# Investigation 6: Galaxy Clusters - Activity 4

## Observations of Galaxy Clusters

Overview: Students undertake a simple guided analysis of galaxy cluster Abell 2029, and learn to use the NASA Extragalactic Database to determine redshifts (and therefore distances) to galaxy clusters.

Physical resources: None

Electronic resources: X-ray image of Abell 2029, NED

Opening observations:

• Observe Chandra images of Hydra A cluster in visible light and X-ray light, and describe differences (Hydra A)
• Read student-generated expert project wiki for galaxy clusters
• Generate questions about galaxy clusters

Student challenge: Basic analysis of Abell 2029 (obsid 891)

• Visible light false color image
• X-ray light false color image
• X-ray light true color image (0.3 - 1.5 keV, 1.5 - 4 keV, 4-7 keV)
• Choose a region → measure angular diameter of cluster
• (Instructors show how to use contour tool to observe general shape of cluster, and set lowest contour to be just above the typical background counts value far from the cluster, in order to get an estimate of the "edge" of the cluster. Process: how to use contours: Learning to Use the ds9 Imaging System.
• Extract and fit a spectrum, using the Thermal Bremsstrahlung model, to estimate the flux
• Instructor review why thermal bremmstrahlung makes sense: gas in cluster is so hot that all atoms are completely ionized so all electrons are free, and can't jump up and down between energy levels. Thermal bremmstrahlung model is for a plasma with no emission lines, so this makes sense. Thermal bremmstrahlung simulation: (Animations and Video: X-ray Astronomy).

Instructors demonstrate how to use NASA Extragalactic Database (NED) to look up redshift of cluster. Process: how to use NED

• Students predict the distance to the cluster (~330 Mpc), using the NED value of recessional velocity (~23,000 km/s) and H0 = 70 km/s/Mpc
• Students use that distance to predict linear diameter of cluster and compare to linear diameter of Milky Way galaxy (= 9 x 1020 meters)
• Students use that distance to predict the X-ray luminosity of cluster, and compare to visible light luminosity of Milky Way galaxy (= 2 x 1011 stars in galaxy X 1 solar luminosity for each star X 4 x 1033 ergs/sec for one solar luminosity = 6 x 1044 ergs/sec)
• Results of student comparisons: (cluster size) Range results from different size regions chosen

Teacher tips/tricks:

• Students should feel comfortable with all these calculations. If not, assign them additional practice before beginning projects!

Assessment ideas:

• How would estimates for linear size and luminosity change if the recessional velocity had been mismeasured, and was actually two times larger? (Distance would be twice as far, so linear size would be twice as large, and luminosity would be four times as large!)