史密斯云：高速吸积和暗物质禁闭
The Smith Cloud: high-velocity accretion and dark-matter confinement
摘要：
史密斯云是一个由贫金属中性电离气体（M_gas >= 2 × 10^6 M_sun）组成的大质量系统，目前相对于银河系以高速（V_GSR ~300 km s^-1）运动，距离太阳12 kpc。星云的彗星尾巴的运动学表明，气体是在吸积到银河系的过程中，首先由洛克曼等人（2008）讨论。在这里，我们重新调查云的轨道，考虑云是由暗物质晕限制的可能性。这是必要的云生存通过银河日冕。我们考虑三种可能的模型的暗物质晕（NFW，Einasto，Burkert），包括潮汐破坏和冲压压力剥离的影响，在云的下落到通过银河系盘。对于NFW和Einasto的暗物质模型，我们能够确定合理的初始条件的史密斯云，虽然这是只有轻微的可能与Burkert模型。对于这三种模型，其前身的初始质量（气体+暗物质）比今天推断的要高出一个数量级。与Lockman等人（2008）的观点一致，云似乎在大约7000万年前穿过了磁盘。对于我们最成功的模型，重子与暗物质的比例在轨道周期内相当恒定，但在穿过圆盘后下降了2-5倍。这片云似乎只在凌日过程中勉强幸存下来，在下一次凌日过程中不太可能保持完整性，大约从现在开始的3000万年。
英文摘要：
The Smith Cloud is a massive system of metal-poor neutral and ionized gas M_gas >= 2x10^6 M_sun) that is presently moving at high velocity (V_GSR ~300 km s^-1) with respect to the Galaxy at a distance of 12 kpc from the Sun. The kinematics of the cloud's cometary tail indicates that the gas is in the process of accretion onto the Galaxy, as first discussed by Lockman et al. (2008). Here, we re-investigate the cloud's orbit by considering the possibility that the cloud is confined by a dark matter halo. This is required for the cloud to survive its passage through the Galactic corona. We consider three possible models for the dark matter halo (NFW, Einasto, Burkert) including the effects of tidal disruption and ram-pressure stripping during the cloud's infall onto and passage through the Galactic disk. For the NFW and Einasto dark-matter models, we are able to determine reasonable initial conditions for the Smith Cloud, although this is only marginally possible with the Burkert model. For all three models, the progenitor had an initial (gas+dark matter) mass that was an order of magnitude higher than inferred today. In agreement with Lockman et al. (2008), the cloud appears to have punched through the disk ~70 Myr ago. For our most successful models, the baryon to dark matter ratio is fairly constant during an orbital period but drops by a factor of 2-5 after transiting the disk. The cloud appears to have only marginally survived its transit, and is unlikely to retain its integrity during the next transit ~30 Myr from now.
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