Using SDSS Navigator
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/dr16/
and click on Navigate (under Data Access).
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.
Note: Clicking "Invert Image" often helps you see things
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
- Roughly how big (in arcminutes) is the cluster?
- What is a rough guess for the redshift of the
- Find a background quasar and look at its spectrum
VERY IMPORTANT: When Navigate says an object has type = STAR,
that does not mean it is actually a star. It only means that it
is an unresolved point source. It might be a star, but it could
also be a small, unresolved galaxy.
Working with SDSS data
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:
(Note: 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.)
Feel free to 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.
- Plot r mag uncertainty versus r mag. If you
want your mags accurate to 10% or better, what is the rough
magnitude limit of your analysis?
- Plot dec vs ra (so a sky map) of resolved
sources (type=3) brighter than r=20. Make another for
unresolved sources (type=6) brighter than r=20. Think about
- Plot redshift versus r-band magnitude for
galaxies that have a measured redshift and are within 30
arcminutes 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?
Tip: Look at the redshift plot, decide what
range of redshifts define the cluster. Make a selection on
galaxies with redshifts in that range, and calculate the
average redshift of those objects.
- 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.
Tip: Given the redshift you calculate
above, use the astropy code in the sample workbook to work
out the angular radius (in arcseconds) that encompasses 1
Mpc in the cluster. Then make a selection on galaxies with
a radial distance less than that angular radius, and plot
the color-magnitude diagram for those clusters.
- Plot the CMD, then overplot in a different
color the CMD for spectroscopically confirmed cluster members.
Again, restrict it to resolved sources projected within 1 Mpc
of the center.
Tip: Do a dual selection: galaxies within
the 1 Mpc projected radius AND within the range of
redshifts that you defined for the cluster. Then plot the
CMD for those galaxies on top of the one you made in the
previous step for all objects projected within 1 Mpc.
- 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 morphologically?
Tip: Define a new column of data for the
SDSS data table to hold the g-r color: SDSS['g-r']=SDSS['g']-SDSS['r']. The do a show_in_browser call
on objects within the cluster redshift range. Sort on the
g-r column to find the bluest objects and look at their
coordinates. Then find them using Navigate.
- Identify the most luminous spectroscopically
confirmed galaxies and look at them in Navigator. What do they
look like morphologically?
Tip: Now sort your show_in_browser table on
the r-magnitude and look at the coordinates of the
brightest object. Then find it using Navigate.
- Identify the highest redshift
objects in the field (they won't be in the cluster,
obviously!) and find them in Navigator. What do they look
like, morphologically and spectroscopically?
Tip: This time do a show_in_browser call on all objects
with a redshift (whether or not they are in the cluster
redshift range), and sort on the redshift to find the
coordinates of the highest redshift object.
and Slicing tips
- When plotting mags and
colors, don't autoscale, or you'll get
unreadable plots. For CMDs, for example, reasonable
limits on the magnitude range would be r = 13-24,
and limits on the color range would be g-r = -1 to
- 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