# Galaxy Clusters

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 tool, then enter the RA and Dec coordinates of Abell 2255: RA = 258.1292, Dec = 64.0925. Click “Get Image.” Click Zoom Out (the minus sign) once or twice, and you should be able to see the whole cluster. Click on any galaxy, and its basic data should appear in the right-hand frame. Click “Add to Notes” to save the galaxy’s data in your online notebook.

Launch Navigate

Think about how you know which galaxies are part of Abell 2255, and which are just other galaxies at different distances in the same part of the sky. Click on 15-20 galaxies that you think are part of the cluster, both spirals and ellipticals, and save them in your notebook. You can save the entire notebook to your computer by choosing CSV as the output type and clicking “Export.” You can then open the resulting CSV file in Excel.

How are these galaxies similar? How are they different?

## Using Galaxy Clusters in Astronomy

Exercise 4: Make a color-color diagram for the galaxies you saved in Exercise 3. You can make the diagram in Excel. Put u-g on the x-axis and g-r on the y-axis. Do you notice any patterns?

In Exercise 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.

You may use one of two tools: SkyServer’s Radial Search or SQL Search. The Radial Search tool will return all objects near Abell 2255; you can then use Excel to select only galaxies.

Your teacher may tell you which one to use, or you could pick one yourself. Click one of the links below to learn how to use the tool of your choice. A new page will open in this window. When you are done, you will return to this page to make and analyze your color-color diagram.

Use the Radial Search tool and Excel

Exercise 5: Use the data you collected to make a color-color diagram of Abell 2255 with u-g on the x-axis and g-r on the y-axis. To learn more about how to make the graph using Microsoft Excel, see SkyServer’s Graphing and Analyzing Data how-to tutorial.

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 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 the PDF link under Download).