Galaxy Clusters
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# Galaxy Clusters

 Abell 2255

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.

 Exercise 5: 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.

 Question 4: 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? Note: Irregular 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).