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):

Intro: why is this an interesting problem? what has been done before? (~1.5 - 2 pages)
Techniques: What you did in your data mining task. Describe how the clusters were chosen (and why those criteria were used). Describe your color-mag plots for each cluster, and explain how you defined the red sequence and how you defined spectroscopically confirmed cluster members. (~ 2 pages)
Results: Did you find AGN in clusters? How many? With what certainty? What kind of galaxies were they in? Other than AGN in clusters, what kind of objects did you find? What were the limitations of what you did? (~ 2 pages)
Future steps: What would the next step be? What data would you need to actually get a better handle on the problem? What interesting scientific questions could you tackle? (~ 2 pages)
References: Your bibliographic reference list, which should consist of journal/technical articles from the scientific literature (again, no popular science websites, textbooks, etc).
Appendix: Your writeup should also include an appendix which describes, for each cluster:

The cluster redshift you determined.
A figure showing the cluster CMD with X-ray sources and spectrsocopically confirmed cluster galaxies overlaid.
A listing of all X-ray sources with r-band magnitudes brighter than r=20.5, listing SDSS position, r mag, g-r color, SDSS-Chandra crossmatch separation, and (if available) spectroscopic redshift.

The writeup should be ~ 8 pages of (single-spaced,12pt, 1" margin) text, not including figures, appendix, and references.

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.

Notes:

As a start to your literature search, I'd suggest looking at a couple of ApJ papers by Martini et al (2006, 2007). This of course should be your starting point, not the end of your reading! From there, you can look at papers they cite as well as papers that cite them, and then continue on down the literature trail. In particular, you should be sure to find more recent papers on the topic, since those Martini papers are getting pretty old now!
We will be working at distances were cosmological effects will start becoming noticeable. So don't use Hubble's law to get "a distance". Instead, use the cluster redshift to get luminosity distances and angular size distances for calculating spatial sizes and luminosities, respectively. Here's how do do that in astropy:

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(DL.to(u.pc).value) + 5

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

r_phys = r_arcsec * DA.value / 206265.