The National Virtual Observatory
2006 Summer School

Correlation Services via the Web

Overview

Through the course of this talk we will be using a few VO tools: Open SkyPortal, Topcat and VOPlot . Our goal in this exercise will be to construct a diagnostic diagram for differentiating galaxies with active nuclei from galaxies with intense star formation.


Summary of Science

We have a list of the brightest cluster galaxies (BCGs) from the C4 catalog compiled by C. Miller. In order to study the properties of BCGs we would like to determine which of them are active galactic nuclei (AGN). The diagnostic diagrams introduced by Osterbrock et al. 1989 and developed further in Kewley et al. 2001 provide the tool necessary to differentiate between AGN and starforming galaxies. We will use VO tools to create the Osterbrock diagram for the BCGs in the C4 catalog.

Step 1:  Get the BCG catalog and upload to Open SkyQuery

http://www.ctio.noao.edu/~chrism/c4/sdss_c4_dr2_published.dat

To use the web-based upload client, your data must be with an XML file, or a comma/space/tab separated ascii file. ADDITIONALLY, you must have columns named RA and DEC.
Take a look at this file. Notice that the BCG coordinates are actually labels RA_BCG and DEC_BCG. Notice that there are no columns intrinsically named RA or DEC. Thus you must alter the names of the RA_BCG and DEC_BCG columns to be RA and DEC respectively. Edit the file and save it to your local disk.

Step 2: Upload the Table

Point your browser to http://www.openskyquery.net:
Click on Import:

OSQ front page

OSQ Import page
Note above that we have named the table (SDSS_C4_BCG), that we have specified the location of the file to upload, and that we have stated a positional uncertainty.
REMEMBER THE NAME OF YOUR TABLE!

You must specify the name of the table (remember it). You must specify the location of the file. You must specifiy the accuracy of your positions.

Step 2:  Query OpenSkyQuery

Now we will use the web interface located here to retrieve line information for each of the BCGs. We want the ADVANCED QUERY.

Once you have the web interface open, execute the following four ADQL queries. Each one will get the height and width information for each of the following lines: OIII, Hβ, NII, Hα respectively. After each query completes, download and save the tables in VOTable format. My suggestion is to save them with the names OIII.xml, Hb.xml, NII.xml, and Ha.xml to keep them straight when merging the tables.

The first challenge is figuring out how to get line information out of the SDSS spectral database.
Questions you need to answer:
1. Is the SDSS the data you want? What else is out there you may want instead (in SkyNodes)?
2. What spectral data is in the SDSS with respect to line information? (www.sdss.org/dr3) (http://cas.sdss.org/astro/en/help/browser/browser.asp)
3. What lines do I want for my research?
4. Whiat other data from what other tables do I need?
5. How do I build my query?

File: OIII.xml
SELECT o.objid, o.ra,
o.dec, o.g, t.*,
s.height, s.sigma, s.wave,
s.restWave
FROM
SDSSDR3:PhotoPrimary o, MyData:SDSS_C4_BCG t,
SDSSDR3:specline s
WHERE XMATCH(o, t) < 3.5 AND
o.specobjid = s.specobjid AND
s.lineID = 5008

File: Hb.xml
SELECT o.objid, o.ra,
o.dec, o.g, t.*,
s.height, s.sigma, s.wave,
s.restWave
FROM
SDSSDR3:PhotoPrimary o, MyData:SDSS_C4_BCG t,
SDSSDR3:specline s
WHERE XMATCH(o, t) < 3.5 AND
o.specobjid = s.specobjid AND
s.lineID = 4863

File: NII.xml
SELECT o.objid, o.ra,
o.dec, o.g, t.*,
s.height, s.sigma, s.wave,
s.restWave
FROM
SDSSDR3:PhotoPrimary o, MyData:SDSS_C4_BCG t,
SDSSDR3:specline s
WHERE XMATCH(o, t) < 3.5 AND
o.specobjid = s.specobjid AND
s.lineID = 6585

File: Ha.xml
SELECT o.objid, o.ra,
o.dec, o.g, t.*,
s.height, s.sigma, s.wave,
s.restWave
FROM
SDSSDR3:PhotoPrimary o, MyData:SDSS_C4_BCG t,
SDSSDR3:specline s
WHERE XMATCH(o, t) < 3.5 AND
o.specobjid = s.specobjid AND
s.lineID = 6565

Step 3:  Merge Tables

Now we will use the Starlink Topcat tool to merge the tables into one table with all the information we need to create the diagnostic diagram. On my linux machine, I start Topcat with the following command. This will vary slightly with platform.

$>topcat [A-Z]*.xml
Listing 3

This should open a window like the one shown in Figure 1. start
Figure 1

Click on the button depicting the two matches. This button is circled in red in Figure 1. Activating the match tool will bring up a new window. The match window is shown in Figure  2.

 match
Figure 2.

We will be doing an exact value match since all the tables have the same sdssdr3_objid field. Once the form is set up to go, hit "Go." The way that I suggest doing the matching is to first match OIII.xml and Hb.xml on sdssdr3_objid. Then match NII.xml and Ha.xml on sdssdr3_objid. Finally, match the two previously matched tables on sdssdr3_objid_1 and sdssdr3_objid_2 respectively. For comparison, a copy of my matched VOTable is available here. If you did not get through the matching portion, download the one I have put together so that you can follow along with the next section. It is a good idea to edit the column name of the sdssdr3_objid to specify which line each block in the matched file represents. This is because when matching in TOPCAT, any columns that have the same name will append and _N (where N is a number) to the column name after the match.

Step 4:  Plot the Osterbrock Diagram

We now have all the information necessary to make our diagnostic diagram in the matched VOTable created in the previous section. Now we will use VOPlot to complete this exercise. Start voplot:

$>voplot
Listing 4

Load the matched VOTable by going to the 'File' button and clicking on 'Open' in the dropdown menu. Chose your VOTable file from the dialog box.

We wish to plot two quantities which are not explicitly in the table, so we must add some columns. The diagnostic diagram compares the line flux ratio of the [OIII] over Hβ to [NII] over Hα. To get the individual line fluxes, we must multiply the height and the measured width (sigma) for each line, the we take the log of each ratio so that we can look at the diagram in log-log space. If you have followed my method for combining the tables, or have downloaded my matched file this would be done clicking on the "Functions" button at the top of the VOPlot window and then choosing "Create new columns." This should pop up a window like the one shown in Figure 3.

coladd
Figure 3

In Figure 3, I have already added a name for the new column and the correct expression for the new column. You should now do the same. The expressions are as follows (again, this is only if you have followed my method above, or have downloaded my matched file):

  • OIII/Hb: log(($6*$7)/($26*$27))
  • NII/Ha: log(($47*$48)/($67*$68))
  • We now finish off the exercise by plotting our new columns against each other (see Figure 4).plot
    Figure 4.

    Most of the points in this diagram are in the area designated to be populated by active galactic nuclei. Models for comparison are available in the further reading section. We have discovered that most of the brightest cluster members with measured lines have active galactic nuclei.  For comparison, I have included another plot showing the same Osterbrock diagram with SDSS galaxies.

    ost
    Figure from Miller et al. 2003 (astro-ph/0307124)

    Student Exercise:

    Use the Osterbrock diagram constructed in this exercise to form a VOTable of the most extreme AGN. Use this new table in conjunction with OpenSkyQuery.org to find out some information about the X-Ray properties of these brightest cluster galaxies.

    Useful Links:

    VOPlot
    TOPCAT
    OpenSkyQuery
    WESIX

    For more information on the theory behind the diagnostic plots see the following and references therein.

    Kewley et al. 2001
    Kauffmann et al. 2003



    The NVO Summer School is made possible through the support of the National Science Foundation and the National Aeronautics and Space Administration.