Tutorial #1: The Coma cluster of Galaxies

 

Goal: Examine the Coma cluster of galaxies (Abell 1656) using VO data and tools in order to perform a quick evaluation of the mean redshift and velocity dispersion of the cluster. We use redshifts and photometry (Petrosian r magnitude) of the SDSS survey and then add redshifts of the CAIRNS survey (Rines et al 2003) in order to improve the completeness of the redshift sample.

 

Note 1: there is a much more detailed printer-friendly step-by-step guide leading through the science case available here http://www.eurovo-ice.eu/twiki/bin/view/EuroVOICE/ThematicTutorial1. 

 

Note 2: the SDSS-DR8 catalog is quite large, we suggest to download it in advance and save it on your pc. You can download it from Vizier or find it here http://adlibitum.oats.inaf.it/iafrate/SDSS-DR8.xml. The faster way to load a local catalog in Aladin is to drag and drop it in the main window of Aladin. As a second option load it from the menu file -> load -> file.

 

1. Display the region of A1656 from the Allsky color atlas of Aladin. We suggest to work with galaxies in a region with radius of about 40’ around the Coma cluster. At the distance of Coma, 40’ corresponds to 1.1Mpc (with H0=71 Mpc-1km s-1, ΩV = 0.73, ΩM = 0.27), a region large enough for our purposes. Tip: in order to check the radius of the region being displayed, the small white box in the bottom left corner of Aladin tells you its area. As a second option draw a 40' long arrow with the "dist" button.

 

2. Load the SDSS-DR8 catalog from the "Surveys" catalog server, radius 40’, check retrieve all columns. Filter SDSS and leave only galaxies (cl=3) that are also SDSS primary sources (mode=1): select the catalog plane and apply the advanced filter ${cl}=3 && ${mode}=1 {draw}. Tip: in the filter window applay the filter and then export the filtered catalog to a new plane and rename it to SDSSgalaxies. The new filtered catalog should have 23771 sources. Broadcast the filtered catalog to Topcat. In Topcat display the column metadata (), deselect all (make all table columns invisible) and tick only the four columns we need: RAJ2000 ($14), DECJ2000 ($16), zsp ($18) and rPmag ($74).

 

3. Create a subset of galaxies with photometry (rPmag) and redshift (zsp) in SDSS. Use the Subset window () to A) create rPmag17 the subset of galaxies with magnitude rPmag  brighter than the completeness limit of the SDSS spectroscopic sample, rPmag lim=17.77 (subset expression $74 <= 17.77). NOTE: Topcat expressions do not differentiate small/capital letters and SDSS contains both rpmag and rPmag. In this case you have to use $ID of the column rather than its name. $ID can be found with Views->Column Info. B) create zsp17 the subset of rPmag17 with redshift (subset expression zsp>0 && $74 <= 17.77).

In the main window of Topcat select the subset zsp17 and duplicate the table (File -> Duplicate Table). Rename the new table to zsp17. Compute the  completeness of the zsp17 sample (482/634=76%).

 

4. Eliminate bright stars from galaxy catalog. select the rPamg17 subset and broadcast it to Aladin (Interop -> Send table to -> Aladin). In Aladin select with the cursor the two brightest galaxies, both are fainter than rPmag=11.7. In Topcat create a subset of probable stars (create subset stars with rPmag<11.7) and verify that they are indeed stars: in the main window of Topcat select the SDSS catalog, then click activaction action ->  transmit row -> Aladin. Display in Topcat the subset stars and move through its rows (): selected entries will be displaied in Aladin. In Topcat, create a subset rPmag17ok of rPmag17 without objects brighter than rPmag=11.7 (or modify the definition and name of rpmag17 directly in the subset window). Completeness rises slightly (482/622).

 

5. Improve completeness with other sources of redshifts in Vizier. In Topcat create the subset rPmag17oknoz of rPmag17ok (use subset selection from table browser or create subset with expression !(zsp>0) together with the expression for rPmag17ok). The new subset has 138 entries, select it and duplicate the table. Rename the new table into rPmag17oknoz. In Topcat search optical catalogs with redshifts (VO -> Vizier Catalog service -> Cone search -> Object name = A1656 -> resolve -> radius = 40 arcmin -> catalog selection -> by keywords). Load Rines+ 2003 galaxies. In Topcat delete che cluster catalog and Keep only the galaxy one.



 

6. Find redshifts in Rines+ for galaxies without redshift in rPmag17. Match the rPmag17oknoz and the Rines catalogs (joins -> pair match). Use sky algorithm with 5” max error. Transmit the catalog of 18 galaxies to Aladin for inspection. 

 

7. Create the “final” catalog including Rines+ redshifts. Add column czsp with the expression toInteger(zsp*300000) to the zsp17 catalog ( -> columns -> new symthetic column). Concatenate zsp17 and match tables (joins -> concatenate tables). The final catalog contains 500 galaxies, completeness is 500/622 = 80%. 

 

8. Determine cz distribution, <cz>, and dispersion in Topcat. Isolate main peak of Coma in the histogram window (). Suggestion: change the bin width to 200. Create new subset Coma (subset -> new subsets from visible), then in the main window of Topcat select it and open the Row Statistics window (): Nmem=492, <cz>=7000 km/s and disp=1131km/s, both in excellent agreement with more refined analyses.