Problem set 4 for ASTR 323/423: The Local Universe
Due in class, Wed November 1.
Group problems for pre-assigned groups:
Galaxy classification using real SDSS data. In this homework you will select a sample of galaxies in a cluster
environment, and one in a very low-density environment such as a void,
and compare their colors and absolute magnitudes.
(1) go to http://skyserver.sdss.org/dr14/en/home.aspx
and read some documentation
(under help and tutorials). For example, click on Archive Intro and
then on Cooking with Sloan, browse the examples. If you have no
experience with SQL, do the SQL tutorial.
(2) Design your experiment:
(3) Make histograms of the color distribution in your dense and
under-dense environments. Do they agree with plots such as those shown
in class notes?
(4) (Graduate students only, Joe and Pengfei work
together). Measure the density of the environment in your two
samples. For this you should read some papers in the literature and
choose what you consider the best density estimate. Justify your
choice. Do your color histograms agree with Blanton's (or others) for
equivalent density values?
(a) Recall from class that if we are not careful about selection
biases, it is easy to make the faulty conclusion that the percentage
of red galaxies increases very fast with redshift. How do you propose
to avoid this bias?
(b) First, read the introduction to Chapter 8 in the Sparke and Gallagher (pages 314
to 321, here). You may choose to find a rich cluster and a low-density region
either from the astronomical literature (remember that SDSS is in the North!) or by downloading your own
"slice of the universe" from the SDSS archive and
pinpointing high and low-density regions by plotting redshift against
RA. Give your SQL query.
If you use the astronomical literature, give references. If you do
this yourself or if the reference does not make a 'slice of the
universe' plot (eg the pie plot in Fig 1 of de Lapparent, Geller and
Huchra 1986), make the plot for both cluster and void. Think about how to plot the diagram to give it a "pie" shape.
Read the documentation at
http://www.sdss.org/dr14/algorithms/
about which of the many
different SDSS magnitudes you should choose to download to measure
colors of your galaxies. Use Petrosian magnitudes in your analysis.
(c) How big should your sample be? Here I mean both how much of the
sky should you use, and how many galaxies should be in the sample.
Be specific about the number you decide on and discuss what considerations
informed your decision.