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

Low-ionization-state Mg II gas has been extensively studied in quasar sightline observations to understand the cool, ∼10⁴ 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 ≈ 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_vir). For example, we show that at an impact parameter of 100 pkpc, a ±500 km s^-1 velocity cut around galaxies with stellar masses of 10⁹-10^9.5 M_⊙ (10¹⁰-10^10.5 M_⊙) 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_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., ≳0.25r_vir) is corotating.