How identifying circumgalactic gas by line-of-sight velocity instead of the location in 3D space affects O VI measurements

Abstract

The high incidence rate of the O VI $\lambda \lambda$1032,1038 absorption around low-redshift, $\sim$$L^{\ast }$ star-forming galaxies has generated interest in studies of the circumgalactic medium. We use the high-resolution EAGLE cosmological simulation to analyze the circumgalactic O VI gas around $z\approx 0.3$ star-forming galaxies. Motivated by the limitation that observations do not reveal where the gas lies along the line-of-sight, we compare the O VI measurements produced by gas within fixed distances around galaxies and by gas selected using line-of-sight velocity cuts commonly adopted by observers. We show that gas selected by a velocity cut of $\pm 300$ km s${}^{-1}$ or $\pm 500$ km s${}^{-1}$ produces a higher O VI column density, a flatter column density profile, and a higher covering fraction compared to gas within one, two, or three times the virial radius ($r_{\mathrm{vir}}$) of galaxies. The discrepancy increases with impact parameter and worsens for lower mass galaxies. For example, compared to the gas within 2$r_{\mathrm{vir}}$, identifying the gas using velocity cuts of 200-500 km s${}^{-1}$ increases the O VI column density by 0.2 dex (0.1 dex) at 1$r_{\mathrm{vir}}$ to over 0.75 dex (0.7 dex) at $\approx 2$$r_{\mathrm{vir}}$ for galaxies with stellar masses of $10^9$-$10^{9.5}$ ${\mathrm{M}}_{\odot }$ ($10^{10}$-$10^{10.5}$ ${\mathrm{M}}_{\odot }$). We furthermore estimate that excluding O VI outside $r_{\mathrm{vir}}$ decreases the circumgalactic oxygen mass measured by Tumlinson et al. (2011) by over 50%. Our results demonstrate that gas at large line-of-sight separations but selected by conventional velocity windows has significant effects on the O VI measurements and may not be observationally distinguishable from gas near the galaxies.

arXiv comment13 pages with 8 figures; accepted for publication in ApJ

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