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

The high incidence rate of the O VI $\lambda\lambda$1032,1038 absorption around low-redshift, $\sim$$L^*$ star-forming galaxies has generated interest in studies of the circumgalactic medium. We use the high-resolution EAGLE cosmological simulation to analyze the circumgalactic O VI gas around $z\approx0.3$ star-forming galaxies. Motivated by the limitation that observations do not reveal where the gas lies along the line-of-sight, we compare the O VI measurements produced by gas within fixed distances around galaxies and by gas selected using line-of-sight velocity cuts commonly adopted by observers. We show that gas selected by a velocity cut of $\pm300$ km s$^{-1}$ or $\pm500$ km s$^{-1}$ produces a higher O VI column density, a flatter column density profile, and a higher covering fraction compared to gas within one, two, or three times the virial radius ($r_\mathrm{vir}$) of galaxies. The discrepancy increases with impact parameter and worsens for lower mass galaxies. For example, compared to the gas within 2$r_\mathrm{vir}$, identifying the gas using velocity cuts of 200-500 km s$^{-1}$ increases the O VI column density by 0.2 dex (0.1 dex) at 1$r_\mathrm{vir}$ to over 0.75 dex (0.7 dex) at $\approx2$$r_\mathrm{vir}$ for galaxies with stellar masses of $10^{9}$-$10^{9.5}$ $\rm M_\odot$ ($10^{10}$-$10^{10.5}$ $\rm M_\odot$). 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.