Finding AGN in cluster galaxies
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, and I'm providing a easy to read data table. From that, we
want to select clusters that we also have optical and hard X-ray
Get the SDSS optical photometry and spectroscopy of galaxies
arcmin radius of the cluster. Work out a good estimate for the
redshift and (RA, Dec) coordinates of the cluster center. Build a
color-magnitude diagram for the cluster, first for all
objects within 1 Mpc of the center, and then overplot
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
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? (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+
- Results: Did you find AGN in clusters? How many? With
what certainty? Other than AGN in clusters, what kind of
objects did you find? What were the limitations of what you
did? (2+ pages)
steps: What would the next step be? What data would you need
actually get a better handle on the problem? What interesting
scientific questions could you tackle? (2+ pages)
- Appendix: Your writeup should also include an
appendix which describes, for each cluster:
- the cluster center (RA, dec) and redshift as determined
by your group.
- a figure showing the cluster CMD with X-ray sources
- a listing of all X-ray sources brighter than r=20.5,
Chandra ID number ('MSID'). position, r mag, g-r color,
probability, and (if available) spectroscopic redshift.
- The writeup should be ~10 pages of single-spaced text,
not including figures, appendix, and references.
The archival data retrieval for steps I, II, and III will be done
in class. The writeup (step IV) should be your own writeup.
- 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:
import WMAP9 as cosmo
cosmo.luminosity_distance(redshift) # in Mpc
cosmo.angular_diameter_distance(redshift) # in Mpc
appmag – 5*np.log10(DL.value) + 5
r_arcsec * DA.value / 206265.