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 < λ_rest < 1700 Å, 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 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_-2.0 Myr and metallicities of 0.4^+0.2_-0.1 Z _⊙. The latter is consistent with the local metallicity of the intracluster medium. We detect the O VI λ1038 line, measuring a flux of f _{O 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 ~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 M _⊙ yr^-1 from the UV continuum, suggesting that star formation is proceeding at 13⁺³_-2% efficiency in this filament. We propose that this inefficient star formation represents a significant contribution to the larger-scale cooling flow problem.