(65535) На сайтеДата: 11.09.2016, 14:29:30 | Сообщение № 1
NGC 16 (другие обозначения — UGC 80, MCG 4-1-32, ZWG 477.61, ZWG 478.33, PGC 660) — линзообразная галактика в созвездии Пегас.
Этот объект входит в число перечисленных в оригинальной редакции «Нового общего каталога».
Галактика была открыта английским астрономом Уильямом Гершелем 8 сентября 1784.
Объект наблюдается с земли как достаточно яркий, круглый, малого размера. Для наблюдения будет достаточен 6-дюймовый телескоп.
(65535) На сайтеДата: 11.09.2016, 14:34:00 | Сообщение № 2
|NGC 16 (= PGC 660)|
Discovered (Sep 8, 1784) by William Herschel
Also observed (Sep 5, 1828) by John Herschel
A magnitude 12.0 lenticular galaxy (type E/SAB0?) in Pegasus (RA 00 09 04.3, Dec +27 43 46)
Historical Identification: Per Dreyer, NGC 16 (= GC 8 = GC 12 = JH 4 = JH 5 = WH IV 15, 1860 RA 00 01 52, NPD 63 03.0) is "pretty bright, small, round, brighter middle". The equivalence of GC 8 (= JH 4) and GC 12 (= JH 5) is discussed in a note at the end of the NGC: "(John Herschel's) h5 (= GC 12) was not seen by d'Arrest and Stephan (XIII); it is = h4 (= GC 8) as they were observed in different sweeps." The position precesses to RA 00 09 04.6, Dec +27 43 46, almost dead center on the galaxy listed above, so the identification is certain.
Discovery Notes: When John Herschel published his "General Catalog", GC 8 was defined as JH 4, and GC 12 was defined as JH 5 and assumed equal to WH IV 15; and since Dreyer published the NGC it has been assumed that GC 12 = GC 8 = NGC 16. However, in 2014 it was suggested that JH 5 and WH IV 15 are not observations of NGC 16, but of some faint star or perhaps even NGC 22. That suggestion is almost certainly wrong, as discussed in the lengthy analysis below; and in any event the identification of NGC 16 as PGC 660 is unaffected by any question about its discovery. So unless you want to read all the details confirming the history of discovery, you can skip that discussion by clicking here.
Analysis of John Herschel's GC 8 = JH 4: John Herschel's GC lists #8 = JH 4 as "pretty bright, small, round, brighter middle", with a position of (1860) RA 00 01 52.1, NPD 63 03 27.5, which precesses to RA 00 09 04.7, Dec +27 43 18, less than half an arcmin south of the center of NGC 16 and within its southern outline, so the identification of GC 8 with that object is certain. Since the JH numbers are in order of the date of observation, JH 4 is the earlier of the two observations in question, and its date of observation (Sep 5, 1828) is the correct one for John Herschel's first observation of NGC 16, regardless of the status of JH 5.
Analysis of William Herschel's IV 15: Per William Herschel's first catalog of 1000 nebulae and clusters, WH IV 15 was observed on Sep 8, 1784 as "A faint star with small chevalure and 2 burs." at a position 2m 6s east and 1° 21' south of 21 (α) Andromedae, or Alpheratz. However, Dr. Corwin has provided me with a copy of the last five observations Herschel took on the night in question, from a "fair copy" made by his sister, Caroline Herschel. That shows that he actually recorded offsets of 2m 6s east and 1° 23' south of 21 Andromeda, and adds other items of interest. Specifically, he noted the rising of the nearly last quarter Moon about half an hour before to stopped observing (presumably because even at that phase the Moon's light would make it difficult to see the faintest objects visible under dark-sky conditions, and the last thing he observed was the very faint object listed as WH IV 15). In the interim Herschel observed three stars: a 7th magnitude star, 21 Andromedae and 85 Pegasi. Since the positions of 21 And and 85 Peg are known, they can both be used to calculate the position of WH IV 15, and in the process provide some information about how consistent Herschel's measurements were on the night in question.
Calculations Using 21 Andromedae: For 21 Andromedae the offsets are 2m 6s east and 1° 23' south. The modern position of Alpheratz is RA 00 08 23.3, Dec +29 05 26, with proper motions of +0.13746"/year in right ascension and -0.16344"/year in declination. This means that in 1784 its J2000 position was 2.0s west and 35" north of its present position, or at RA 00 08 21.3, Dec +29 06 01. This precesses to (1784) RA 23 57 15.7, Dec +27 53 51, and applying Herschel's offsets for IV 15, whatever he observed should be located near (1784) RA 23 59 21.7, Dec +26 30 51. This precesses to J2000 RA 00 10 28.6, Dec +27 43 00, in a completely stellar region about 1m 24s east and 0.8 arcmin south of NGC 16, and about 40s east and 7 arcmin south of NGC 22.
Calculations Using 85 Pegasi, and a Comparison of the Results: For 85 Pegasi, the offsets for IV 15 are 8m 18s east and 34' north. The modern position of 85 Pegasi is RA 00 02 10.2, Dec +27 04 56, with proper motions of +0.78022"/year in right ascension and -0.91775"/year in declination, meaning that in 1784 its J2000 position was 11.2s west and 3' 7" north of its present position, or at RA 00 01 59.0, Dec +27 08 03. This precesses to (1784) RA 23 50 57.5, Dec +25 55 54, and applying Herschel's offsets, whatever he observed should be located near (1784) RA 23 59 15.5, Dec +26 29 54. This precesses to J2000 RA 00 10 22.2, Dec +27 42 03, only a few seconds of time and an arcmin away from the position obtained from comparison with 21 Andromedae, and since William Herschel only recorded right ascensions to the nearest tenth of a minute of time and declinations to the nearest arcmin, "errors" of up to 0.2 minutes of right ascension and 2 arcmin of declination could be recorded even if the observations themselves were absolutely perfect. For example, consider the 7th-magnitude star recorded by Herschel 1.5 minutes before 85 Pegasi, at an NPD 15' south of that star. That must be the magnitude 6.5 star HD 224758, which had a position in 1784 (using similar methods of calculation and rounding to the same accuracy as Herschel's measurements) 1.5 minutes west and 13' south of 85 Pegasi, within the range of rounding errors stated above, verifying that for all practical purposes the position of WH IV 15 is exactly the same regardless of which comparison star is used.
Analysis of John Herschel's GC 12 = JH 5 (declared = WH IV 15 in the GC): John Herschel's GC lists #12 = JH 5 = WH IV 15 as "very faint, very small, stellar" with a position of (1860) RA 00 03 14.6, NPD 63 04 58.5, which precesses to RA 00 10 27.8, Dec +27 41 46, in the same completely stellar region as the elder Herschel's positions, and in fact less than an arcmin from the average position (RA 00 10 25.4, Dec +27 42 31) of the two values for WH IV 15 calculated above, making its position exactly the same to the accuracy of the earlier observations. So there can be no doubt that JH 5 = WH IV 15, as stated in the GC.
What Is GC 12 = JH 5 = WH IV 15?: NGC 16 and NGC 22 are the only objects reasonably close to the positions obtained above, so one of those galaxies is almost certainly what the Herschels observed. And the answer is almost certainly NGC 16, since its declination is almost exactly the same as that measured by the Herschels, while NGC 22 is over 7 arcmin to the north, which is a far less likely error than a minute or so of right ascension in historical observations, and NGC 16 is far brighter. Admittedly, the recorded magnitudes of the two galaxies only differ by a factor of 4 in brightness, which appears to agree with the difference in the description of GC 8 as "very bright" and GC 12 as "very faint", but that is for their entire extent as recorded by modern photographs. For visual observers only the bright core is visible, and the large bright core of NGC 16 is many times brighter than the much smaller, fainter core of NGC 22. In fact, there is considerable doubt whether William Herschel could have seen NGC 22 even under the best of conditions, let alone in the growing light accompanying even a nearly quarter Moon rising in the eastern sky. So we are left with three options: (1) Assume that GC 12 represents an observation of a nonexistent object, despite the fact that the Herschels' essentially identical positions suggest they must have seen something, (2) assume that GC 12 represents an observation of NGC 22, despite the fact that it was probably just as hard for them to see as a nonexistent object and the large error in declination is hard to accept, or (3) assume as Dreyer did that GC 12 = GC 8 = NGC 16. Given these choices I see no reason to abandon Dreyer's suggestion, and feel reasonably comfortable accepting the probable fact that GC 12 is simply one of many examples in which the Herschels misrecorded the right ascension of an object.
(Temporary) Final Discovery Note: Although the discussion above covers everything available for William Herschel's observation, it only relies on the published record for John Herschel's observation, and there are additional original data available for that. Per Corwin, those original data are very confused. It looks like JH probably made a minute of time error in recording his position, and if William did the same thing (which cannot be known, since there is nothing left to review for his observation), the identification with NGC 16 becomes certain. But since I have not had time to review JH's records in detail, even though I am already essentially convinced of the identification with NGC 16, the warning that I am still working on this entry will remain in place for a little while.
Physical Information: Based on a recessional velocity of 3100 km/sec, NGC 16 is about 145 million light years away, in fair agreement with redshift-independent distance estimates of 105 to 120 million light years. Given that and its apparent size of 1.9 by 1.1 arcmin, it is about 80 thousand light years across. NGC 16 appears to be a completely isolated galaxy, at least 20 million light years from any other galaxy of significant size.