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 $z\approx0.3$ 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 $\pm500$ 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., $\gtrsim0.25 r_\mathrm{vir}$) is corotating.

中文翻译：

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EAGLE 模拟中环星系 Mg ii 气体的形态和旋转结构及其对星系特性的依赖性

低电离态的 Mg II 气体已在类星体视线观测中得到广泛研究，以了解银河系周围介质中的冷 $\sim$$10^4$K 气体。受近期观测显示低红移星系周围的 Mg II 气体具有显着角动量的启发，我们使用高分辨率 EAGLE 宇宙学模拟来分析 $z\approx0.3$ 环绕银河系的 Mg II 气体的形态和旋转结构和检查它们如何随着宿主星系的特性而变化。在恒星形成星系周围，我们发现 Mg II 气体具有轴对称而不是球形分布，并且对称轴与 Mg II 气体旋转的对称轴对齐。类似的旋转结构在模拟静止星系的小样本中不太常见。我们还检查了使用视线速度切割选择的星系周围的 Mg II 气体有多频繁地包含物理上位于维里半径 ($r_\mathrm{vir}$) 之外的气体。例如，我们表明，在 100 pkpc 的撞击参数下，$\pm500$ km s$^{-1}$ 速度在星系周围的恒星质量为 $10^9$-$10^{9.5}\mathrm{M_ \odot}$ ($10^{10}$-$10^{10.5}\mathrm{M_\odot}$) 80% (6%) 的时间选择 Mg II 气体超出维里半径。因为观察者通常使用这样的速度切割来选择目标星系周围的 Mg II 气体，我们讨论了这个问题如何影响环星系 Mg II 气体特性的研究，包括检测共转。虽然同向旋转的 Mg II 气体通常超出 $0.5r_\mathrm{vir}$，