A good way to study the properties of many galaxies is by
looking at a galaxy cluster. There are many galaxy clusters in the
SDSS data, which may contain hundreds or even thousands of galaxies.
The picture at the right shows a famous cluster called Abell 2255.
The cluster is named after George Abell, an American astronomer who published
a catalog of galaxy clusters in 1958. In this project, you will study the
galaxies that make up Abell 2255.
Exercise 3: Use the Navigation tool to look up
a few galaxies in Abell 2255. Open the Navigation tool, then click on the
ra and dec in the left-hand frame. Type in the coordinates of Abell 2255:
ra = 258, dec = 64 and click OK. The field containing Abell 2255 will
appear in the Zoom window.
Think about how you know which galaxies are part of the cluster, and which
are just other galaxies at different distances in the same part of the sky.
Click on 10 or so galaxies that you think are part of the cluster, both
spirals and ellipticals, and save them in your notebook. How are these
galaxies similar? How are they different?
Launch the Navigation tool
If you have completed the
Color project, you are now ready to use Abell 2255 to learn about the
nature of galaxies.
Using Galaxy Clusters in Astronomy
Exercise 4: Make a color-color diagram for the
galaxies you saved in Exercise 3. Put u-g on the x-axis and g-r on the
y-axis. Do you notice any patterns?
In Exericse 4, you looked at only a few galaxies. To draw convincing
conclusions about galaxies, you need to examine hundreds or thousands
of galaxies - far more than you could look up individually. Therefore,
in the next exercise, you will use a search tool to automatically look
up information on all the galaxies in Abell 2255.
The search tool you will use is called MAST, which stands for Multi-mission
Archive at Space Telescope. You can use MAST to access data from
many different science missions, including the SDSS and the Hubble Space Telescope.
Launch the MAST search tool. When you click the link, the tool will open in a new window.
Type Abell 2255 into the Target Name field and
click Get Coordinates. You should see the RA and dec boxes fill in
for you automatically. Under "Object Type," highlight Galaxy to make sure you do not get
other objects in your data. Click Search. Download the
results as Comma-separated text; your results will be saved as
a .csv (comma separated value) file. Give the file a
filename such as Abell 2255.
Open the file using a graphing program such as Microsoft Excel (if you can't
see the file name, select "All" under "Files of Type"). Create two new
columns : u-g and g-r. Create a color color diagram of Abell 2255 with
u-g on the x-axis and g-r on the y-axis.
Where on the color-color diagram are the bluer galaxies?
Where on the diagram are the redder galaxies?
Question 5: Look at your graph along with the
graph you made in Exercise 4. Which part of
the graph corresponds to the early (elliptical) galaxies? Which
part corresponds to the late (spiral) galaxies?
galaxies are difficult to classify by colors and may be scattered on
your diagram. But only 3% of observed galaxies are irregular, so this
should not be a problem.
SDSS astronomers recently analyzed over 147,000 galaxies in the Early
Data Release and created a diagram similar to the one you made in Exercise 5.
If you are interested in some challenging reading, you can download the
paper they published
here (to see the paper as a .pdf file, click Other Formats, then Download PDF).