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Page last updated Aug 12, 2017

NGC 27 (= PGC 742)
Discovered (Aug 3, 1884) by
Lewis Swift
A magnitude 13.5 spiral galaxy (type Sbc?) in Andromeda (RA 00 10 32.8, Dec +28 59 46)
Historical Identification: Per Dreyer, NGC 27 (Swift list I (#1), 1860 RA 00 03 15, NPD 61 47.3) is "extremely faint, very small, extended, bright star near". The position precesses to RA 00 10 28.3, Dec +28 59 27, only an arcmin southwest of the galaxy listed above, and less than half an arcmin from its apparent companion, PGC 731. The bright star directly below PGC 742 makes the identification of NGC 27 with one of the two galaxies certain, and although Swift's position is closer to PGC 731 than PGC 742, the two magnitude difference in their brightness makes it certain that what Swift observed was the brighter galaxy.
Observational Note: Although the modern photographs below make it look like Swift could just as easily have seen both galaxies as only the brighter one, the fact that galaxies are much fainter as visual objects than stars makes it extremely unlikely that he could have seen the fainter one. To show how difficult (or even impossible) it would have been for Swift to see PGC 731, the last image for this entry is a simulation of the 33 arcmin field of view used by him, more nearly as seen visually than in a photograph. But even though Swift almost certainly could not see the fainter galaxy, since it is so obvious in modern photographs it is discussed immediately following this entry.
Physical Information: Based on a recessional velocity of 7035 km/sec (and H0 = 70 km/sec/Mpc), a straightforward calculation indicates that NGC 27 is about 325 to 330 million light years away, in good agreement with redshift-independent distance estimates of 270 to 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 315 to 320 million light years away at the time the light by which we see it was emitted, about 320 to 325 million years ago (the slight 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 1.15 by 0.5 arcmin (from the images below), it is about 105 to 110 thousand light years across. (NED lists a second recessional velocity measurement of 7850 km/sec, but that is undoubtedly a misassigned value for PGC 731, and a good example of why it is necessary to be careful in identifying galaxies by one designation or another.) Although NGC 27 and PGC 731 appear to be close to each other, the fainter galaxy is probably 40 million light years further away, making them merely an optical double.
SDSS image of region near spiral galaxy NGC 27
Above, a 12 arcmin wide SDSS image centered on NGC 27, also showing PGC 731
Below, a 1.2 arcmin wide SDSS image of PGC 742 (= NGC 27)
SDSS image of spiral galaxy NGC 27
Below, a 33 arcmin wide SDSS image simulating the appearance of Swift's field of view
NGC 27 is the faint object in the center of the image, just above a "bright" star
SDSS simulation of Swift's view of the region near spiral galaxy NGC 27

PGC 731
Not an NGC object but listed here since discussed in the entry for NGC 27
A magnitude 15.5(?) spiral galaxy (type Sc?) in
Andromeda (RA 00 10 26.4, Dec +28 59 16)
Historical Identification: As discussed in the entry for NGC 27, Swift could not have seen PGC 731, despite its reasonably impressive appearance in modern photographs. So it is discussed here only because it is so obvious in such photographs.
Physical Information: Based on a recessional velocity of 7850 km/sec (and H0 = 70 km/sec/Mpc), a straightforward calculation indicates that PGC 731 is about 365 million light years away, in good agreement with redshift-independent distance estimates of 340 to 500 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 355 million light years away at the time the light by which we see it was emitted, about 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 about 1.75 by 0.2 arcmin (from the image below), it is about 180 thousand light years across. Although NGC 27 and PGC 731 appear to be close to each other, the fainter galaxy is probably 40 million light years further away, making them most likely merely optical companions.
SDSS image of spiral galaxy PGC 731
Above, a 2.0 arcmin wide SDSS image of PGC 731; for a wider view, see NGC 27

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IC 5350 - 5386 ← NGC Objects: NGC 1 - 49 → NGC 50 - 99