Abell Galaxy Cluster 1631

A Deep Sky Challenge
Centered on NGC 4756

For 16-inch or larger telescopes

We now turn to the distant galaxy cluster Abell 1631, which at 600 million light years lies ten times deeper than most of the galaxies observed visually in Corvus.  Within a 1.5o field lie at least 33 tiny galaxies brighter than 16th magnitude. The tiny, faint galaxies of this cluster make a good challenge for users of large aperture telescopes. The brightest galaxy in the region is the 13.3 magnitude NGC 4756, which lies roughly at the center of the cluster. It has a mean surface brightness of 23.3 mag/arc sec2. NGC 4756 is a good point to start your search, although at a distance of only 160 million light years this galaxy is nothing but an interloper.

About 15' to the southwest lie a small grouping of 10-11th magnitude stars. In actuality, all of these apparent stars are galaxies in the cluster. Use as much magnification as conditions will permit in this area--how many individual galaxies can you make out? I was able to discern four; IC 829, MCG -2-33-36, MCG -2-33-37 and MCG -2-33-35. Although tiny, these compact galaxies are relatively easy, with mean surface brightnesses in the 20 to 21 mag/arc sec2 range.

About 8' to the northwest of this group lies MCG -2-33-33, another relatively bright interloper.

The next most obvious galaxy is PGC 43660, which lies about 15' to the northwest. This is a 14.8 magnitude, edge-on spiral that subtends 1.8' x 0.3'. Its mean surface brightness is a very faint 24.9 mag/arc sec2.

The combination of PGC 43530 and MCG -2-33-28 is also a good target. These two galaxies lie about 30' to the west-northwest and appear like a faint, slightly fuzzy double star.

Beyond that you are on your own! As the final diagram shows, there are literally dozens of tiny galaxies here. Use high magnification and sweep the area close to NGC 4756.

Good hunting!

The field in an 18-inch with a 27mm Panoptic (76x). North is down and east is to the right.

This image from the DSS shows a 1o x 1o  field centered on NGC 4756. North is down and east is to the right.

The amazing richness of this cluster is revealed in this 30' field centered on NGC 4756 showing galaxies down to 18th magnitude. Note that many of the "stars" in the DSS image are actually galaxies.
Millennium Star Atlas Vol II Chart 844
Sky Atlas 2000 Chart 14
Uranometria 2000 Vol II Chart 284