Richard Mushotzky (NASA/GSFC) and Amalia K. Hicks (University of Colorado)

1. Star Formation Rates in Cooling Flow Clusters Richard Mushotzky (NASA/GSFC) and Amalia K. Hicks (University of Colorado) Abstract Method An enduring enigma in X-ray astronomy is the "missing mass" in cooling flow clusters of galaxies. Though the X-ray temperature often drops by a factor of three within the central 100 kpc, there is seldom an indication of enough mass at temperatures cooler than ~2 keV to uphold mass deposition rates predicted by the cooling flow model. One possible depository for recently cooled gas is in recent star formation. Here we present recent results from an investigation of star formation rates in cooling flow clusters of galaxies utilizing archival XMM-Newton Optical Monitor UV data and J band IR flux information from the 2MASS survey. To establish what constitutes excess star formation, we first calibrate the relationship between these two wavebands in passively evolving cluster galaxies. We then apply the technique to cD galaxies in the cores of clusters, focusing primarily on cooling flow clusters. A high UV/IR ratio is a strong indication of the existence of hot young stars and thus a direct indication of copious recent star formation. Our initial results demonstrate a clear UV excess in many, but not all, cooling flow cDs. This finding is largely consistent with the outcome of earlier studies. An expansion of the study toward higher redshift is also discussed, including preliminary results at moderate redshifts (z~0.25). UVW1 background corrected fluxes were calculated via application of an OM count conversion factor. Our initial flux values were then corrected for galactic extinction using a filter weighted average. To confirm that the OM did not have a red leak we verified our fluxes by comparison to UV fluxes from other instruments (IUE, FOCA). We then examined the UV/IR luminosity ratio of low redshift elliptical galaxies in the IC1860 and Coma clusters. The relationship between these two wavebands in passively evolving cluster galaxies was calibrated by obtaining a least squares fit to that initial data. This parameterization was then used to determine the amount of excess UV luminosity in a selection of cD galaxies in the cores of cooling flow clusters. Starburst99 models were redshifted to coincide with the redshift of the individual galaxies, and folded through the UVW1 filter, providing an estimate of the continuous star formation rate of each galaxy over various time periods. Figure 1: HST WFPC2 1780 second exposure image of Abell 1795. This image has been overlaid with UV contours from an XMM-Newton Optical Monitor UVW1 filter (220-400 nm) observation. Logarithmic contours in blue were produced with a 2499 second exposure image, and have count values of 14, 25.03, 44.75, and 80. The large splotch of UV contours to the lower right of the cD galaxy is due to scattered light. Table 1: Excess UV luminosity is given along with estimates of the mass deposition rate for each cluster. These rates were calculated using a Starburst99 model of continuous star formation over 900 Myr with a powerlaw index of 3.3. X-ray mass deposition rates are also given. The duration of continuous star formation and IMF powerlaw index are constrained by setting the UV mass deposition rate equal to the X-ray value and determining which model best describes that scenario. Figure 3: UV/IR luminosity ratio of selected galaxies vs. their 2MASS J band luminosities. All luminosities were calculated in 7 arcsecond radius regions, and corrected for galactic extinction. The squares indicate passively evolving early-type galaxies at low redshift. The triangles represent low redshift non-cooling flow cD galaxies, and the diamonds indicate cooling flow cluster cDs, all of which are low redshift (z < 0.1) except for the three points farthest to the right which have redshifts of ~0.25. The high UV/IR ratio of the cooling flow clusters is a strong indication of the existence of hot young stars and thus a direct indication of copious recent star formation. Figure 2: UV vs. IR luminosity for galaxies in our sample. The line indicates the best fitting relationship between UV and IR luminosity for passively evolving and non-cooling flow cD galaxies, represented by squares and triangles, respectively. Diamonds indicate cooling flow cD galaxies and in general show an obvious UV luminosity excess, clearly indicating recent star formation. Results • A UV luminosity excess exists in many, but not all, cooling flow cDs (Figure 2)  significant star formation is occurring in many of the galaxies at the centers of cooling flows. • Our findings are qualitatively consistent with the results of earlier studies (McNamara & O’Connell 1989, Cardiel, Gorgas & Aragon-Salamanca 1998). • For ~2/3 of our clusters it is possible to match the new Chandra/XMM cooling flow rates with UV inferred star formation rates for some combination of lifetime of the star burst and the IMF slope. • At present we do not understand the objects without excess UV light, however this has been a pilot study, and we hope to investigate the subject more thoroughly by studying the many clusters that remain in the archive. References Burstein, D. et al., 1988, ApJ, 328, 440. Cardelli, J. A., Clayton,G. C., Mathis, J. S., 1989, ApJ, 345, 245. Cardiel, N., Gorgas, J. & Aragon-Salamanca, 1998, MNRAS, 298, 977. Deharveng, J. 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