Complement to VII/239A

NGC 2363

and NGC 2366. Well, folks, it's bad news for those of us who have always identified NGC 2363 as the giant HII region in the low surface brightness irregular galaxy NGC 2366. WH's original description clearly refers to the HII region as the principal object with the bit of fuzz to the north as an incidental appendage.

This view was further solidified by Ralph Copeland, observing with Lord Rosse's 72-inch reflector. Copeland identified the HII region as the center of an greatly extended object, stretching 9 or 10 arcmin to the northeast. He lists micrometric measurements of seven different objects in the surrounding area, and all are clearly refered to the HII region. Here is a list of his measurements, along with mine (made from the POSS1 blue print):

Copeland Corwin Note Object P.A. Dist. P.A. Dist. * 9.5 6.3 214.9 7.5 211 * 10 337.9 191.9 335 193 * 10 340.8 235.8 339 236 * 8 351.0 396.6 351.5 400 Dif neb 265.9 71.4 274 70.5 Copeland quadrant error? Neb * 318.0 77.6 318 75.2 Star sup on F ext of galaxy "Tail" 30.9 [9-10min] 31 8min+- Main body of the galaxy

So, the historical record is unmistakeable: NGC 2366 is the HII region. We can, of course, also include the rest of the galaxy under this number, since it was certainly seen. For the sake of the modern catalogues, this is also certainly the best thing to do.

But what then is NGC 2363? One of Copeland's micrometric measures above – for the "Diffused nebulosity preceding" – is the one that Dreyer put into the NGC with the note "III 748 s[outh] f[ollowing]." This, combined with Copeland's measurement which Dreyer used, clearly points to the smaller object that we now call UGC 03847 = MCG +12-07-039 (N2366 is U3851) – it is NGC 2363, not the HII region. I have usually taken this object to be a detached star cloud of N2366, but Steve Odewahn has shown through his detailed study of the velocity fields of the objects that it is indeed a separate galaxy interacting with N2366.

So, we have two galaxies here, along with two NGC numbers clearly attached to each one. We shall just have to get used to calling the HII region "Markarian 71" (or one of its other names) since it is not N2363 as we've thought all these years.

There is still one other nebulous object seen by Copeland in the area. This is the "Nebulous star or nebulous knot" which is listed in the table above. Why didn't Dreyer include it in the NGC, too? Other objects with just that sort of description were included. While this is an unanswerable question, it's possible that Dreyer had access to other notes that were not published. Or, since he and Copeland were colleagues at the time, the two of them may well have decided that the object was a star. The object is indeed a star superposed on a faint extension of NGC 2363. There may also be a distant background galaxy adding to the appearance of nebulosity – see the lovely 200-inch photographs in the Revised Shapley Ames Catalogue (page 113) and in the Carnegie Atlas (Panel 327).

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