Harold Corwin

NGC 2646 = IC 511, which see. This galaxy also has a curious history in my own work with the three Reference Catalogues, going all the way back to the summer of 1965, my first episode of working with Gerard and Antoinette de Vaucouleurs.

My curiosity was piqued when I found that IC 520 was included in the Shapley- Ames Catalogue while NGC 2646 was not. What's going on here, that the brighter of a pair of galaxies just 12.5 arcminutes apart is an IC object object while the fainter is an NGC object? Shouldn't it be the other way around?

In principle, perhaps, but Wilhelm Tempel, the discoverer of NGC 2646, has the following description for it on the night of 27 July 1883: "Nebel III, klein, es folgen sudlich, 2 1/2' entfernt, 2 Sternchen 12-13m". In my translation, this reads, "Nebula [Herschel class] III, small, followed south, 2 1/2' away [by] 2 stars 12-13m." The two stars are just where Tempel says they are for the galaxy usually taken as the NGC object; they are very definitely NOT on the sky near the brighter IC 520.

Even though Tempel's position is flagged by "+-" in both coordinates, and is indeed not very accurate, the two stars clinch the identification. IC 520 was not discovered until five years later when Lewis Swift went over the field. He picked up NGC 2646, too, and also noted the two stars to the southeast. Because of the poor NGC position, Swift did not realize that his second "new" object had been discovered earlier. So, it now carries an IC number as well as the NGC number. See the note on IC 511 for more on that number.

So, here is a case where I simply have to accept that the IC object is the brighter of the pair. These things happen...

― NGC Notes by Harold Corwin

Harold Corwin

IC 511 = NGC 2646. After Steve Gottlieb and I had convinced ourselves that Swift's positions for IC 895, 1028, and 1045 (all of which see; IC 895 has the full story) have RA's that are ten minutes of time too small, Courtney Seligman suggested that this final nebula discovered by Swift on 1 September 1888 shares the same error.

I was skeptical at first as Swift's nominal position for IC 511 is almost exactly coincident with UGC 4510, the usual galaxy taken as IC 511. Given, however, that the corrected position is also nearly coincident with NGC 2646, and that the NGC galaxy actually does have a "wide D * nr sf" -- the UGC galaxy does not -- Courtney has made a strong case for the identity.

Courtney also points out that this clears up another mystery with Swift's description (in full, it reads, "vF, S, cE; wide D * nr sf; sp of 2") -- how can this be the "sp of 2"? The galaxy with the notation "nf of 2" is IC 520, nearly a degree away if we take Swift's positions as accurate. If, however, we take the position for IC 511 and correct it by +10 minutes, the separation becomes a much more understandable 12.5 arcminutes.

This may also explain Swift's note for IC 520, "... another suspected." He found IC 520 on 29 August 1888, three nights before IC 511. I wonder if he returned to the field on 1 September to search for his "suspected" nebula. If so, his recovery of it could well have led to the "sp/nf of 2" in the descriptions. It does leave open the question of why Swift did not recognize the wide separation implied by his incorrect position for IC 511.

Whatever happened, I am confident that Courtney is correct and that IC 511 is indeed NGC 2646. I've left UGC 4510 in the position table, though with double question marks, to flag the previous identification. Also see NGC 2646 and IC 520 for a curiosity that has bothered me since the days of my work with de Vaucouleurs in 1965 on galaxy groups in RC1.

― IC Notes by Harold Corwin

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