Does Circumgalactic O vi Trace Low-pressure Gas Beyond the Accretion Shock? Clues from H i and Low-ion Absorption, Line Kinematics, and Dust Extinction
Does Circumgalactic O vi Trace Low-pressure Gas Beyond the Accretion Shock? Clues from H i and Low-ion Absorption, Line Kinematics, and Dust Extinction
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DOI：
10.3847/1538-4357/aac884
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发表时间：
2018-03
期刊：
影响因子：
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通讯作者：
J. Stern;C. Faucher-Giguère;J. Hennawi;Zachary Hafen;S. Johnson;D. Fielding
中科院分区：
文献类型：
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作者：
J. Stern;C. Faucher-Giguère;J. Hennawi;Zachary Hafen;S. Johnson;D. Fielding
在形成恒星的低红移星系周围观察到大的ovi柱，依赖于撞击参数，表明大多数粒子位于光晕维里半径的一半之外（）。为了约束所追踪的气体的性质，我们分析了外晕的附加观测值，即与O vi柱的比值为1-10，缺乏低离子吸收，平均微分消光，以及O vi柱与O vi速度宽度之间的线性关系。我们将这些观测结果与两种物理情景进行了对比：(1)ovi追踪高压（）碰撞电离气体从虚激波阶段冷却，(2)ovi追踪吸积激波之外的低压（）气体，其中气体与紫外线背景处于电离和热平衡状态。我们证明了高压情景需要多个气相来解释观测结果，并且需要大量的能量沉积来抵消冷却气体辐射的能量。相反，如果重子过密度与暗物质过密度相当，并且气体富集到与ism类似的尘金属比，那么低压情景可以解释所有考虑到的热平衡中单个气相的观测结果。低压情景意味着O vi的质量流速率为，与中心星系的恒星形成速率相当。ovi线宽度与该流中预期的速度剪切一致。在低压情况下，由于吸积激波上的压力跳跃，吸收线比在。
Large O vi columns are observed around star-forming low-redshift galaxies, with a dependence on impact parameter indicating that most particles reside beyond half the halo virial radius ( ). In order to constrain the nature of the gas traced by , we analyze additional observables of the outer halo, namely to O vi column ratios of 1–10, an absence of low-ion absorption, a mean differential extinction of , and a linear relation between the O vi column and the O VI velocity width. We contrast these observations with two physical scenarios: (1) O vi traces high-pressure ( ) collisionally ionized gas cooling from a virially shocked phase, and (2) O vi traces low-pressure ( ) gas beyond the accretion shock, where the gas is in ionization and thermal equilibrium with the UV background. We demonstrate that the high-pressure scenario requires multiple gas phases to explain the observations and a large deposition of energy at to offset the energy radiated by the cooling gas. In contrast, the low-pressure scenario can explain all considered observations with a single gas phase in thermal equilibrium, provided that the baryon overdensity is comparable to the dark-matter overdensity and that the gas is enriched to with an ISM-like dust-to-metal ratio. The low-pressure scenario implies that O vi traces a cool flow with a mass flow rate of , comparable to the star formation rate of the central galaxies. The O vi line widths are consistent with the velocity shear expected within this flow. The low-pressure scenario predicts a bimodality in absorption line ratios at , due to the pressure jump across the accretion shock.