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 within 30
arcmin radius of the cluster. Work out a good estimate for the cluster
redshift and (RA, Dec) coordinates of the cluster center. 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
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+ pages)
- 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 to
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 overlaid.
- a listing of all X-ray sources brighter than r=20.5, listing
Chandra ID number ('MSID'). position, r mag, g-r color, match
probability, and (if available) spectroscopic redshift.
- The writeup should be ~10 pages of single-spaced text, not including figures, appendix, and
The archival data retrieval for steps I, II, and III will be done in
class in groups. Any subsequent analysis and discussion should also be
done in your groups. 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. Ned
Wright's cosmology calculator will be helpful for this, or use the astropy routines to calculate them.