AMIGA计划：Andromeda Galaxy周围光学厚环星系气体的最小覆盖因子
Project AMIGA: A Minimal Covering Factor for Optically Thick Circumgalactic Gas Around the Andromeda Galaxy
摘要：
作为AMIGA项目的一部分，我们提出了一个深入的搜索HI 21厘米发射的气体晕的梅西耶31，一个大型计划哈勃太空望远镜计划，研究环星系介质的仙女座星系。我们的观察与罗伯特C。伯德绿岸望远镜瞄准48个背景活动星系核，其中一半以上是用宇宙起源摄谱仪在紫外线下观测到的，其撞击参数为$25 \lesssim ρ\lesssim 330$ kpc（$0.1 \lesssim ρ/ R_{\rm vir} \lesssim 1.1$）。我们没有检测到任何21厘米的发射对这些活动星系核的限制$N({\rm HI}) \approx 4 \times10^{17}$厘米$^{-2}$（$5σ$，每2千秒差距直径束）。这个列密度对应于$\sim2.5$在莱曼极限的光学深度，因此我们的观测结果与莱曼极限系统在更高红移的吸收线研究重叠。对于$ρ\leq R_{\rm vir}$，我们的未检测限制了M31周围这种光学厚气体对$f_c < 0.051$的覆盖因子（90\%置信度）。虽然之前在M31和M33之间的区域发现过个别云，但强烈光学厚度气体的覆盖系数相当小。我们的覆盖因子的上限与最近的宇宙学“变焦”模拟的预期一致。最近COS-Halos对$z \approx 0.2$星系系综的\HI \吸收的紫外测量表明，在相同的$N({\rm H I})$下，$ρ\lesssim 0.5 R_{\rm vir}$内的覆盖因子明显更高，尽管金属离子与H I的比率似乎与M31中看到的一致。
英文摘要：
We present a deep search for HI 21-cm emission from the gaseous halo of Messier 31 as part of Project AMIGA, a large program Hubble Space Telescope program to study the circumgalactic medium of the Andromeda galaxy. Our observations with the Robert C. Byrd Green Bank Telesope target sight lines to 48 background AGNs, more than half of which have been observed in the ultraviolet with the Cosmic Origins Spectrograph, with impact parameters $25 \lesssim ρ\lesssim 330$ kpc ($0.1 \lesssim ρ/ R_{\rm vir} \lesssim 1.1$). We do not detect any 21-cm emission toward these AGNs to limits of $N({\rm HI}) \approx 4 \times10^{17}$ cm$^{-2}$ ($5σ$, per 2 kpc diameter beam). This column density corresponds to an optical depth of $\sim2.5$ at the Lyman limit, thus our observations overlap with absorption line studies of Lyman limit systems at higher redshift. Our non-detections place a limit on the covering factor of such optically-thick gas around M31 to $f_c < 0.051$ (at 90\% confidence) for $ρ\leq R_{\rm vir}$. While individual clouds have previously been found in the region between M31 and M33, the covering factor of strongly optically-thick gas is quite small. Our upper limits on the covering factor are consistent with expectations from recent cosmological "zoom" simulations. Recent COS-Halos ultraviolet measurements of \HI\ absorption about an ensemble of galaxies at $z \approx 0.2$ show significantly higher covering factors within $ρ\lesssim 0.5 R_{\rm vir}$ at the same $N({\rm H I})$, although the metal ion-to-H I ratios appear to be consistent with those seen in M31.
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