Arp 138, NGC 4015, is a pair of galaxies in Coma Berenices about 210 million light-years away. It is right on the border with Leo. In fact, the right 25% of my image is in Leo. Arp put this pair in his category Elliptical and elliptical-like galaxies: Material emanating from elliptical galaxies. His comment reads: "Absorption leads directly into E galaxy." I presume this refers to the dark cloud off the end of the spiral galaxy's tip. It's hard to tell if it is coming from the spiral or the elliptical. Arp apparently considers it coming from the elliptical or appearing to at least by the category it put it in. Since the elliptical shows no other signs of dust and spirals are often very dusty I'd suspect the spiral as being the more likely source of the dust. The redshifts of the galaxies are virtually the same so no help there.

Other than the dust cloud I see no sign the two galaxies are even interacting. If they are it must be just beginning. Apparently, they aren't quite as close as they appear to be. NED classes them as S? and E with no further refinement. It appears the spiral is closer to us than the elliptical though its redshift is very slightly larger. Are they on a collision course? No way to know as there's no way to measure their proper motion across the field. They may be moving sideways far faster than their approach velocity. It was discovered by John Dreyer on April 26, 1878. Yes, the same guy who compiled the NGC and related IC catalogs.

The field has several NGC galaxies in it. I've identified those on the annotated image as well as redshift distances in billions of light-years when known. There are distant galaxy clusters. Their position is marked by the large cD galaxy at the heart of the cluster. They are shown as having 18 or so members but I don't begin to see that many around either cD galaxy. They are marked C/GC followed by their distance in billions of light-years. NGC 4005 is also NGC 4007. Such double names are surprisingly common in the NGC.

There are two asteroids in the image. The bright one is (43477) 2001 BX7 at 18.7 magnitude. The fainter at my estimated magnitude of about 19.8 is -- I did it again -- unknown. Found and let another one get away. This was taken March 20, 2010 so a bit late to try and find it again. Minor planet center doesn't have it in their database is all I know. Edit: Since then it has been discovered in 2016. It is 2016 LN21. Since then it has faded per the MPC magnitude estimate to magnitude 20.8 a full magnitude below what I measured.

Arp's image
http://ned.ipac.caltech.edu/level5/Arp/Figures/big_arp138.jpeg

Sloan image
http://astronomerica.awardspace.com/SDSS-30/NGC4015.php

14" LX200R @ f/10, L=4x10' RGB=2x10'x3, STL-11000XM, Paramount ME