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arXiv 1507.05237astro-ph.GA
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M31附近Local Group中性氢的21厘米灵敏观测

Sensitive 21cm Observations of Neutral Hydrogen in the Local Group near M31

Spencer A. Wolfe, Felix J. Lockman, D. J. Pisano

备注：
Revised text and added figure. In total: 13 pages, 11 figures, 5 tables. Accepted for publication in ApJ
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摘要：

用绿岸望远镜（GBT）在本星系群M31周围的几个位置进行了非常灵敏的21厘米HI测量，角分辨率为9.1 '，探测能级为$\rm{N_{HI} = 3.9 \times 10^{17}~cm^{-2}}$的30 $\rm{km~s^{-1}}$线的5 $σ$。在M31和M33之间几乎等距的12平方度区域内，大部分HI包含在9个离散的云中，这些云的典型大小为几千秒差距，HI质量为$10^5$ M $_{\odot}$。它们的静止速度在本地星系群标准中介于-100和+40 $\rm{km~s^{-1}}$之间，与M31和M33的系统速度相当。这些云在运动学上和空间上似乎彼此孤立。对于800千秒差距的采用距离，所有九个云的HI总质量是$1.4 \times 10^6$ M $_{\odot}$，在较小的云或更弥散的发射中可能还有另一个$0.2 \times 10^6$ M $_{\odot}$。每个云的HI质量通常比通过引力束缚云所需的动力学（维里）质量小三个数量级。虽然它们具有矮星系的大小和HI质量，但这些云不太可能是本星系群卫星系统的一部分，因为它们缺乏恒星。在M31以北，在粗网格上进行的灵敏HI测量发现了可能与M31高速云系扩展到预计距离$\sim 100$千秒差距有关的辐射。没有观察到M31高速云群在向南类似距离处向M33延伸。

英文摘要：

Very sensitive 21cm HI measurements have been made at several locations around the Local Group galaxy M31 using the Green Bank Telescope (GBT) at an angular resolution of 9.1', with a 5$σ$ detection level of $\rm{N_{HI} = 3.9 \times 10^{17}~cm^{-2}}$ for a 30 $\rm{km~s^{-1}}$ line. Most of the HI in a 12 square degree area almost equidistant between M31 and M33 is contained in nine discrete clouds that have a typical size of a few kpc and HI mass of $10^5$ M$_{\odot}$. Their velocities in the Local Group Standard of Rest lie between -100 and +40 $\rm{km~s^{-1}}$, comparable to the systemic velocities of M31 and M33. The clouds appear to be isolated kinematically and spatially from each other. The total HI mass of all nine clouds is $1.4 \times 10^6$ M$_{\odot}$ for an adopted distance of 800 kpc with perhaps another $0.2 \times 10^6$ M$_{\odot}$ in smaller clouds or more diffuse emission. The HI mass of each cloud is typically three orders of magnitude less than the dynamical (virial) mass needed to bind the cloud gravitationally. Although they have the size and HI mass of dwarf galaxies, the clouds are unlikely to be part of the satellite system of the Local Group as they lack stars. To the north of M31, sensitive HI measurements on a coarse grid find emission that may be associated with an extension of the M31 high-velocity cloud population to projected distances of $\sim 100$ kpc. An extension of the M31 high-velocity cloud population at a similar distance to the south, toward M33, is not observed.

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