The interesting NGC 4756 group

by Steve Gottlieb

Last Saturday night at Lake Sonoma (4/29/06), I took a quick look at the galaxy NGC 4756 in Corvus (12 52.9, -15 25) in my 18-inch Starmaster. At mag 12.4V, the galaxy was quite easy and I estimated the size as 1.3'x0.8'. NGC 4756 was once thought to be the brightest galaxy in the core of Abell Galaxy Cluster 1631, but redshift studies by Alan Dressler in the late 1980's revealed that NGC 4756 was the brightest member of a closer superimposed group with a recessional velocity (3728 km/sec) of only 1/3 the more distant Abell cluster (roughly 225 million light years vs. 650 million light years).

When I peered into the eyepiece, I immediately noticed several faint galaxies littered about the 225x field. But besides NGC 4756, the brighter members of both clusters are in the same magnitude range (14th-15th) and that makes it impossible to sort out the two clusters at the eyepiece (I later checked the redshifts from NED). Especially interesting was a clump of three galaxies about 7' north of NGC 4756. It turns out this quartet (I missed one galaxy) is at the same redshift of NGC 4756 and would meet the criteria to qualify as a Hickson Compact Group.

So, in the same eyepiece field is a distant Abell cluster, the more nearby NGC 4756 galaxy group and a subgroup similar to a Hickson Compact Group! With an 18-inch scope about a dozen galaxies can be tracked down in the region with a good finder chart although I wasn't able to make a careful search of the region. I plan to return to the same field next year.

Posted on sf-bay-tac May 02, 2006 10:36:42 PT
Converted by report.pm 1.4 May 04, 2006 21:20:50 PT

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