Harold Corwin

IC 4946 is probably identical with Shapley-Ames's "New 5", ESO 285-G007. Swift's declination and description fit pretty well: "eF, S, R, 3 or 4 sts f, form with the neb, a circle; sp of 2"; the stars are there. (The "nf of 2" is IC 4948, which I'll write about in the next couple of paragraphs.) Swift's position is 20 02 30, -44 10.9 (for 1950); there are no nebulae near that position that Swift could have seen. He dates his discovery to 11 Sept 1897, from his last year of observing at Mt. Lowe in Southern California.

The position of the galaxy is 20 20 31.6, -44 09 28 (1950). This leads to the major problem: Swift's RA is a full 18 minutes of time off. However, the presence of the other object which Swift noted gives us a chance to test the hypothesis. There is indeed another galaxy in the relative position given by Swift's observations, NGC 6902. If IC 4946 is indeed New 5, then IC 4948 is identical to NGC 6902.

Here is what Swift has to say about IC 4948: "vF, vS, R, bet a wide D * f and a * np; nf of 2". His position is 20 02 59, -43 50.9 (1950); that for NGC 6902 is 20 21 02.2, -43 48 57. Again, the RA is 18 minutes out. Swift gives the discovery date as 17 Sept 1897, a week later than for IC 4946.

How could he make the same unlikely 18 minute error on two different nights? I wonder if it is possible that he got his date wrong for IC 4948. He has no other objects recorded on 17 Sept, but there are two others on 11 Sept 1897, IC 4998 (which see), and IC 5018 (but these may be identical -- more confusion!). If "17" is a transcription error for "11", then Swift's observations make more sense. It's possible, however, that, on the 17th, he zeroed his setting circles on the galaxy that he found on the 11th. In this case, his relative position would be good (as it is), but his absolute position would be off once again by the same amount.

Also, Swift's description of the star field around IC 4948 is not a good match to the sky. It is, in fact, a better match to the stars around IC 4946, particularly the "wide D * f" (these two are the brightest of the circle of stars he notes for I4946). This leads me to suggest that IC 4948 may possibly be a duplicate observation of IC 4946, this time with a large Dec error as well as an even larger RA error. This isn't very likely at all, of course, but given the problems here, even this may be possible.

Finally, adding to the mess is one of Delisle Stewart's nebulae. He did not number it, but gave the discovery credit to Swift. Dreyer followed Stewart's lead, and included both observers in IC2 for IC 4948. Stewart's RA is the same as Swift's, but his declination is 5.3 arcmin north (Dreyer adopted Stewart's Dec). His description reads "F, S, R, bM" from a one-hour Bruce plate (number 3701). There are only stars in Stewart's position -- but, interestingly, a wide double star follows it by an arcminute or so.

If this were the only observation of the nebula, I'd say that Stewart has another plate defect (see IC 4922, IC 4924, and IC 4940, the only other objects that Stewart found on plate 3701; all are probably defects). Also, he does not mention the double star, and I at first took it to be his object. In fact, the double could well be his object, but we will need to examine the plate to know for sure. At the moment, though, it looks like Stewart's I4948 is indeed another defect or perhaps the double star.

So, that's the evidence. I'm leaning toward the 18 minute of time error for both objects, but there are enough pieces of contradictory evidence that I can't be sure about either one.

In the end, I've marked the IC 4946 identification with colons, and that for IC 4948 with question marks. That about sums it up.

― IC Notes by Harold Corwin

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