Harold Corwin

IC 997 and IC 998 are perhaps the brightest and faintest galaxies in a group of four (the others are IC 4401 and MCG -01-37-002). Swift has these two objects entered in two of his lists ("(X)" and XI), but -- guessing here -- I think that he only observed them once, on 16 May 1892. His second "observation," claimed to be on 16 Sept 1896, reported in his long 11th list, looks to me like a hasty updating of the first, though perhaps after a quick second look at the field (it would have been low in the west just after sunset, though, not ideally placed for re-examination). The positions are just 10 seconds of time and 1.4 arcmin different, roughly the numbers one would expect from precession over the 10-year equinox difference (assuming that the short list in MNRAS 53, 273, 1893 has positions given for 1890.0). The descriptions are identical aside from the brightness of IC 997 ("vF" rather than "pF" in the earlier list). In particular, his note about the "star with distant companion near north" is worded the same in both lists. In other re-observations where Swift notes nearby stars, his wording is different (see e.g. NGC 5502 = NGC 5503).

Howe has an extensive note about the group (in which he found a third object, IC 4401) in MNRAS 61, 46, 1900. Here he locates and measures the three galaxies that he and Swift saw, as well as the nearby star and its companion. He also notes the identity of Swift's nebulae in the two different lists. I have adopted his identities for the three galaxies -- though with questions. Keep reading.

Malcolm has argued pretty persuasively that Swift probably saw the brightest two objects in the group, IC 997 and IC 4401. However, this would make Swift's positions not only further off in absolute terms, but his differences between his two objects would be off, too. Instead of the pretty good agreement with the actual differences of 20 seconds and 2.1 arcmin (Swift has 18 seconds and 1.0 arcmin), the differences would be 35 seconds and 4.2 arcmin. Also, the stars Swift mentions are much further from IC 4401 than they are from IC 997, though still within the 5 arcmin or so that he usually means when he uses the word "near" in his descriptions. Finally, IC 997 and IC 4401 are virtually equal in brightness (and surface brightness, too), while Swift's descriptions are quite different. He says "pF" or "vF" for I997 and "eeF" for I998. The galaxy that Howe and I take as I998 is a magnitude fainter than I997, more in keeping with Swift's descriptions.

I also note that Howe saw only three of the four galaxies here, missing the MCG object, in spite of the fact that it is half a magnitude brighter than I998. So, we do indeed have a mystery: how could two seasoned observers miss seeing brighter objects in a group while picking up the faintest? Malcolm's argument is not without a solid basis, so I've put colons and question marks on the identifications.

Earlier, I had said the following about the positions:

Finally, the accurate positions for the object I've taken as IC 997 are not well-determined in the optical -- the galaxy is an interacting double system with a complex optical structure. The components are clear in the infrared, however, so I've adopted the 2MASS PSC positions as best representing the two objects.

With the Carlsberg Meridian, Gaia DR2, Pan-STARRS1, URAT1, and 6dF lists now (October 2018) available -- and agreeing quite well -- my note about the optical positions is no longer defensible. I've adopted the Pan-STARRS1 positions for the two objects, with the mean taken for the pair.

― IC Notes by Harold Corwin

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