The properties of the gas around galaxies from metal-line absorption: insights into galaxy formation and the role of energetic feedback

We use quasar absorption lines to study the physical conditions in the circumgalactic medium of z~2.3 star-forming galaxies taken from the Keck Baryonic Structure Survey (KBSS). The pixel optical depth technique is applied to 15 background QSOs and used to construct the first galaxy-centered 2-D maps of the median absorption by OVI, NV, CIV, CIII, and SiIV. We find that metal-line absorption is strongly enhanced within 240 km/s from 21 galaxies with impact parameters <180 proper kpc from the sight lines to the background quasars. Furthermore, the optical depth of OVI at fixed HI, CIV and SiIV is observed to be enhanced with respect to the full pixel sample. We use ionization models to show that the observed strength of OVI as a function of HI would lead to implausibly high metallicities at low densities if the gas were photoionized by the background radiation. We conclude that the galaxies are surrounded by gas that is sufficiently hot to be collisionally ionized (T>10^5 K) and that a substantial fraction of the hot gas has a metallicity >10^(-1) of solar. Finally, we turn to the EAGLE cosmological hydrodynamical simulations to study the evolution of galaxy column density profiles as a function of impact parameter for different metal ions and halo masses.

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