Galaxy Clusters
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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 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.
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?
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. 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?
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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
Use the SQL Search tool
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.
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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 Other Formats, then Download PDF).
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