史密斯云：高速云与Milky Way碰撞
Smith's Cloud: A High-velocity Cloud Colliding with the Milky Way
摘要：
用绿岸望远镜进行的新的21厘米HI观测表明，被称为史密斯云的高速云有着惊人的彗星外观和许多与银河ISM相互作用的迹象。相互作用速度给出的运动学距离为12.4 +/-1.3千秒差距，与其他方法得出的距离一致。星云的大小大于3 x 1 kpc，它的尖端位于（l，b）=（39 deg，-13 deg），距离银河系中心7.6 kpc，距离银河系平面以下2.9 kpc。它在HI的质量大于10^6 M太阳质量。它的前部有一个接近300 km/s的总空间速度，正以73+/-26 km/s的速度向银河系平面移动，并正在向银河系脱落物质。在没有阻力的情况下，云将在大约2700万年后穿过平面。史密斯云可能是银河系吸积气体的一个例子，用来解释银河系化学演化中某些持续的异常现象。
英文摘要：
New 21cm HI observations made with the Green Bank Telescope show that the high-velocity cloud known as Smith's Cloud has a striking cometary appearance and many indications of interaction with the Galactic ISM. The velocities of interaction give a kinematic distance of 12.4 +/-1.3 kpc, consistent with the distance derived from other methods. The Cloud is >3 x 1 kpc in size and its tip at (l,b)=(39 deg,-13 deg) is 7.6 kpc from the Galactic center and 2.9 kpc below the Galactic plane. It has greater than 10^6 M solar masses in HI. Its leading section has a total space velocity near 300 km/s, is moving toward the Galactic plane with a velocity of 73+/-26 km/s, and is shedding material to the Galaxy. In the absence of drag the Cloud will cross the plane in about 27 Myr. Smith's Cloud may be an example of the accretion of gas by the Milky Way needed to explain certain persistent anomalies in Galactic chemical evolution.
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