Scavenger Hunt #2   (the 3rd one...; IDL arrays start at 0)

1. To get started, see the website http://cas.sdss.org/astro/en/tools/search/sql.asp to learn about this toolkit. Read about accessing the SDSS by performing an SQL search. There are many parameters and many galaxies; you need to refine the search by selecting carefully.

2. Let's start by searching over the region of interest to find only those galaxies in our drift which have been detected by their H-alpha lines and which have redshifts in the ALFALFA spectral window (z < 0.06). You delete the line that appears in the box ("select stripe, run from Segment order by stripe") and replace it with your query. In this case, try a sample query that looks like this:
   SELECT
         p.ra, p.dec, s.z, p.expAB_r, p.petroMag_r, p.petroR90_r,
         p.lnLExp_r, p.lnLDeV_r, l.sigma, l.sigmaErr, l.ew, l.ewErr,
         l.chisq, l.nu, s.zWarning
   FROM PhotoObj p, SpecObj s, SpecLine l
   WHERE
         p.SpecObjID = s.SpecObjID AND
         p.SpecObjID = l.specobjID AND
         s.specClass=2 AND
         l.ew > 5 AND
         l.lineID = dbo.fSpecLineNames('Ha_6565') AND
         (p.ra >= 202.50 AND p.ra <= 225.75) AND
         (p.dec >= 5.0 AND p.dec <=5.6) AND
         s.z <= 0.06
   order by p.ra
   Notice:
   
   • "p" stands for "photometry"; "s" for spectroscopy.
     
   • We set the box a bit bigger than the drift:
     13.5h to 15.05h = 202.50 to 225.75d
     5.0d to 5.6d
     
   • We could also have selected by some other spectral line (to get the galaxies with late type stellar absorption lines but no Hα). We should do this if we want to get all the galaxies with redshifts, not just ones with H-alpha.
   • You can get the answer back in several formats. In fact, we most often use "csv" which can then be manipulated by programs.

   Click here to download a simple text version.
   Try to understand the syntax of the query, what parameters are output, what each item means. Browse the on-line documentation and look at the sample queries.

3. Run the query with the output sent back in one of the formats. Note that the query will take a while to run... depending on how many others are running at the same time on the server. Make sure you understand what you get back.

4. Here is where we leave you to explore the SDSS database on your own. Try varying the query parameters in one or more ways. In exploring, you may want to cut down the size of the box (RA, Dec) you search to speed up each query. We have made a useful list of the SDSS spectral line identifications.

5. If you are suitably inspired, figure out how to use what is sent back from the server to plot the distribution of galaxies on the sky, in some way coding objects of different redshift. It might take a little while to figure out how best to do this, but it would be a useful thing to know how to do!