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 λλ1032, 1038 absorption around low-redshift, ~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 ≈ 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 ±300 km s^-1 or ±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 1, 2, or 3 times the virial radius (r _vir) of galaxies. The discrepancy increases with impact parameter and worsens for lower-mass galaxies. For example, compared to the gas within 2 r _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 _vir to over 0.75 dex (0.7 dex) at ≈ 2 r _vir for galaxies with stellar masses of 10⁹-10^9.5 M _⊙ (10¹⁰-10^10.5 M _⊙). We furthermore estimate that excluding O VI outside r _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.