SDSS Quasars

The SDSS has found over a hundred thousand of quasars, including 26 of the 30 most distant quasars ever seen. Those quasars were been found in only the first few percent of the sky to be mapped!

To look at the properties of large numbers of quasars, you are going to use a search tool to retrieve information on a thousand quasars at once. The query tool uses a programming language called Structured Query Language (SQL) to retrieve the data. Don’t worry – the next exercise gives you a sample query to get you started.

Exercise 3. Go to SkyServer’s SQL Search tool. Copy the query listed below, and paste it into the box (overwrite the default query). Get your results as CSV (comma-separated value). Click Submit.

select top 1000
objid, modelmag_u, modelmag_g, modelmag_r, modelmag_i, modelmag_z,z

from
SpecPhoto

where
class='QSO'

Results will open in a new window. Wait for the query to finish – it should take less than a minute. You will get a a long list of data, separated by commas. The data is information on 1,000 quasars. Save the file to your computer with the “Save” command in your browser. Be sure to save it as plain text rather than HTML.

Import the file into a graphing program such as Excel. If you don’t have a graphing program, you can download a free program such as Open Office.

Use Excel’s “Import Text File” option, or a similar option in another program, to import the data (see SkyServer’s Graphing and Analyzing Data how-to tutorial to see how to import the file). Look at the results in Excel.

The first column tells you the SDSS Object ID of the quasar. You can use this number to look the quasar up in SkyServer’s Object Explorer. The next five columns give the SDSS’s five magnitudes (u,g,r,i,z) for each quasar. The last column gives the quasar’s redshift.

If you want more than 1,000 quasars, remove the “top 1000” from the query, then run it again. It will take longer to run, maybe 20 to 30 minutes, but it will return more than 60,000 quasars.

You can analyze these quasars in many different ways using these magnitudes and redshifts. You can also study the quasars’ spectra with the Object Explorer. The two projects on the next page let you do quasar analysis that was only recently published by the SDSS collaboration. The projects are a little advanced, but they reflect research that professional astronomers are doing right now!