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 (105.5 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 Myr and metallicities of 0.4 Z⊙. The latter is consistent with the local metallicity of the intracluster medium. We detect the O VI λ1038 line, measuring a flux of fO VI, 1038 = 4.0 ± 0.9 × 10-17 erg s -1 cm-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⊙ yr-1 at ∼105.5 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⊙ yr-1 from the UV continuum, suggesting that star formation is proceeding at % efficiency in this filament. We propose that this inefficient star formation represents a significant contribution to the larger-scale cooling flow problem. © 2014. The American Astronomical Society. All rights reserved.