Research Using Quasar Data
In the two exercises below, you will use data from
the query you did in the last section to learn about quasars.
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
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
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
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?