Morphological and Rotation Structures of Circumgalactic Mg II Gas in the EAGLE Simulation and the Dependence on Galaxy Properties

Abstract

Low-ionization-state Mg II gas has been extensively studied in quasar sightline observations to understand the cool, $\sim$$10^4$ K gas in the circumgalactic medium. Motivated by recent observations showing that the Mg II gas around low-redshift galaxies has significant angular momentum, we use the high-resolution EAGLE cosmological simulation to analyze the morphological and rotation structures of the $z\approx 0.3$ circumgalactic Mg II gas and examine how they change with the host galaxy properties. Around star-forming galaxies, we find that the Mg II gas has an axisymmetric instead of a spherical distribution, and the axis of symmetry aligns with that of the Mg II gas rotation. A similar rotating structure is less commonly found in the small sample of simulated quiescent galaxies. We also examine how often Mg II gas around galaxies selected using a line-of-sight velocity cut includes gas physically outside of the virial radius ($r_{\mathrm{vir}}$). For example, we show that at an impact parameter of 100 pkpc, a $\pm 500$ km s${}^{-1}$ velocity cut around galaxies with stellar masses of $10^9$-$10^{9.5}{\mathrm{M}}_{\odot }$ ($10^{10}$-$10^{10.5}{\mathrm{M}}_{\odot }$) selects Mg II gas beyond the virial radius 80% (6%) of the time. Because observers typically select Mg II gas around target galaxies using such a velocity cut, we discuss how this issue affects the study of circumgalactic Mg II gas properties, including the detection of corotation. While the corotating Mg II gas generally extends beyond $0.5r_{\mathrm{vir}}$, the Mg II gas outside of the virial radius contaminates the corotation signal and makes observers less likely to conclude that gas at large impact parameters (e.g., $\gtrsim 0.25r_{\mathrm{vir}}$) is corotating.

26 pages with 18 figures and 3 tables; accepted for publication in ApJ