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Answers for Star Formation and U/HLXs in the Cartwheel Galaxy ds9 Activity

Determining the size of the ring:

12. The diameter of the ring is ~170 pixels.
13. 170 pixels (0.5 arcsec / 1 pixel)(1 rad / 206,625 arc sec) = 0.00041 rad
14. (0.00041 rad)(380 ly) = 160,000 ly

Conclusions and Analyses:
• 1. Most of the X-ray sources are along the lower part of the ring where Hubble observed bright blue knots that are gigantic clusters of newborn stars.

• 2. Using the expansion rate given in the background material and the distance the ring has moved from the center (half your answer to #14):
(200,000 mi/h) / (5.88 X 1012 mi/ly) X (24 h/day) X (365.25 days/y) = 0.0003 ly/y
v = d/t so t = d/v = (80,000 ly) / ( 0.0003 ly/y) = 300 million years

• 3. No, there is no X-ray source corresponding to the galactic nucleus as seen in the optical image.

• 4. Answers may vary, but X-ray sources along the ring could be supernova remnants, neutron stars or black holes because the lifetime of a massive star is less than 300 MY which is approximately when the galaxy collision occurred and new star formation was triggered.

• 5. From Nonnuclear Hyper/Ultraluminous X-Ray Sources:
• A. "It has been argued on observational and theoretical grounds (Appleton & Struck-Marcell 1996; Bransford et al. 1998) that the triggering of newly formed stars in ring galaxies occurs approximately simultaneously as the wave propagates out through the disk -the outer ring representing the most recently formed stars, with representative ages < 107 yrs. In this picture, the ring represents the outermost progress of a wave that began at the disk-center some 300 Myrs previously, created by the central perturbation of the intruder, either G3 or G1."

• B. "Almost all the X-ray emission in the Cartwheel originates from point-like sources within the southern quadrant of the outer ring. The sources are nearly coincident with the strong H, radio continuum emission and blue super-star clusters (SSCs)."

• C. "The companion galaxy G1 (spiral) contains 6 point-like X-ray sources, and the early-type spiral G2 is seen as a fainter diffuse source (Fig 1). The farthest companion galaxy G3 is also significantly detected, with one ULX in the eastern edge of its disk. In addition, a faint, diffuse X-ray envelope which includes the Cartwheel, G1 and G2 is marginally detected."

• D. "The absence of any point-like X-ray source in the nuclear region of the Cartwheel rules out the existence of AGN."

• E. "A point-like source 31, 10 kpc north of G2, is likely a background galaxy or AGN as it has a faint optical counterpart in the HST image."

• F. "The two most likely sources of X-ray emission associated with massive young star-forming regions are probably supernovae (SNe) or extremely young SN remnants (SNRs) and the high-mass X-ray binaries (HMXBs). We can almost rule-out low-mass X-ray binaries (LMXBs) to be the significant sources for H/ULXs along the Cartwheel narrow ring, although intermediate mass black holes (IMBHs, see review by Miller & Colbert 2003) are likely viable. It is conceivable that LMXBs and/or background sources could be responsible for the three ULXs interior to the ring. Three "ULXs" outside the Cartwheel with faint optical counterparts are likely background galaxies."