In the two exercises below, you will use data from the query you did in the last section to learn about quasars.
Exercise 4. In this exercise, you can explore the relationship between colors and redshifts of quasars. Make a color-redshift diagram using the data on 1000 quasars you found in Exercise 3. Put u-g on the x-axis and redshift on the y-axis. Repeat the graph for the colors g-r, r-i, and i-z.
For a tutorial in how to use Microsoft Excel to make graphs, see SkyServer’s Graphing and Analyzing Data How-To tutorial.
In December 2000, the SDSS collaboration published a paper with graphs like the ones you made. To compare your graphs to theirs, click here to download their paper from the Los Alamos Astrophysical Preprint Server. Click “Other Formats,” then select “Download PDF.” (You will need to download Adobe Acrobat Reader to view this file. Click here to download the program, free of charge, from Adobe’s web site.)
The researchers’ graphs are on page 34 of their paper. You may want to change the scales on your graphs to make yours look more like theirs.
The practical application of the graphs you just created is that they can be used to find the redshift of a quasar photometrically; that is, using only the quasar’s image.
Most quasars have their redshifts determined by looking at their spectra. However, the SDSS will discover so many quasars that we cannot take the spectra of every quasar. We do know the colors of the quasars, and we can use the colors to estimate the redshifts. Although this method is not as accurate as determining redshifts from the spectra, we can find the redshifts of a large number of quasars for which no spectral measurements exist.
Exercise 5. In this exercise, you will make color-color diagrams for quasars. Use the spreadsheet to make a graph with u-g on the x-axis and g-r on the y-axis. Make a second diagram with g-r on the x-axis and r-i on the y-axis. Make a third diagram with r-i on the y-axis and i-z on the x-axis. Where are the very high redshift quasars on your diagrams? Where are the very low redshift quasars?