Detection of hot, metal-enriched outflowing gas around z ≈ 2.3 star-forming galaxies in the Keck Baryonic Structure Survey

Abstract

We use quasar absorption lines to study the physical conditions in the circumgalactic medium of redshift z ≈ 2.3 star-forming galaxies taken from the Keck Baryonic Structure Survey (KBSS). In Turner et al. (2014) we used the pixel optical depth technique to show that absorption by Hi and the metal ions OvI, Nv, CIv, CIII and SiIv is strongly enhanced within │Δv│≾. 170 km s^(-1) and projected distances │d│≾. 180 proper kpc from sightlines to the background quasars. Here we demonstrate that the OvI absorption is also strongly enhanced at fixed HI, CIv, and SiIv optical depths, and that this enhancement extends out to ~ 350 km s^(-1). At fixed HI the increase in the median OvI optical depth near galaxies is 0.3-0.7 dex and is detected at 2-3-σ confidence for all seven HI bins that have log_(10) T_(HI) ≥ -1.5. We use ionisation models to show that the observed strength of OvI as a function of HI is consistent with enriched, photoionised gas for pixels with T_(HI) ≳ 10. However, for pixels with T_(HI) ≾ 1 this would lead to implausibly high metallicities at low densities if the gas were photoionised by the background radiation. This indicates that the galaxies are surrounded by gas that is sufficiently hot to be collisionally ionised (T > 10^5 K) and that a substantial fraction of the hot gas has a metallicity ≳ 10^(-1) of solar. Given the high metallicity and large velocity extent (out to ~ 1.5 x v_(circ)) of this gas, we conclude that we have detected hot, metal enriched outflows arising from star-forming galaxies.

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