Finding AGN in cluster galaxies

Advisor: For your thesis, we should look at the properties of AGN in clusters. Why don't you go dig through the data archives and see what you come up with?

Student: uh, OK.

So what do you do? This is an excellent example of a multiwavelength data mining task. We'll combine optical and X-ray data to study AGN in clusters.

Step I: Identify some galaxy clusters. We'll start with the soft X-ray selected ROSAT Brightest Cluster Sample. From that, we want to select clusters that we also have optical and hard X-ray data for.

Step II: Get the SDSS optical photometry and spectroscopy of galaxies within 30 arcmin radius of the cluster. Work out a good estimate for the cluster redshift. Build a color-magnitude diagram for the cluster, first for all (non-stellar) objects within 1 Mpc of the center, and then overplot spectroscopically confirmed cluster members (you will need to determine the cluster redshift and then also give your criterion for "spectroscopically confirmed cluster member"). Use these plots to identify the red sequence, and thereby assign "photometric cluster membership".

Step III: Search the Chandra source catalog for hard X-ray point sources around the position of the cluster -- these are likely AGN. Cross-match them with your optical galaxy catalog, and plot them on your cluster color-magnitude diagram. What can you say about their cluster membership? What kind of galaxies do they live in? Make sure you look individually at every cross-matched source brighter than an apparent r-band magnitude of 20.5.

Step IV: Write up your results in the form of a short thesis proposal to your advisor. Your write-up should have the following format (page lengths do not include figures; they should go at the end of your writeup):

The archival data retrieval and analysis for steps I, II, and III will be done in class. The writeup (step IV) should be your own writeup.


from astropy.cosmology import WMAP9 as cosmo
import astropy.units as u

redshift = <whatever>
appmag, size_arcsec = <whatever>, <whatever>

# use the cosmology modules to get luminosity distance (DL) and angular size distance (DA).
# these will be astropy objects with units attached

DL = cosmo.luminosity_distance(redshift) # in Mpc

DA = cosmo.angular_diameter_distance(redshift) # in Mpc

# to calculate an absolute magnitude, convert DL to parsecs and extract just the value (no units) to pass into the log10 function

absmag = appmag - 5*np.log10( + 5

# convert angular size to physical size using the angular size distance

r_phys = r_arcsec * DA.value / 206265.