HST-COS Spectroscopy of the Cooling Flow in Abell 1795 - Evidence for Inefficient Star Formation in Condensing Intracluster Gas
We present far-UV spectroscopy from the Cosmic Origins Spectrograph on the Hubble Space Telescope of a cool, star-forming filament in the core of Abell 1795. These data, which span 1025A - 1700A, allow for the simultaneous modeling of the young stellar populations and the intermediate-temperature (10^5.5 K) gas in this filament, which is far removed (~30 kpc) from the direct influence of the central AGN. Using a combination of UV absorption line indices and stellar population synthesis modeling, we find evidence for ongoing star formation, with the youngest stars having ages of 7.5 +/- 2.0 Myr and metallicities of 0.4 +/- 0.2 Zsun. The latter is consistent with the local metallicity of the intracluster medium. We detect the O VI (1038) line, measuring a flux of 4.0 +/- 0.9 x 10^-17 erg s^-1 cm^-2. The O VI (1032) line is redshifted such that it is coincident with a strong Galactic H2 absorption feature, and is not detected. The measured O VI (1038) flux corresponds to a cooling rate of 0.85 +/- 0.2 (stat) +/- 0.15 (sys) Msun/yr at ~10^5.5 K, assuming that the cooling proceeds isochorically, which is consistent with the classical X-ray luminosity-derived cooling rate in the same region. We measure a star formation rate of 0.11 +/- 0.02 Msun/yr from the UV continuum, suggesting that star formation is proceeding at 13 +/- 3% efficiency in this filament. We propose that this inefficient star formation represents a significant contribution to the larger-scale cooling flow problem.