Let's look at the properties of Abell 2065. The
cluster's sky position is
(RA,Dec) = (230.62156, +27.70763)
where both coordinates are given in decimal degrees.
Go to SDSS Skyserver: http://skyserver.sdss.org/
and under "Data Access" click on Navigate.
Go to Navigate, and put in the cluster coordinates. Click
"Search". Hopefully you'll see a cluster.
If you click on "Grid" (in Drawing Options), you'll get a scale
bar which helps you work out the angular scale of the image. You
can zoom in and out using the magnifying class +/- buttons, and
you can pan by click-dragging.
Click on a galaxy. Over to the right, you'll see the galaxy in a
little zoomed-in window, with the ugriz magnitudes given above
it. If you click the "Explore" button underneath the zoom
window, a new webpage will open up with more detailed properties
of the galaxy, including (if it exists) a spectrum.
If you go back to the Navigate page, and (under Drawing Options)
click "Objects with Spectra", it will highlight all sources that
have spectroscopy, and you can click on one of them, click on
Explore again, and you will see both photometry and spectroscopy
Play around. Scroll, zoom, click on galaxies and hit "Explore",
etc. Look at a few spectra of galaxies in the field. Work out
- How big (in arcminutes) is the cluster?
- What is a rough guess for the redshift of the
OK, so now you have a feel for the cluster, lets get some data
and try and work out some quantitative analysis.
Instructions for downloading data
Jupyter notebook to get you started
Try a few of these things:
I say plot "this" versus "that", "this" goes on the y-axis and
"that" goes on the x-axis. So "plot color versus magnitude"
means that color goes on the y-axis and magnitude goes on the
- Plot redshift versus r-band magnitude for galaxies
that have a measured redshift and are within 30 arcmin of
the cluster center. Using the plot as a guide, work out a
quantitative, statistical estimate of the cluster redshift.
If you wanted to define a "spectroscopically confirmed
cluster member", how might you do it?
- Plot dec vs ra (so a sky map) of resolved sources
(type=3). Make another for unresolved sources (type=6).
Think about the differences.
- Plot r mag uncertainty versus r mag. If you
want your mags good to 10% or better, what is the rough
magnitude limit of your analysis?
- Plot g-r color versus r magnitude for all resolved
sources projected within 1 Mpc of the cluster center -- this
is a color-magnitude diagram (CMD) for galaxies.
- Plot the CMD, then overplot in a different color the
CMD for spectroscopically confirmed cluster members. Again,
restrict it to sources projected within 1 Mpc of the center.
- Plot the CMD, then overplot in red things that are
more likely to be ellipticals and in blue things that are
more likely to be disks. Again, just for things
projected <1 Mpc from the center.
- Try other things, even if they might seem
non-sensical at first. If a pattern shows up, think about
it! And remember, there are other source properties
available in the SDSS database; you can browse the PhotoObj
tables to see what else is there and add them to your
download request if you want.
Advanced exercises to try:
- Identify the bluest spectrosocopically confirmed
galaxies, find their ra and dec, and then find them using
Skyserver's "Navigate" function. What do they look like
- Identify the most luminous spectrosocopically
confirmed galaxies and find them in Navigator.
- Identify the highest redshift objects in the field
(obviously they wont be in the cluster). Find them in
Navigator. What do they look like?
and Slicing tips
- When plotting mags and
colors, don't autoscale, or you'll get
unreadable plots. For CMDs, for example, reasonable
limits would be r = 13-24, and g-r = -1 to +2.
- When plotting lots of data
points, make the marker sizes small so that the
density of points doesn't make it so you can't see
all the data. try something like scatter(x,y,s=1)
- Also, to plot subsamples,
the easiest way to do this is to set a selection
flag like this:
# for selecting objects with a g mag brighter than 20
followed by, for example,
# for selecting objects in the redshift range 0.005 to
# for selecting objects with an r magnitude uncertainty
less than 0.2
SDSS['g']-SDSS['r'], s=1) # if you want to plot the
SDSS['g'][want]-SDSS['r'][want], s=20,color='red') # to then overplot the
- You can also "stack" selections like this:
want=SDSS['g']<18 # bright
SDSS['g']-SDSS['r']>0.7) # red
!= -999) # has
which would give you a "want" selection that is bright red
galaxies with measured redshifts
- Many of these tips are employed in the template
plotting code, so it's a good place to start