Arecibo Observatory

1.   Estimating the mass of the cluster of galaxies Abell 1238

In the 1950's, George Abell performed a systematic visual examination of the Palomar Observatory Sky Survey photographic plates to identify rich clusters of galaxies. Practically no redshifts were measured for galaxies at the time, so it was all based on his ability to estimate galaxy distances from the visible images. It turns out he did a fantastic job of identifying only real 3-d structures; there are some that turned out to be the chance projection of structures at different distances, but not many. His original catalog of 2712 northern hemisphere clusters is known as the "Abell Catalog" (Abell 1958, ApJS 3, 211). He later extended this work to a similar survey of the southern sky which was completed by his colleagues after his untimely death.

In this exercise, we will use the NASA Extragalactic Database, NED to find a sample of clusters of galaxies and then to make an estimate of the mass of the cluster Abell 1238. We try to provide specific instructions to guide you through the exercise, but you are invited to explore this database and its tools yourself.

We've been (deliberately) selective in restricting the volume searched so that you only would find a few objects, and in particular, so that the region searched is contained in the current data release of the Sloan Digital Sky Survey (SDSS) which we will use in another exercise. Of the 9 clusters, let's explore Abell 1238. You may wish to note the basic information returned by this NED query on that one cluster for future reference.

Last modified: Tue Jan 23 08:58:34 2007 by martha