D. Multiple Galaxies in Clusters and in the General Field
If the formation of interconnected multiple galaxies is mainly due to close encounters resulting in mutual capture we should expect physically connected double nebulae should be very much more frequent in clusters than in the general field. This conclusion is correct if the proper reservations are made. Actually we must distinguish between the general field, open clusters and very compact clusters in the centers of which several dozen member galaxies may be in virtual contact and all of them imbedded in a vast luminous cloud of intergalactic stars. In compact clusters it is therefore not any more possible to isolate many double and triple galaxies, just as it is difficult to isolate molecules (multiple atoms) in a liquid or in a crystal. In the open clusters the individual units are separated by several diameters and double and triple nebulae are therefore very conspicuous. Plate X shows a 200-inch photograph of a small part of an open cluster within which half a dozen very beautiful physical double galaxies can be recognized.
Plate X. Field within the Hercules cluster of galaxies located at R.A. 16h 2m 55s and Decl. +17° 54' (Epoch, 1950). Photograph obtained with the 200-inch telescope, exposure time 30 minutes on Eastman 103a-0 emulsion. Scale indicates one minute of arc.
Dr. HUMASON has obtained the following values for the symbolic apparent velocities of recession Vs of seven galaxies in the Herculis cluster. NGC 6041, 6044, 6045, and 6047 have respectively Vs = 10469, 9936, 9935 and 9470 km/sec. For IC 1183, 1185 and 1194 it is respectively Vs = 10038, 10452 and 11642 km/sec.