Harold Corwin

IC 917 may be a star, but its companions (IC 918, IC 919, IC 921, IC 922, IC 923, IC 925, IC 926, IC 928, IC 929, IC 930, IC 931, IC 932, IC 934, IC 935, IC 936, IC 937, and IC 938) are galaxies in Abell 1783. These were found by Burnham with the Lick 36-inch refractor in 1890.

Barnard provides crude positions for them in AN 125, 380, 1890, copied correctly into the first IC. Barnard's positions and descriptions are not clear enough to positively identify more than four or five of the 18 galaxies in his table. One, IC 919, is called "cB" while the others are "vF" or "faint" and "vS, R." There are two pairs noted: IC 923 and IC 925, and IC 937 and IC 938. However, the positions imply the existence of two other pairs: IC 918 and IC 919, and IC 934 and IC 936.

Faced with this puzzle, I wrote to Don Osterbrock at Lick to ask if Burnham's or Barnard's original observations are still extant. They are, but they turn out to be even more sketchy than the AN article, so are no help. Brent Archinal reports that Leos Ondra who, during an extended visit to several US observatories, has found that Barnard's original observing books at Yerkes are still kept in the library there. These may not help solve a mystery originating in the 36-inch dome at Lick, but they might be usefully examined, anyhow.

All we can do now, however, is to assign the numbers, in approximate order of right ascension as given in AN, to the brightest 18 objects in the cluster, paying attention to the declinations when we can, as well as to the meager clues given in the descriptions. As I noted above, it seems likely that IC 917 is a star, but the remainder of the numbers can be assigned to galaxies without too much bending of Barnard's positions. However, I have to make it clear that there is a lot of guesswork going on here, and that these identifications really are tentative, pending uncovering more information.

Hubble, by the way, in his PhD thesis at the University of Chicago (Publ. Yerkes Obs. IV, Part II, 1917) describes this area and lists positions of some of the galaxies in the field. He listed tentative identifications for only three of the galaxies: IC 921, IC 925, and IC 938. The identifications that I've adopted agree with Hubble's for IC 921 and IC 938, but not for IC 925: Hubble has this as the galaxy at 13 43 14.1, +55 36 11 = my IC 923, while I have the galaxy at 13 43 16.1, +55 36 36 as IC 925.

Malcolm and Wolfgang also assign different IC numbers to different galaxies in the cluster. See their notes and tables for their interpretations.

― IC Notes by Harold Corwin

Drawings, descriptions, and CCD photos are copyright Andrew Cooper unless otherwise noted, no usage without permission.

A complete list of credits and sources can be found on the about page