NGC/IC Project Restoration Effort

===== NGC 6491 is probably also NGC 6493. It has seemed "obvious" to all of us for the nearly 130 years since Swift found his 78th and 79th objects that he must have seen the two galaxies, UGC 11008 and UGC 11011 near the positions he gives. This would make U11008 = N6491, and U11011 = N6493. All the recent catalogues (aside from RC1 which adopted Keenan's ApJ 82, 62, 1935 designation "N6493A" for UGC 11008; and RNGC which switched the NGC identifications so that "N6491" = U11011, and "N6493" = U11008) have adopted these IDs. But, as Steve Gottlieb has recently (December 2014) pointed out, there are problems with this simple assumption. First, Swift's position for N6493 is not very good -- it is -17 seconds of time in RA, and -3 arcminutes in Dec off of UGC 11011, the canonical ID. Second, the comment "sf of 2" for U11011 is wrong -- it is in fact northeast of U11008, the canonical N6491. (Similarly, the companion comment "np of 2" for N6491 is wrong.) Steve suggests that this is probably another case (see e.g. N6497 = N6498) where Swift added the comments on the relative positions while he was preparing his list of nebulae for publication. Finally, Steve had some difficulty seeing the northeastern galaxy with his 18-inch reflector; it has a much lower surface brightness than the considerably brighter southwestern object. So, Steve suggests that Swift's two observations are actually of the same object, UGC 11008. The descriptions are similar: Swift 1-78 (13 June 1885): "vvS, pF, vF * in or just in contact with it; np of 2"; and 1-79 (5 June 1885): "F, vvS, R, planetary, F * v nr; sf of 2". Only the star in or near the object might raise a warning flag as there is no such star near the brighter galaxy, and two such stars near U11011, either of which Swift might have seen. Steve suggests instead that Swift might have mistaken the bright nucleus of U11008 for a star. I'm pretty well convinced by Steve's argument, though the problem of the nearby star is a bit bothersome. For that reason alone, I am going to put colons on Steve's suggested identities, and leave UGC 11011 as a possible identification for N6493, though with a prominent question mark. Finally, concerning the visibility of the objects, take a look at the red and blue DSS2 images using SkyView (http://skyview.gsfc.nasa.gov/). I used a default field 500 pixels on a side centered on NGC 6491. The difference in appearance of UGC 11011 between the two images is striking, while the bright core of UGC 11008 remains essentially unchanged.

===== NGC 6491 18" (7/27/03): fairly faint, small, round, sharply concentrated with a very small bright core and stellar nucleus. The outer halo has a very low surface brightness and core may be offset within the halo. Brightest of a close trio with NGC 6493 3.1' NE and UGC 11007 4.6' NNW. 17.5" (8/4/94): fairly faint, fairly small, round, even concentration to a small bright core, stellar nucleus. Forms a pair with NGC 6493 3.1' ENE. The identifications of NGC 6491 and NGC 6493 are reversed in the RNGC. 17.5" (6/11/88): very faint, very small, slightly elongated. NGC 6493 3.1' NE not visible.