Let's look at the properties of Abell 2065
(These are rough properties, you will need to refine them with the SDSS data!)
Go to SDSS Skyserver: http://skyserver.sdss.org/dr12/en/home.aspx
Go to Navigate, and put in the cluster
coordinates. Click "get image." Play around. Scroll, zoom, click
on galaxies and hit "Explore", etc. Look at a few spectra of
galaxies in the field. Work out the following:
Download and examine a fits image. Pick an object, hit
explore, then under PhotoObj, hit FITS. Grab one of the g-band
corrected frames, open it up in ds9, and see if you can match it
up to the navigate view.
- Find the rough center of the cluster.
- How big (in arcminutes) is the cluster?
- What is the rough redshift of the cluster?
OK, so now you have a feel for the cluster, lets get some data
and try and work out some quantitative analysis.
Go back to Skyserver main page.
Go to Search, Imaging Query, and do a search for galaxies within a
radius of 30 arcmin of the cluster center that you found. Don't select
stars (look over the search form to see how you can do this). Make sure
you set "Limit number of output rows" to zero so that you get
everything, and tell it to give the data to you in a FITS table.
Highlight the following information and execute the search:
(If you need a copy of the fits table on your laptop, you can
regenerate it using the procedure above, or just use the version I
Grab this template python code for reading in the SDSS fits table and plotting data. And try making the following plots:
when 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 x-axis.)
Other exercises to try:
- First plot redshift
(z1) versus r magnitude (r). Find the cluster on the plot and work out
(quantitatively) the cluster redshift. If you wanted to define a
spectroscopically confirmed cluster member, how would you do it?
- Next make an dec versus ra plot (in other words, the positions of the objects), and overplot in a different color
the galaxies that are spectroscopically confirmed cluster
members. Work out (quantitatively) the cluster center in RA and Dec.
- Plot r mag uncertainty (rerr) versus r mag (r). If you want your mags good to 10% or better, what is the
magnitude limit of your analysis?
- Plot g-r color (gmr) versus r magnitude (r) for all galaxies 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.
- Plot the CMD, then overplot in red the ellipticals
and in blue the disky things (only for objects within 1 Mpc).
- Identify the bluest spectrosocopically confirmed
galaxies, print their ra and dec, and then find them using Skyserver's
"navigate" function. What do they look like morphologically?
- 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?
- 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:
want=(g<20) # 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
scatter(r,gmr,s=1) # if you want to plot the whole sample
scatter(r[want],gmr[want],s=20,color='red') # to then overplot the subsample
- Many of these tips are employed in the template plotting code, so it's a good place to start