity is only 405 km/sec, which is too small in comparison to peculiar (non-Hubble-expansion) velocities to be a reliable indicator of its distance. An estimate of its distance based on its recessional velocity would be about 18 million light years, much further than redshift-independent distance estimates of 3.3 to 5.9 million light years, so its peculiar (non-Hubble expansion) velocity is indeed substantially larger than local universal expansion effects. Assuming a distance of about 4.5 million light years, NGC 3109's apparent size of 20 by 4 arcmin implies that it is about 25 thousand light years across. Originally classified as an irregular galaxy, it now seems likely that it is actually a dwarf spiral seen at an angle that makes its spiral structure difficult to detect, as it appears to have a halo and disk structure typical of spiral galaxies. It is relatively close to the Antlia dwarf (another member of the Sextans Group), and is believed to be tidally interacting with it in the same way that our galaxy interacts with the Magnellanic Clouds and its other companions.

Historical Identification: Per Dreyer, NGC 3110 (Stephan list XIII (#54), 1860 RA 09 57 03, NPD 95 47.0) is "faint, very small, irregularly round, mottled but not resolved". The position precesses to RA 10 04 03.3, Dec -06 27 34, less than an arcmin north northeast of the center of the galaxy listed above and barely beyond its northeast outline, the description fits and there is nothing comparable nearby so the identification is certain.
Discovery Notes: Per Corwin, Stephan's measurement was good, and the error in his declination was primarily due to an incorrect position for his comparison star. Herschel's position (see NGC 3122 for more) was considerably off the mark due to a misidentification of his comparison star. Though not noted in the NGC, Stephan suggested that his XIII#54 might be the same as Herschel's II 305 (= NGC 3122), and Dreyer's 1912 paper confirmed that identity. However (as also noted by Corwin), the equality with NGC 3518 (which see for more) was a more complicated affair.
Physical Information: Based on a recessional velocity of 5055 km/sec, NGC 3110 is about 235 million light years away, in reasonable agreement with redshift-independent distance estimates of 175 to 235 million light years. Given that and its apparent size of 1.65 by 0.7 arcmin, it is 110 to 115 thousand light years across.

Historical Identification: Per Dreyer, NGC 3111 (= GC 2005 = JH 667, 1860 RA 09 57 15, NPD 42 03.5) is "pretty bright, small, round, suddenly much brighter middle equivalent to 12th magnitude star".
Physical Information: Based on a recessional velocity of 7385 km/sec, a straightforward calculation indicates that NGC 3111 is 340 to 345 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 335 million light years away at the time the light by which we see it was emitted, a little less than 340 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 1.1 by 1.0 arcmin, the galaxy is 105 to 110 thousand light years across.
Alternate Physical Information: The NED lists a second recessional velocity of 4415 km/sec, which may be a value for another galaxy misattributed to NGC 3111; but in the absence of any redshift-independent distance estimates, there is no way to know whether the lower radial velocity is correct or incorrect, so a brief discussion feels necessary. If correct, the lower velocity would yield a Hubble expansion distance of only 205 million light years, and (since the apparent size is the same regardless of the distance), would only be about 65 thousand light years across.

Historical Identification: Todd 19a (1860 RA 09 57 24, NPD 76 31.5) was merely "suspected" of being nebular. It was noticed by Todd during his unsuccessful search for a trans-Neptunian planet. His position for his 1.3 degree wide field of view and a comparison of his sketch of the field with modern photographs of the region clearly show that the star listed above must be his 19a.
Discovery Notes: Presumably omitted from the NGC by Dreyer because he did not like the description and (correctly) suspected it was not an object worthy of inclusion. Per Corwin, its modern identification was originally made by Klaus Wenzel.

Historical Identification: Per Dreyer, NGC 3112 (Ormond Stone list I (#163), 1860 RA 09 57 35, NPD 110 06.3) is "extremely faint, extremely small, round, perhaps nebulous". The position precesses to RA 10 04 10.6, Dec -20 46 54, about 2.7 arcmin east of the galaxy listed above, but that is actually a modest error for Leander McCormick Observatory objects, the description though not perfect is a reasonable fit, and there is nothing else anywhere near the position that Stone could have possibly seen, so the identification is essentially certain.
Physical Information: Based on a recessional velocity of 3975 km/sec, NGC 3112 is about 185 million light years away. Given that and its apparent size of 0.8 by 0.2 arcmin, it is 40 to 45 thousand light years across.

Historical Identification: Per Dreyer, NGC 3113 (= GC 2006 = JH 3222, 1860 RA 09 58 03, NPD 117 46.2) is "extremely faint, large, makes a triangle with two 8th magnitude stars". The position precesses to RA 10 04 23.8, Dec -28 26 50, within the western extension of the nucleus of the galaxy listed above, the description fits perfectly and there is nothing else nearby so the identification is certain.
Physical Information: Based on a recessional velocity of 1085 km/sec, NGC 3113 is about 50 million light years away, in reasonable agreement with widely varying redshift-independent distance estimates of 45 to 105 million light years. Given that and its apparent size of 3.6 by 1.2 arcmin, the galaxy is 50 to 55 thousand light years across.

Historical Identification: Per Dreyer, NGC 3114 (= GC 2007 = JH 3224, Dunlop #297, 1860 RA 09 58 11, NPD 149 26.8) is a "cluster, extremely large, a little compact, bright, stars from 9th to 14th magnitude". The position precesses to RA 10 02 41.1, Dec -60 07 21, a few arcmin northwest of 6th magnitude HD 87436, which is one of the objects Herschel used to define the position of the cluster, and as discussed in the Discovery Notes below, there is no doubt that the region listed above is what Herschel was looking at when he made his observations.
Discovery Notes: Per Corwin, Herschel gave three positions for the cluster, one based on a trio of stars that form a small equilateral triangle on its northwestern periphery (near RA 10 02 21, Dec -60 00.3) and two based on HD 87436 (at RA 10 02 59.9, Dec -60 10 43.2), though only one of them is accurate, the other being 20 seconds of time too far west. That does little to determine the "actual" position of the cluster, but does prove that we are looking at the right region. Corwin's position for the cluster (used in the description at the start of this entry) is about 30 seconds of time west of other references he quotes, but this may be due to uncertainty about the apparent size of the cluster. The more eastern positions probably refer to a smaller circular region centered to the north of HD 87436, while Corwin's position corresponds to a much larger region outlined by a number of additional stars forming (as he describes it) an elongated "home plate" similar to the shape of the constellation of Auriga. The larger region seems a better fit to Herschel's description of the cluster, hence my decision to use Corwin's position and description of the cluster's outline.
Physical Information: NGC 3114 is aoubt 3000 light years away. Using the specified (larger) definition of its boundaries, its few dozen or so bright and moderately bright stars are scattered across a region about 40 by 35 arcmin in size, or about 35 light years. The characteristics of stars that are radial-velocity confirmed members of the cluster suggests an age of about 160 million years. LEDA lists it as a cluster with the designation PGC 3518280, but a search of the database for that designation returns no result.

Historical Identification: Per Dreyer, NGC 3115 (= GC 2008 = JH 668 = JH 3223 = WH I 163, 1860 RA 09 58 16, NPD 97 02.5) is "very bright, large, very much extended 46 , very gradually suddenly much brighter middle and extended nucleus". The position precesses to RA 10 05 14.3, Dec -07 43 11, right 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 of 665 km/sec, NGC 3115 is about 30 million light years away, in good agreement with redshift-independent distance estimates of 15 to 40 million light years. Given that and its apparent size of 8.9 by 4.0 arcmin, it is about 80 thousand light years across. Short-exposure images of the galaxy show a very thin disk, and if it were not for the extended halo seen in longer exposures, it might be considered a "superthin" galaxy.

 Below, an X-ray image (and inset) of the galaxy showing numerous point sources of X-rays due to gas from binary stars falling into stellar mass black hole companions, and a broad plateau of X-radiation near the center, due to material falling into a 2 billion solar mass (supermassive) black hole at the center of the galaxy. Changes in the spectrum of the material falling into the central black hole indicates that gases falling into the black hole are substantially heated by collision with similar gases at a distance of about 700 light years from the black hole; this distance confirms mass estimates obtained by other methods, and makes this the closest known billion-plus solar mass black hole. (Image Credit X-ray: NASA/CXC/Univ. of Alabama/K.Wong et al, Optical: ESO/VLT)

Historical Identification: Per Dreyer, NGC 3116 (Palisa (#6), 1860 RA 09 58 39, NPD 58 12.7) is a "nebulous star, 13th magnitude". The position precesses to RA 10 06 45.3, Dec +31 06 31, only 0.6 arcmin north of the galaxy listed above, the description is appropriate and there is nothing else nearby so the identification is certain.
Physical Information: Based on a recessional velocity of 6400 km/sec, NGC 3116 is 295 to 300 million light years away. Given that and its apparent size of 0.45 by 0.45 arcmin, it is about 40 thousand light years across.

Historical Identification: Per Dreyer, NGC 3117 (Stephan list IX (#21), 1860 RA 09 58 55, NPD 86 24.2) is "very faint, very small, round, small star involved". The position precesses to RA 10 06 10.5, Dec +02 55 02, on the northern rim of the galaxy listed above, the description is reasonable and there is nothing else nearby so the identification is certain.
Physical Information: Based on a recessional velocity of 6785 km/sec, NGC 3117 is about 315 million light years away. Given that and its apparent size of about 0.9 by 0.9 arcmin, it is 80 to 85 thousand light years across.

Historical Identification: Per Dreyer, NGC 3118 (Stephan list XIII (#55), 1860 RA 09 59 02, NPD 56 17.5) is a "small group of very faint stars in very faint nebula". The position precesses to RA 10 07 12.4, Dec +33 01 41, right on the galaxy listed above, and although the description seems odd (perhaps superimposed stars and a slightly irregular structure misled the observer?), there is nothing else nearby so the identification is certain.
Physical Information: Based on a recessional velocity of 1340 km/sec, NGC 3118 is 60 to 65 million light years away, in fair agreement with redshift-independent distance estimates of 85 to 95 million light years. Given that and its apparent size of about 2.5 by 0.45 arcmin, it is about 45 thousand light years across.

Historical Identification: Per Dreyer, NGC 3119 (= GC 5520, Marth #194, 1860 RA 09 59 14, NPD 74 58) is "very faint". The position precesses to RA 10 06 48.2, Dec +14 21 11, about 1.5 arcmin southwest of the pair of galaxies listed above (and as NGC 3121), and about 2.4 arcmin north of PGC 29381, which is (mis?)identified as NGC 3119 by several (mostly older) references. Unfortunately, the description is of no help in determining which of the two objects Marth observed, so although the brighter pair of galaxies (NGC 3121) is almost always identified as a duplicate of NGC 3119, that identification can only be listed as almost certain, instead of absolutely certain.
Discovery Notes: Per Corwin, Marth could have observed either NGC 3121 or PGC 29381, as the telescope he used was capable of showing either object. But although he could have observed either of them, the fact that his position is closer to the brighter one, combined with its brightness, makes it far more likely to be what he observed. For one thing, if he saw the fainter galaxy he would have also seen the brighter one, and if so why didn't he bother to mention it? But Marth's failure to mention Lassell's earlier discovery does not prove that it is (or is not) his #194, so it is possible that the fainter galaxy was his "nova". So although it seems appropriate to list NGC 3119 as an almost certain duplicate of NGC 3121, it is also appropriate to discuss PGC 29381 (immediately below) as a far less likely but not impossible identification of Marth's object.
Physical Information (presuming NGC 3119 = NGC 3121): Given the almost certain physical interaction between the two members of NGC 3119/3121 and their nearly equal recessional velocities (8825 km/sec for the brighter component and 8985 km/sec for the fainter one), it is appropriate to use their average recessional velocity to determine their Hubble distance. Based on that average of 8900 km/sec, a straightforward calculation indicates that NGC 3119/3121 is about 415 million light years away, in reasonable agreement with redshift-independent distance estimates of 415 to 445 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 pair was about 400 million light years away at the time the light by which we see it was emitted, about 405 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 0.9 by 0.9 arcmin, PGC 29387 is about 105 thousand light years across, and PGC 93103's apparent size of about 0.25 by 0.25 arcmin corresponds to about 30 thousand light years. (Although the fainter companion could not have made much if any difference in the various observations, its almost certain interaction with the brighter galaxy means it should probably be treated as part of the NGC object; however, many references treat it as merely a companion.)

Historical Identification: As discussed in the entry for NGC 3119, although that is almost certainly a duplicate of NGC 3121, there is a remote possibility that NGC 3119 is actually PGC 29381, and it is treated as such in several references, making it necessary to discuss it here no matter how unlikely the identification might be.
Physical Information: Based on a recessional velocity of 8945 km/sec, a straightforward calculation indicates that PGC 29381 is about 415 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 400 to 405 million light years away at the time the light by which we see it was emitted, 405 to 410 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 0.5 by 0.45 arcmin, the galaxy is 55 to 60 thousand light years across.

Historical Identification: Per Dreyer, NGC 3120 (= GC 2009 = JH 3225, 1860 RA 09 59 15, NPD 123 32.6) is "faint, pretty small, round, gradually brighter middle". The position precesses to RA 10 05 23.6, Dec -34 13 21, well within the eastern outline of the galaxy listed above, the description is reasonable and there is nothing else nearby so the identification is certain.
Physical Information: Based on a recessional velocity of 2790 km/sec, NGC 3120 is about 130 million light years away, in good agreement with redshift-independent distance estimates of 65 to 145 million light years. Given that and its apparent size of 2.4 by 1.4 arcmin, it is about 90 thousand light years across.

Historical Identification: Per Dreyer, NGC 3121 (Lassell, 1848, 1860 RA 09 59 20, NPD 74 56.5) is "pretty faint, pretty large, gradually a little brighter middle, 9.5 magnitude star to northwest (Auwers 26)". The position precesses to RA 10 06 54.3, Dec +14 22 41, only about 0.6 arcmin east northeast of the pair of galaxies listed above, the description fits, there is nothing else nearby and the star to the northwest (BD+15 2165) makes the identification certain.
Discovery Notes: Though not giving the date of his own observation (almost certainly between 1850 and 1862, but otherwise indeterminate), Auwers states (translating from the German) that the object is "very faint, round, 1.5 arcmin diameter, 9.1 magnitude star 4 arcmin north, 14 to 15 seconds to west; the star and its position are in the Bonn Catalog". Auwers also notes that Lassell discovered the nebula during observations of Comet Mauvais (1847 IV). That is confirmed by Lassell's communication published on page 171 of the Astronomische Nachrichten #635 of April 1848.
Physical Information: As discussed in the entry for NGC 3119, that entry is almost certainly a duplicate of NGC 3121, so although the equality is not absolutely certain it seems certain enough, and the reader should see NGC 3119 for anything else about NGC 3121.

Historical Identification: Per Dreyer, NGC 3122 (= GC 2011 = WH II 305, 1860 RA 09 59 26, NPD 95 51.3) is "faint, small, a little extended, extremely mottled but not resolved". The position precesses to RA 10 06 26.3, Dec -06 32 06, but there is nothing in the nearly stellar field that Herschel could have seen. However, Dreyer's notes for 1912's The Scientific Papers of Sir William Herschel include "II.305. Looked for but not found in 1787. It was the only object compared with "20 Sextantis", but the star was in reality B(ode) 1414 (= HD 88372). This gives for 1860 (RA) 09 57 04, (NPD) 95 49, in perfect agreement with NGC 3110 (Stephan XIII.)." Dreyer said much the same thing in a briefer manner in his 1912 summary of changes to the NGC required by his reassessment of Herschel's papers, stating "(NGC) 3110 is = II. 305" and "(NGC) 3122 to be struck out (= 3110)", so the duplicate entry has been known about for more than a century.
Discovery Notes: (1) Per Corwin, Dreyer's identification of the correct comparison star was obtained from Caroline Herschel's note that the comparison star used for her reduction of her brother's observation was B.1414. (2) Also per Corwin, in the 1973 RNGC Sulentic chose a faint object just over 3 arcmin southwest of Herschel's incorrect original position as NGC 3122, and although as noted above that object is too faint for Herschel to have seen (Corwin states that if Herschel "looked for it again in 1787 at roughly this location, then I'm not surprised that he did not recover it"), the fact that PGC 139241 has been misidentified as NGC 3122 means it must be discussed (below), if only as a warning.
Physical Information: Given the duplicate entry, see NGC 3110 for anything else.

Historical Misidentification: As stated in the entry for NGC 3122, it has been known for more than a century that NGC 3122 is a duplicate of NGC 3110; so the 1973 RNGC's misidentification of PGC 139241 as NGC 3122 serves as a warning about how common such misidentifications can be, and is why I point out such mistakes when I notice them.
Physical Information: Based on a recessional velocity of 3540 km/sec, PGC 139241 is about 165 million light years away. Given that and its apparent size of 0.55 by 0.25 arcmin, it is about 25 thousand light years across.

Historical Identification: Per Dreyer, NGC 3123 (= GC 5067, Sidney Coolidge (#13, HN15), 1860 RA 09 59 51, NPD 89 15.1) is a "nebula, no description". The position precesses to RA 10 07 01.9, Dec +00 04 03, but there is nothing there save for a completely stellar field. Precessing Coolidge's original position gives a very slightly different position (RA 10 07 01.9, Dec +00 04 03), but does not change the fact that there is nothing near the position that is obviously what he observed. Since most of the early Harvard Nebula observations (and all of Coolidge's) turned out to be stars, it would usually be assumed that one of the stars near Coolidge's position was what he observed, but there is no way of telling which star, and in most respects it makes no difference, since the NGC/IC catalogs were supposed to be for nebulae and clusters, not stars. As a result, an earlier version of this entry stated that NGC 3123 was an unidentifiable object near Coolidge's position. However, in March 2015 Steve Gottlieb did an analysis of the original Harvard records that showed the position listed in the Harvard Annals must be incorrect, and NGC 3123 is almost certainly the star listed above, so I have changed this entry to reflect that discovery.
Discovery Notes: Per Corwin, in March 2015 Steve Gottlieb wrote that he had examined the "Harvard Zones", which is where the original observations later published as Harvard Nebulae were first recorded. In Zone 117, which is where HN 15 is supposed to lie, only one object (#47) has any kind of note suggesting nebulosity (namely "Has a perceptible disc?"). This suggests that NGC 3123 is probably Harvard Zone 117-47, whose position is listed as (1859) RA 10 10 58.91, Dec +00 44 39.3, which precesses to RA 10 18 12.8, Dec +00 02 27. This is very close to the position of the star listed above, and taking into account the proper motion of the star, its 1859 position differs from the Harvard Zone position by only 0.6 seconds of right ascension and 1 arcsec of declination, making it absolutely certain that HZ 117-47 is indeed that star. The position published by George Bond (director of the Harvard Observatory) for HN 15 has a nearly identical declination, making it reasonably certain that this is the correct NGC 3123; but there is nothing in Harvard Zone 117 with a right ascension anywhere near the one published in Bond's paper, so how that obviously incorrect position was obtained is a mystery, and likely to remain one.
Final Note: LEDA lists NGC 3123 as a star near the incorrect NGC position, with the designation PGC 5067620; but aside from being based on the older incorrect position and therefore certainly incorrect identification, a search of the database for that designation returns no result.

Historical Identification: Per Dreyer, NGC 3124 (= GC 2012 = JH 3226, 1860 RA 10 00 02, NPD 108 33.6) is "faint, pretty large, round, a little brighter middle, double star to south". The position precesses to RA 10 06 41.1, Dec -19 14 27, within the southeastern outline of the galaxy listed above, there is nothing else nearby and the double star due south makes the identification certain.
Physical Information: Based on a recessional velocity of 3560 km/sec, NGC 3124 is about 165 million light years away, in good agreement with redshift-independent distance estimates of 120 to 180 million light years. Given that and its apparent size of 3.3 by 2.8 arcmin, it is about 160 thousand light years across.

Historical Identification: Per Dreyer, NGC 3125 (= GC 2013 = JH 3227, 1860 RA 10 00 15, NPD 119 15.2) is "considerably faint, small, round, very gradually brighter middle". The position precesses to RA 10 06 33.5, Dec -29 56 03, right 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 of 1115 km/sec, NGC 3125 is about 50 million light years away. Given that and its apparent size of 1.1 by 0.9 arcmin, it is about 15 thousand light years across, making it a "dwarf" irregular, similar to one of the Magellanic Clouds, but unfathomably brighter and more energetic. NGC 3125 is a multiple starburst galaxy, with two 3 to 4 million year old superclusters each containing a pair of 100 to 175 thousand solar mass subclusters, much of which is contained within exceptionally large numbers of stars of 120 or more solar masses, at the upper limit of known stellar masses. The brightest of the clusters (NGC 3125-A1) is considered to be the "most extreme Wolf-Rayet star cluster" within its 50 million light year distance from us (referred to as the "local Universe" in one of the papers studying its extraordinary properties). As shown in the HST image below, the radiation from these stars is strongly heating and compressing complex clouds of gas which should soon produce another generation of supermassive stars.

 Below, a 25 arcsec wide image of the core of the galaxy. Superclusters A1 and A2 are the core of the star-forming region at upper right, while superclusters B1 and B2 are the core of the star-forming region at lower left. (Image Credit ESA/Hubble/NASA, Judy Schmidt)

Historical Identification: Per Dreyer, NGC 3126 (= GC 5521, d'Arrest, Struve, 1860 RA 10 00 17, NPD 57 27.5) is "faint, small, a little extended, nucleus = 15th magnitude star". The position precesses to RA 10 08 24.2, Dec +31 51 34, just east of the southern 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 of 5180 km/sec, NGC 3126 is about 240 million light years away, in good agreement with redshift-independent distance estimates of 200 to 245 million light years. Given that and its apparent size of 3.0 by 0.45 arcmin, it is about 210 thousand light years across.

Historical Identification: Per Dreyer, NGC 3127 (Leavenworth list I (#164), 1860 RA 10 00 35, NPD 105 27.3) is "extremely faint, pretty large, much extended 45 ". The position precesses to RA 10 07 19.6, Dec -16 08 12, in a completely stellar region. However, there is a galaxy that perfectly fits the description almost exactly one minute of time to the west, and since such errors in right ascension are common in Leander McCormick Observatory positions and there is nothing else that fits the description anywhere near its parallel of declination, the identification is considered reasonably certain.
Discovery Notes: Leavenworth recorded exactly the same position for his #164 (= NGC 3127) and #165 (= NGC 3128), so Dreyer presumably listed them in the order he did simply because that's what Leavenworth did. As it turns out the order should have been reversed, as the descriptions make it clear that #165 (with a nearly north-south orientation) is the western member of the pair, and #164 (with a 45 position angle) is the eastern member; but considering the relatively large error in their positions and the very rough measurement implied by giving them the same position when they are nearly 6 arcmin apart, the fact that they can be identified at all seems sufficient.
Physical Information: Based on a recessional velocity of 4645 km/sec, NGC 3127 is about 215 million light years away. Given that and its apparent size of 1.3 by 0.25 arcmin, it is 80 to 85 thousand light years across. Since the recessional velocities of NGC 3127 and 3128 and their corresponding distances are nearly identical, it is quite possible that they are a physical pair.

Historical Identification: Per Dreyer, NGC 3128 (Leavenworth list I (#165), 1860 RA 10 00 35, NPD 105 27.3) is "extremely faint, pretty large, much extended 170 , a little brighter middle". The position precesses to RA 10 07 19.6, Dec -16 08 12, in a completely stellar region. However, there is a galaxy that perfectly fits the description a little over a minute of time to the west, and since such errors in right ascension are common in Leander McCormick Observatory positions and there is nothing else that fits the description anywhere near its parallel of declination, the identification is considered reasonably certain.
Discovery Notes: Leavenworth recorded exactly the same position for his #164 (= NGC 3127) and #165 (= NGC 3128), so Dreyer presumably listed them in the order he did simply because that's what Leavenworth did. As it turns out the order should have been reversed, as the descriptions make it clear that #165 (with a nearly north-south orientation) is the western member of the pair, and #164 (with a 45 position angle) is the eastern member; but considering the relatively large error in their positions and the very rough measurement implied by giving them the same position when they are nearly 6 arcmin apart, the fact that they can be identified at all seems sufficient.
Physical Information: Based on a recessional velocity of 4675 km/sec, NGC 3128 is 215 to 220 million light years away. Given that and its apparent size of 2.4 by 0.8 arcmin, it is 150 to 155 thousand light years across. Since the recessional velocities of NGC 3127 and 3128 and their corresponding distances are nearly identical, it is quite possible that they are a physical pair.

Historical Identification: Per Dreyer, NGC 3129 (= GC 2014 = JH 669 = WH III 65, 1860 RA 10 00 40, NPD 70 53.7) is "extremely faint, considerably small, very little extended, mottled but not resolved". A note at the end of the NGC states "3129: (JH) 669 = (WH) III 65. Not found by Lord Rosse three times, not looked for by d'Arrest. It was found by (JH) in its place per working list, so that (WH)'s place was correct ((JH) took only the place roughly)." The position precesses to RA 10 08 21.0, Dec +18 25 21, only 0.6 arcmin southeast of the pair listed above, the description fits perfectly and there is nothing else nearby that could possibly fit the description, so the identification is considered certain.
Database Error Warning: Steinicke lists WH III 65 as = JH 671, not JH 669, but the positions are completely different, whereas WH's position for his III 65 is virtually identical to that given for GC 2014, so there is no doubt that III 65 = NGC 3129. Steinicke's error is therefore probably just an accidental placing of the information in the wrong cell of his database.
Physical Information: The northeastern member of the pair is magnitude 13.8, and the southwestern is magnitude 14.1. The pair is separated by only 12 arcsec, so as noted above it perfectly fits the description. LEDA lists NGC 3129 as a double star with the designation PGC 5067738, but a search of the database for that designation returns no entry.

Historical Identification: Per Dreyer, NGC 3130 (= GC 2015 = JH 670, 1860 RA 10 00 45, NPD 79 20.7) is "extremely faint, small, pretty suddenly brighter middle, 5th magnitude star to southeast" (Corwin notes that Herschel's 1833 catalog correctly states that the galaxy is southeast of 31 Leonis, but the GC and NGC accidentally reversed that, placing the star to the southeast instead). The position precesses to RA 10 08 11.7, Dec +09 58 21, on the southwestern rim of the galaxy listed above, the description fits save for the accidental reversal of the position of 31 Leo already noted above, and there is nothing else nearby so the identification is certain.
Discovery Notes: The date of Herschel's observation is uncertain because "sweep 123" is merely listed as the last sweep in January, with the previous sweep having been done on Jan 19.
Physical Information: Based on a recessional velocity of 8190 km/sec, a straightforward calculation indicates that NGC 3130 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 370 million light years away at the time the light by which we see it was emitted, about 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 0.9 by 0.55 arcmin, the galaxy is 95 to 100 thousand light years across.

Historical Identification: Per Dreyer, NGC 3131 (= GC 2016 = JH 671, 1860 RA 10 00 57, NPD 71 05.2) is "pretty bright, pretty small, pretty much extended, gradually brighter middle". The position precesses to RA 10 08 37.6, Dec +18 13 49, on the southeastern rim of the nuclear bulge of the galaxy listed above, the description fits and there is nothing else nearby so the identification is certain.
Database Error Warning: Steinicke lists WH III 65 as = JH 671, not JH 669, but the positions are completely different, whereas WH's position for his III 65 is virtually identical to that given for GC 2014, so there is no doubt that III 65 = NGC 3129. (The error is probably due to accidentally placing the information in the wrong cell of the database, and will undoubtedly be corrected in a later iteration.)
Physical Information: Based on a recessional velocity of 5100 km/sec, NGC 3131 is 235 to 240 million light years away, in good agreement with redshift-independent distance estimates of 205 to 255 million light years. Given that and its apparent size of about 2.15 by 0.7 arcmin, the galaxy is 145 to 150 thousand light years across.

Historical Identification: Per Dreyer, NGC 3132 (= GC 2017 = JH 3228, 1860 RA 10 01 08, NPD 129 45.1) is "a most remarkable object, planetary, very bright, very large, a little extended middle equivalent to 9th magnitude star, 4 seconds of time diameter". The position precesses to RA 10 07 02.0, Dec -40 26 01, right on the nebula listed above, the description fits and there is nothing comparable nearby so the identification is certain.
Note About the Name: The nicknames of astronomical objects often bear little relationship to their appearance, but "Southern Ring Nebula" is a clear reference to the similar appearance of the Ring Nebula in Lyra. "Eight-Burst Planetary Nebula" appears to have no known origin, although overexposed images of the nebula do bear a slight resemblance to the number 8.
Database Error Warning: An early version of Steinicke's database attributed Sidney Coolidge's observation of NGC 3123 to NGC 3132 (a typical typographical error). That error has long been corrected, but may have been perpetuated elsewhere; so this is a warning about that possibility.
PGC Designation Note: For purposes of completeness, HyperLEDA assigned a PGC designation to this object even though it is not a galaxy; but a search of the database for that designation returns no result, so it is shown in quotes.
Physical Information: NGC 3132 is thought to be about 2000 light years away. Given that and its apparent size of 2.1 by 1.1 arcmin (including fainter outer regions), it is about 1.2 light years across. The two stars at the center of the nebula form a binary system, with the smaller fainter one being the extremely hot white dwarf that ejected the nebula and is providing the energy that keeps it glowing; but its brighter companion is in the last stages of its life and may eventually create a nebula of its own. LEDA lists NGC 3132 as a planetary nebula with the designation PGC 3517762, but a search of the database for that designation returns no result.

Historical Identification: Per Dreyer, NGC 3133 (Leavenworth list II (#419), 1860 RA 10 01 38, NPD 101 18.3) is "extremely faint, very small, round". The position precesses to RA 10 08 29.7, Dec -11 59 19, in a completely stellar field. However, Leander McCormick Observatory positions are often well off in right ascension, and there is an appropriate object about 75 seconds west of Leavenworth's position, namely the galaxy listed above, and there is nothing else in the region so the identification seems reasonably certain. (There is a suitable candidate somewhat closer, to the northeast of Leavenworth's position, but that is already accounted for by his observation of NGC 3138.)
Physical Information: Based on a recessional velocity of 9120 km/sec, a straightforward calculation indicates that NGC 3133 is about 425 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 410 million light years away at the time the light by which we see it was emitted, about 415 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 0.6 by 0.3 arcmin, the galaxy is 105 to 110 thousand light years across.

Historical Identification: Per Dreyer, NGC 3134 (Todd (#21), 1860 RA 10 02, NPD 76 59) is "very faint, disc". The position precesses to RA 10 09 30.4, Dec +12 19 55, in a completely stellar field. The problem appears to be that since Todd's records were based on the setting circle position of a finder with a 1.3 degree wide field of view, his position for the field of view could be off by quite a bit. In this case it turns out that the position was 3 minutes of time west and over 3 arcmin south of the object he observed, as there is a candidate that far to the north and east that has a surrounding field of view that almost perfectly matches Todd's sketch of the region (as shown in the first image below) and his description of the object, namely the galaxy listed above. Aside from that, Todd measured the position of a star to the east of the nebula a total of 24 times, with a consistent difference of 28 seconds of time, exactly matching the position of the 13th magnitude star shown as Todd 21b in the second image, making the identification certain.
Discovery Notes: Todd's observations of nebulae were merely incidental to his search for a trans-Neptunian planet, and his records of such observations, though meticulously kept, are in a form that makes them difficult to identify; namely, he did not measure their positions relative to objects of known position, and the positions recorded for his finder were surprisingly crude for a project that required multiple observations of a region to identify anything moving relative to the stellar background. However, that is not why he did not succeed in finding his trans-Neptunian planet. His search was doomed to failure because the object being looked for was expected to be massive and therefore large and relatively bright, and the largest trans-Neptunian object (Pluto) is hundreds of times less massive, hundreds of times smaller in surface area, and much fainter than what he was searching for. It would take more than 50 years after Todd gave up his search before Clyde Tombaugh finally found the only trans-Neptunian object known for more than another half-century.
Physical Information: Based on a recessional velocity of 5365 km/sec, NGC 3134 is about 250 million light years away. Given that and its apparent size of about 1.15 by 0.25 arcmin, it is 80 to 85 thousand light years across.

Historical Identification: Per Dreyer, NGC 3135 (= GC 2019 = JH 672, 1860 RA 10 02 10, NPD 43 21.2) is "faint, small, round, gradually brighter middle". The position precesses to RA 10 10 53.3, Dec +45 57 39, less than 0.7 arcmin north northwest 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 of 7230 km/sec, a straightforward calculation indicates that NGC 3135 is 335 to 340 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 325 to 330 million light years away at the time the light by which we see it was emitted, just over 330 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 1.05 by 0.55 arcmin, the galaxy is about 100 thousand light years across.

Historical Identification: Per Dreyer, NGC 3136 (= GC 2018 = GC 2020 = JH 3229 = JH 3231, 1860 RA 10 02 10, NPD 156 41.5) is "pretty bright, pretty small, round, gradually brighter middle, 13th magnitude star to north". The position precesses to RA 10 05 46.1, Dec -67 22 25, barely outside the northwestern rim of the galaxy listed above, the description fits and there is nothing else nearby so the identification is certain.
Discovery Notes: Dreyer noted that GC 2018 was = GC 2020 in his 1877 supplement to the GC, stating that the place for JH 3229, listed by Herschel as "only a very rude approximation", involved a misprint in the Cape Observations.
Physical Information: Based on a recessional velocity of 1715 km/sec, NGC 3136 is about 80 million light years away, in good agreement with redshift-independent distance estimates of 45 to 85 million light years. Given that and its apparent size of about 3.5 by 3.2 arcmin, it is about 80 thousand light years across. Generally listed as an elliptical galaxy, but its central dust lanes (seen in the HST image below) have also led to a listing as a dusty E/S0 galaxy.

Physical Information: Based on a recessional velocity of 2010 km/sec, PGC 29160 is 90 to 95 million light years away, in poor agreement with redshift-independent distance estimates of 45 to 50 million light years. Given that and its apparent size of about 1.8 by 0.2 arcmin, the galaxy is 45 to 50 thousand light years across.

Physical Information: Based on a recessional velocity of 1780 km/sec, PGC 29597 is 80 to 85 million light years away, in good agreement with redshift-independent distance estimates of 70 to 115 million light years. Given that and its apparent size of about 1.4 by 1.0 arcmin, it is 30 to 35 thousand light years across.

Historical Identification: Per Dreyer, NGC 3137 (= GC 2021 = JH 3230, 1860 RA 10 02 30, NPD 118 22.4) is "very faint, small, a little extended". The position precesses to RA 10 08 51.2, Dec -29 03 28, over 3.5 arcmin west northwest of the galaxy listed above, but the description fits and there is nothing else nearby so the identification is certain. (Keep in mind that Herschel's description applies only to the core of the galaxy, which is much smaller and fainter than the galaxy's appearance in modern photographs might lead one to believe, especially in the case of a low surface-brightness galaxy such as this one.)
Physical Information: Based on a recessional velocity of 1105 km/sec, NGC 3137 is 50 to 55 million light years away, in good agreement with redshift-independent distance estimates of 45 to 75 million light years. Given that and its apparent size of about 6.3 by 2.5 arcmin, it is about 95 thousand light years across.

Historical Identification: Per Dreyer, NGC 3138 (Leavenworth list II (#420), 1860 RA 10 02 30, NPD 101 15.3) is "extremely faint, very small, round, 1st of 2", the other being NGC 3139. The position precesses to RA 10 09 21.9, Dec -11 56 24, about 1.6 arcmin northeast of the galaxy listed above, but the description fits (even the "1st of 2", although that is well to the north) and there is nothing comparable nearby so the identification is certain.
Physical Information: Based on a recessional velocity of 7820 km/sec, a straightforward calculation indicates that NGC 3138 is about 365 million light years away, in good agreement with redshift-independent distance estimates of 305 to 465 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 350 to 355 million light years away at the time the light by which we see it was emitted, 355 to 360 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 1.3 by 0.25 arcmin, the galaxy is 130 to 135 thousand light years across.

Historical Identification: Per Dreyer, NGC 3139 (Leavenworth list II (#421), 1860 RA 10 02 30, NPD 101 07.3) is "extremely faint, very small, round, 2nd of 2", the other being NGC 3138. The position precesses to RA 10 09 22.2, Dec -11 48 24, almost 45 seconds of time west of the galaxy listed above, but right ascensions recorded at the Leander McCormick Observatory were notoriously poor, and Leavenworth used the same right ascension for this and his "1st of 2" despite nearly a minute of time difference in their actual positions. However, the difference in declination between it and Leavenworth's previous entry is a reasonable fit to their relative positions, the description fits and there is nothing else nearby, so the identification is reasonably certain and universally accepted.
Physical Information: Based on a NED recessional velocity of 1410 km/sec, NGC 3139 is about 65 million light years away. Given that and its apparent size of about 1.6 by 1.3 arcmin, it is about 30 thousand light years across, which seems unusually small for an object of its appearance. However, LEDA lists a different recessional velocity of 9200 km/sec, which is also noted in the NED as a second value; so it is very possible that the smaller radial velocity actually belongs to another object and was misattributed to NGC 3139. If that is the case, then a straightforward calculation based on the larger recessional velocity would yield a distance for NGC 3139 of 425 to 430 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 places the galaxy 410 to 415 million light years away at the time the light by which we see it was emitted, about 420 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 1.6 by 1.3 arcmin, the galaxy would be about 190 to 195 thousand light years across.

Historical Identification: Per Dreyer, NGC 3140 (Leavenworth list I (#166), 1860 RA 10 02 35, NPD 105 57.2) is "extremely faint, pretty small, round, suddenly brighter middle and nucleus, 1st of 2", the other being NGC 3141. The position precesses to RA 10 09 19.2, Dec -16 38 18 (the same as for NGC 3141, since Leavenworth recorded the same position for both objects), near a pair of galaxies, about 0.9 arcmin north northwest of the southwestern galaxy and a little over 2 arcmin west southwest of the northeastern galaxy. Given their description (and per Corwin, Leavenworth's sketch of the pair), the northeastern galaxy, which is the one listed above, must be NGC 3140, so its identification is certain.
Discovery Notes: Per Corwin, since Leavenworth recorded the same position for this and his #167, Dreyer presumed the prior number was the western, hence "1st of 2", and the latter number the eastern, hence "2nd of 2". But Leavenworth's sketch and description show that the brighter galaxy is the northeastern one, so the entries for NGC 3140 and 3141 are in reverse order of their actual right ascension.
Physical Information: Based on a recessional velocity of 8460 km/sec, a straightforward calculation indicates that NGC 3140 is 390 to 395 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 380 to 385 million light years away at the time the light by which we see it was emitted, 385 to 390 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 1.05 by 0.7 arcmin, the galaxy is 115 to 120 thousand light years across. (Note: It appears that at least some data for NGC 3140 has been switched with that for NGC 3141 in various databases, perhaps because of the original confusion about their relative positions; so although the information shown here appears to be correct, future study may require some alterations.)

Historical Identification: NGC 3141 (Leavenworth list I (#167), 1860 RA 10 02 35, NPD 105 57.2) is "extremely faint, small, round, 2nd of 2", the other being NGC 3140. The position precesses to RA 10 09 19.2, Dec -16 38 18 (the same as for NGC 3140, since Leavenworth recorded the same position for both objects), near a pair of galaxies, about 0.9 arcmin north northwest of the southwestern galaxy and a little over 2 arcmin west southwest of the northeastern galaxy. Given their description (and per Corwin, Leavenworth's sketch of the pair), the southwestern galaxy, which is the one listed above, must be NGC 3141, so its identification is certain.
Discovery Notes: Per Corwin, since Leavenworth recorded the same position for this and his #166, Dreyer presumed the prior number was the western, hence "1st of 2", and the latter number the eastern, hence "2nd of 2". But Leavenworth's sketch and description show that the brighter galaxy is the northeastern one, so the entries for NGC 3140 and 3141 are in reverse order of their actual right ascension.
Physical Information: Based on a recessional velocity of 8520 km/sec, a straightforward calculation indicates that NGC 3141 is 395 to 400 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 380 to 385 million light years away at the time the light by which we see it was emitted, about 390 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 0.4 by 0.2 arcmin, the galaxy is about 45 thousand light years across. (Note: It appears that at least some data for NGC 3140 has been switched with that for NGC 3141 in various databases, perhaps because of the original confusion about their relative positions; so although the information shown here appears to be correct, future study may require some alterations.)

Historical Identification: Per Dreyer, NGC 3142 (= GC 2022 = 3232, (d'Arrest), 1860 RA 10 03 10, NPD 97 47.9) is "faint, round (d'Arrest PD (97) 43.3, 1 observation)". Herschel's position precesses to RA 10 10 07.6, Dec -08 29 04, just off the southeastern outline of the galaxy listed above, the description fits and there is nothing else nearby so the identification is certain.
Discovery Notes: Corwin notes that d'Arrest's polar distance matches that of the star immediately to the north (17 Sextantis), and proposes that d'Arrest accidentally recorded the position of his comparison star, instead of the nebula.
Physical Information: Based on a recessional velocity of 5405 km/sec, NGC 3142 is 250 to 255 million light years away. Given that and its apparent size of about 0.95 by 0.9 arcmin, it is about 70 thousand light years across. Usually listed as a lenticular galaxy, but based on the images below it appears more likely to be a spiral galaxy, and perhaps a starburst galaxy.

Historical Identification: Per Dreyer, NGC 3143 (Common (#1), 1860 RA 10 03 12, NPD 101 58) is "faint, small". The second IC lists a corrected NPD (per Howe) of 101 53.6. The corrected position precesses to RA 10 10 03.0, Dec -12 34 46, on the western 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 of 3650 km/sec, NGC 3143 is about 170 million light years away. Given that and its apparent size of about 0.9 by 0.85 arcmin, it is about 45 thousand light years across.

Historical Identification: Per Dreyer, NGC 3144 (= GC 5522, d'Arrest, 1860 RA 10 03 17, NPD 15 04.5) is "very faint, small, round, 13th magnitude star attached on east". The position precesses to RA 10 15 41.8, Dec +74 14 04, just over an arcmin northeast of the galaxy listed above, the description fits and the star immediately to the east makes the identification certain.
Discovery Notes: Although d'Arrest's position was good, Herschel's observations of Apr 2, 1801 were made with his telescope out of alignment with the meridian, making the fifteen nebulae he discovered on that night difficult or impossible to identify. Dreyer's 1912 revision of the NGC based on his reassessment of William Herschel's papers resolved the situation by including the notes "3144 is = (WH) III 964", and "3174 to be struck out (= 3144)"; but see NGC 3174 for more about the double listing.
Physical Information: Based on a recessional velocity of 6445 km/sec, NGC 3144 is about 300 million light years away, in good agreement with redshift-independent distance estimates of 275 to 335 million light years. Given that and its apparent size of 1.4 by 0.6 arcmin, it is 120 to 125 thousand light years across.

Historical Identification: Per Dreyer, NGC 3145 (= GC 2023 = JH 673 = WH III 518, 1860 RA 10 03 18, NPD 101 44.3) is "faint, pretty large, round, very gradually then suddenly a little brighter middle". The position precesses to RA 10 10 09.3, Dec -12 25 28, within the northwestern outline of the galaxy listed above and there is nothing else nearby, so the identification is certain. (The second IC adds "Not round but much extended, according to Swift. λ Hydrae is to the northwest"; Swift's description is correct, and although λ Hydrae is actually to the northeast, such an inversion of direction is not uncommon.)
Discovery Notes: Swift's observation was appended as a note to his 8th list of nebulae discovered at the Lowe Observatory on Echo Mountain. No date is mentioned, but since all the objects in the list were observed in January or February of 1898, his observation of NGC 3145 must have also been during that time.
Physical Information: Based on a recessional velocity of 3650 km/sec, NGC 3145 is about 170 million light years away, in good agreement with redshift-independent distance estimates of 115 to 200 million light years. Given that and its apparent size of about 2.95 by 1.25 arcmin, it is about 145 thousand light years across.

Historical Identification: Per Dreyer, NGC 3146 (Ormond Stone list I (#168), 1860 RA 10 04 35, NPD 110 11.2) is "extremely faint, small, round, gradually brighter middle". The position precesses to RA 10 11 12.3, Dec -20 52 29, only about 0.6 arcmin east southeast 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 of 3960 km/sec, NGC 3146 is about 185 million light years away. Given that and its apparent size of about 0.9 by 0.85 arcmin, it is 45 to 50 thousand light years across.

Historical Identification: Per Dreyer, NGC 3147 (= GC 2024 = JH 674 = WH I 79, 1860 RA 10 04 48, NPD 15 54.4) is "very bright, large, round, very gradually then very suddenly very much brighter middle." The position precesses to RA 10 16 52.5, Dec +73 24 02, right 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 2860 km/sec (and H₀ = 70 km/sec/Mpc), NGC 3147 is about 130 to 135 million light years away, in good agreement with redshift-independent distance estimates of 90 to 180 million light years. Given that and its apparent size of about 3.95 by 3.4 arcmin (from the images below), it is 150 to 155 thousand light years across. It is listed as a Seyfert galaxy (type Sy 2), and has strong radiation by interstellar gas heated by rapid star formation (making it an HII galaxy, as well).
Note About HST images Below: The first is a "natural-color" image, while the second adds infrared data and was processed to emphasize star-forming regions and dust lanes. Both images were processed by Judy Schmidt (Geckzilla) using Hubble Legacy Archive images, which are full of artifacts caused by cosmic rays and the CCD imaging system. The lack of those artifacts in her images is a testament to the extremely high quality of her work, which is often featured on the HST Picture of the Week.

Historical Identification: Per Dreyer, NGC 3148 (= GC 2025 = JH 675, 1860 RA 10 04 48, NPD 38 49.4) is a "7th magnitude star in photosphere 2 or 3 arcmin in diameter". The position precesses to RA 10 13 44.9, Dec +50 29 11, only about 0.6 arcmin south southeast of the star listed above, so the identification is certain. The only problem (per Corwin) is that no one else has ever seen a hint of a "photosphere" or any other kind of nebulosity, so Herschel was mistaken in thinking that it existed, probably as a result of atmospheric haze too thin to notice without a bright star to illuminate it. (The bright disc around the star in the image below is due to overexposure, and would not be seen by a visual observer.)
Physical Information: The star is a semi-detached Beta Lyrae type eclipsing variable (that is, a pair of stars orbiting each other so closely that they are distended by their mutual gravity, and in many cases share a common envelope of gases), and as a result is designated as GM UMa. Its brightness varies from magnitude 6.66 to 6.81, which combined with a distance of a little under a hundred parsecs (best estimate 95 parsecs or about 310 light years) means it has an absolute magnitude slightly brighter than magnitude 2, consistent with its combined spectral type of A. It is listed in LEDA as a star with designation PGC 2903853, but a search of the database for that designation returns no result.

Historical Identification: Per Dreyer, NGC 3149 (= GC 2027 = JH 3234, 1860 RA 10 05 11, NPD 169 43.9) is "faint, small, a little extended, very little brighter middle, 15th magnitude star involved". The position precesses to RA 10 03 44.8, Dec -80 24 52, within the northern outline of the galaxy listed above, the description is a reasonable fit and there is nothing else nearby so the identification is certain.
Physical Information: Based on a recessional velocity of 2015 km/sec, NGC 3149 is 90 to 95 million light years away. Given that and its apparent size of about 2.5 by 2.3 arcmin, it is 65 to 70 thousand light years across.