2. The Cartwheel galaxy's unusual wagon-wheel shape was created by a nearly head- on collision with a smaller galaxy. The image below right is a composite of an optical image and high resolution radio observations of neutral hydrogen (traced by the green contours). The neutral hydrogen trail suggests that the intruder galaxy could be the galaxy located at the lower left of the image.The collision sent a ripple of energy into space, plowing gas and dust in front of it. Expanding at 200,000 mi/h Credit: (NRAO, ISU, Hughes STX, STScI, NASA) this cosmic tsunami leaves in its wake a firestorm of new star creation. Hubble resolves bright blue knots that are gigantic clusters of newborn stars.

3. STELLAR VOLUTIONVOLUTION INCREASINGLIFETIMEINCREASINGLIFETIME INCREASINGMASSINCREASINGMASS

4.  Open ds9. From the menus, choose Analysis>Virtual Observatory>Chandra-Ed Archive Server.  In the new window that comes up, scroll down to and click ObsID 2019 – THE CARTWHEEL ’ S RING.  When the image is loaded, go back to the SAOImage ds9 window. Maximize your screen. Choose Scale>Square Root and Color>b (Credit: NASA/CXC/ A.Wolter & G.Trinchieri et al.)

5. 1.Analysis>Image Servers>SAO-DSS and then click Retrieve 2.Zoom>Zoom 4 3.Frame>Match Frames>WCS 4.Colors>Colormap Parameters (Try Contrast – 4.4, Bias – 0.25) 5.To help define the x-ray sources, go to Analysis>Contours and then Analysis>Contour Parameters 6.Choose Contour levels – 10, Contour Smoothness – 1, Low – 0, High – 550 and then click Generate, Apply and Close. 7.Edit>Crosshair and Frame>Lock Crosshairs>WCS 8.Move the crosshairs around. Notice the location of the x-ray sources in comparison to the optical image. On the right, optical image: On the left, x-ray image:

6. Distance between 2 points: With the right frame chosen (and Edit>Pointer), draw a circular region around the Cartwheel galaxy. With the left frame chosen (and Edit>Crosshairs), find the diameter of this region by finding the x- and y- coordinates of two points on either side and using the distance formula. Make sure you move crosshairs over the left image.

7. Convert the size of the Cartwheel from pixels to radians: 1 pixel = 0.5 arc sec 1 radian = 206,265 arc sec Use the small angle formulua to determine the size of the ring in light years: Distance to Cartwheel = ~380 million light years Angular size (radians) = (actual size of object)/(distance to object) How does the size of the ring compare to that of the Milky Way? (~100,000 ly across)

8. How long ago might the galaxy collision have occurred? Hubble scientists give a rate of expansion at 200,000 miles per hour. 1 light year = 5.89 X 10 12 miles How do the locations of the majority of x-ray sources compare to the areas of new star formation in the optical image? What might these ultra- and hyperluminous x-ray sources be? Note: the lifetime of a blue, 10 solar mass star is ~32 million years.

9. In some galaxies, known as "active galactic nuclei" (AGN), the nucleus (or central core) produces more radiation than the entire rest of the galaxy! X-ray studies are particularly useful in helping us figure out what is going on in an AGN, since they can penetrate from far within the center of a galaxy. Based on X-ray (and other) observations, a good guess is that the power source in AGN is a supermassive black hole. Look back at the x-ray and optical images of the Cartwheel to determine if it has an AGN. Is there an AGN candidate elsewhere in the field? Artist’s conception of an AGN

10. Nonnuclear Hyper/Ultraluminous X-Ray Sources in the Starbursting Cartwheel Ring Galaxy, Yu Gao, 1 Q. Daniel Wang, 1 P. N. Appleton, 2 and Ray A. Lucas 3, The Astrophysical Journal Letters, 596:L171–L174, 2003 October 20.