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.
- -- Go to the NED home page http://nedwww.ipac.caltech.edu/. You
may wish to bookmark this for yourself.
- -- Click on "Advanced AllSky". We want to restrict some, but not all, options.
- -- Restict a redshift range to BETWEEN 0.05 and 0.1
- -- Restrict Object Typeto: Galaxy Cluster
- -- Include objects with the Name Prefix: ABELL
- -- Restrict the RA range to between 10h and 14; (units here are hours)
- -- Restrict the Dec range to between 0 and +3; (units here are degrees)
- -- Submit the query. Depending on traffic on the NED system, it may take a while to run, so be patient.
As of June 2006, should get back 9 objects. If not, you've done something
wrong, and you should try again until you find exactly 9 objects. If you get more than 9, then
new redshifts may have been added to NED since we wrote up this exercise!
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.
- -- Go back to the NED home page.
- -- Click on "By Name"
- -- Enter "Abell 1238"
- -- Click "yes" for "Extended Name Search"
- -- Submit the query; you should get back a list of 42 objects. If not, you've done something
wrong, and you should try again until you find exactly 42 objects.
1.1. For 20 objects (in June 2006) designated as "G" (galaxies), there are redshifts. Are there some that are
probably not cluster members? Explain your answer.
- -- Go back to the NED home page.
- -- Click on "Near Name"
- -- Enter "Abell 1238"
- -- Enter 30 arcmin for the "Search Radius"
- -- Submit the query; you should find a lot more objects. It will be advantageous to
refine the search a bit, so let's invoke more options.
You found more objects this time, because you did not require that the object be identified (by someone)
as a member of the cluster A1238; it is important in using a database to understand its limitations. In
this case, it is safer to perform a thorough search than assuming that someone interested in studying
Abell clusters has made the linkage between a galaxy and the cluster.
- -- Go back to the "Near Name" page and add a selection in
redshift to "less than 0.1" but make the search radius
30 arcmin and set"Object Type" to
Galaxies.
- -- Submit the query; you should get back a list of 106 objects (in June 2006). If not, you've done something
wrong, and you should try again until you find exactly 106 objects. If you find more than 106 objects, it
may be that more redshifts have been added to NED since we made up this exercise. NED is a dynamic database!
1.2. Use the "SKYPLOT" facility (at the top of the page; it will
only plot the first 100 objects, but ignore that detail. Use the default scale but say "No" to the
SAO stars option.) What do you note about the distribution?
1.3. In order to determine the membership of the cluster, we can examine how the observed
heliocentric velocities are distributed as a function of radial distance from the central position (given
in the column labeled "Dist. arcmin".)
Make such a plot, with radial distance of the X-axis, and heliocentric velocity on the Y-axis.
(Hint: There are various ways you can download the data
(e.g. cut and paste), but if you can't figure out easily how to do
it yourself, you are welcome to use this file.)
Comment on how this method allows you to identify outliers.
1.4. Supppose you could observe the radial velocity of every galaxy in the cluster; assume
the cluster is spherical and the orbital motions are isotropic (randomly oriented). How do you expect
the observed velocity distribution of cluster members to change with radial distance from the center?
Why? ( Hint: this is not what you see in this case. We'll see why shortly.)
1.5. Make a histogram of the heliocentric velocities, exclude the outliers and
calculate the mean heliocentric velocity and its standard deviation. How does your mean velocity
compare to that given in NED for Abell 1238?
1.6. The conversion of angular scale to linear distance depends on the geometry of the
universe; we will discuss these relations in a couple of weeks. For the moment, we can use
Ned Wright's JavaScript cosmology
calculator to find the angular to linear conversion scale. Using the CosmoCalc tool with its
default parameters for a flat universe, what is the angular extent corresponding to
2 Mpc at the distance of Abell 1238?
1.7. Suppose the cluster has a radius of 2 Mpc; estimate its mass. Explain your methodology.
Last modified: Tue Jan 23 08:58:34 2007
by martha