HST-COS SPECTROSCOPY OF THE COOLING FLOW IN A1795—EVIDENCE FOR INEFFICIENT STAR FORMATION IN CONDENSING INTRACLUSTER GAS

Abstract

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 A1795. These data, which span 1025 Å < λrest < 1700 Å, allow for the simultaneous modeling of the young stellar populations and the intermediate-temperature (10[superscript 5.5] K) gas in this filament, which is far removed (~30 kpc) from the direct influence of the central active galactic nucleus. 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 [+2.5 over -2.0] Myr and metallicities of 0.4 [+0.2 over -0.1] Z[subscript ☉]. The latter is consistent with the local metallicity of the intracluster medium. We detect the O vi λ1038 line, measuring a flux of f[subscript O VI, 1038] = 4.0 ± 0.9 × 10[superscript −17] erg s[superscript −1] cm[superscript −2]. The O vi λ1032 line is redshifted such that it is coincident with a strong Galactic H2 absorption feature, and is not detected. The measured O vi λ1038 flux corresponds to a cooling rate of 0.85 ± 0.2 (stat) ± 0.15 (sys) M[subscript ☉] yr[superscript −1] at ~10[superscript 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 M[subscript ☉] yr[superscript −1] from the UV continuum, suggesting that star formation is proceeding at13 [+3 over -2]% efficiency in this filament. We propose that this inefficient star formation represents a significant contribution to the larger-scale cooling flow problem.