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NGC 4637 (18,682 of 18,816)

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NGC 4637

NGC 4637, GC 3172

History and Accurate Positions for the NGC/IC Objects (Corwin 2004)

NGC 4637 and NGC 4638. The brighter of the two galaxies now carrying these numbers was found by WH (he actually found it twice, so it has two entries in his catalogue). The fainter, a much smaller spiral of fairly low surface brightness with a faint star superposed west of the nucleus, was seen only once in 1854 by Lord Rosse (or his observer), who noted only a "Double nebula; faint nebulosity connects them." Given this sparce description, Dreyer assigned an approximate position to the fainter and called it NGC 4637, giving NGC 4638 to WH's brighter object. He also added a note in the NGC suggesting that Lord Rosse had actually seen M60 (NGC 4649) and NGC 4647, which are just 12 arcmin northeast. This would explain why no other observers (aside from LdR and Herman Schultz) recorded the object as a double nebula.

Schultz's observation is an interesting one. He has an extensive note in which he claims that the nebula is clearly double (in spite of relatively poor seeing on the night of observation), nearly on the parallel, and with a star of 10th magnitude north-preceding (which there indeed is; there is no such star north-preceding N4647 and M60). Like LdR, he says nothing about the relative brightness of the objects, but records his surprise that neither of the Herschel's noticed that the object was double and extended. Curiously, he gives measurements (on two different nights) of only one of the nebulae, though he specifically mentions that the micrometer wire, aligned with the equator, nearly bisected both objects. His reduced position is that of NGC 4638, the brighter object.

In his Virgo Cluster survey, Schwassmann listed only one object here and assigned it the first NGC number of the pair. His description fits the brighter object, however, and he noted that the identity was uncertain and that the object could be NGC 4638 instead. His position is peculiar, too: the RA is that of the fainter eastern galaxy, while the declination is that of the brighter western object. Remembering that Schwassmann's plate was taken with a 6-inch lens, I suspect that the plate recorded only the brighter object and that Schwassmann made a measurement or reduction error in his RA.

Dreyer, however, had only Schwassmann's entry to go on, not a modern sky survey. So, he could not know about the potential problems in the Heidelberg observation. Thus, he adopted Schwassmann's observation as applying to the fainter object, and put a note in IC2 to that effect.

My own guess, without Schultz's confirmation of the duplicity of the object, would have been that Dreyer was correct in his supposition about LdR having misidentified the objects he observed in 1854. If this is the case, then NGC 4637 is a reobservation of NGC 4647 (found by JH) rather than the very faint companion to NGC 4638. However, Schultz's observation seems to clearly point not to M60 and its companion, but to N4638 and its companion.

Still, LdR and Schultz could have seen the fainter object -- both have others just as faint in their lists -- especially since it is enhanced by the superposed star, so the "classic" numbering for this pair of galaxies is still a possibility. I should note, too, that there has been some confusion in the modern catalogues as to which number applies to which object. Dreyer unfortunately confused the issue a bit with his IC2 note, and also with his original numbering: JH had the fainter companion coming second in the GC. Nevertheless, Dreyer clearly meant NGC 4638 to apply to WH's object, so that is the identification I've adopted, leaving NGC 4637 as probably applying to the faint companion -- or possibly to NGC 4647.

Published comments

Sulentic & Tifft (1973)

The RNGC (Sulentic and Tifft 1973) notes that this is a 12.0 mag galaxy. Their coded description reads EL,F,DIF,BM,38NR.

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