Object Followup



Having cross-matched Chandra X-ray and SDSS optical sources and put the X-ray host(?) galaxies onto the CMD, we can now follow up by looking at individual sources to see if we have found many true cluster AGN. First, get your codes working to the point where you can generate a color-magnitude diagram for each cluster that shows:
In all cases, restrict your objects to those within 1 Mpc of the cluster center. Your plot should have a legend that describes what's being plotted, as well as a label giving the name of the cluster and the redshift you determined. So that you can see what I'm looking for, here's my plot for Abell 2065:



So yours should look similar, and then you should also run the analysis for the other two clusters as well. Remember, when you run your analysis on the other clusters, they are at different redshifts, so in each case you will need to make sure you have estimated the redshift and used it to evaluate the 1 Mpc radius (in arcsec) for each cluster.

Then for Monday's class, from each cluster pick 5 or so objects that are brighter than r=20.5 (the dotted line on my plot) and follow them up in SDSS Navigator. Use their coordinates (their SDSS coordinates, remember) to search on them and see what the objects look like (disky? elliptical? interacting?), where they are in the cluster (near the center? in the outskirts?), make a note of their color, see if they have a spectrum (if so, what is the redshift? what does the spectrum look like?), etc.  Any evidence for AGN activity in the spectrum? If they dont have a true spectroscopic redshift, take a look at the photometric redshift estimate and see if it is consistent with the cluster redshift.

When you do this, pick a variety of objects-- some on the red sequence, some in the blue cloud, and even some that (based on their position on the CMD) might not even be in the cluster at all. This latter point is important, since we want to understand the types of objects we are finding when we do this X-ray selection, even if they aren't in the cluster. And if you want, you might also look at a couple that are fainter than r=20.5 -- don't spend too much time on these, but they will give you an idea of what lurks below the magnitude limit we are imposing....

Helpful tip: I find it useful to add a column to the crossmatch table that holds the g-r color of the object, that way you can sort on it if you are looking for red or blue things. Like this:
CROSSMATCHED['gmr']=CROSSMATCHED['g']-CROSSMATCHED['r']
inside_1Mpc=CROSSMATCHED['angdist_arcsec']<One_Mpc_In_Arcsec
good_match=CROSSMATCHED['match_sep']<3 # or whatever your criterion is....
want=np.logical_and(inside_1Mpc,good_match)
CROSSMATCHED[want].show_in_browser(jsviewer=True)
Then in the browser window you can click on the 'r' column to sort by brightness, or click on the 'gmr' column to sort by color, etc. It also makes it easier to find a particular object that you've selected off the CMD (for example, "I want to look at that object that has a r mag of 19.5 and g-r color of 0.5")