标题： 关于史密斯高银含量云的金属丰度及其起源 显示英文标题

标题： On the Metallicity and Origin of the Smith High-Velocity Cloud

摘要： 史密斯云是一个处于吸积晚期的高能气体云（HVC），距离银河系平面仅2.9千秒差距，并将在27百万年内撞击盘面。它与其他HVC的不同之处在于其已知的距离（12.4±1.3千秒差距）和精确的三维速度（296公里/秒），但其起源长期以来一直是个谜。在这里，我们利用位于云后方的三个活动星系核（AGN）的HST/COS紫外光谱以及相同方向的绿岸望远镜21厘米谱线数据，首次测量了其金属丰度。通过Voigt轮廓拟合硫的1250、1253和1259三重峰，并结合从光致电离建模推导出的电离修正值，我们在每个方向上得出了硫的丰度；这三个测量值的加权平均值为[S/H]=-0.28±0.14，即0.53+0.21-0.15倍太阳金属丰度。发现史密斯云富含金属支持了它代表的是循环使用的银河系物质而非矮星系遗迹或吸积的星系际气体的情景。云的金属丰度和轨迹都表明它起源于外盘。然而，它的大质量和前进运动仍需进一步解释。如果该云在其喷泉轨迹过程中从日冕吸积冷却气体，正如最近理论研究预测的那样，那么它的当前质量将高于发射时的质量，从而缓解质量方面的担忧。 显示英文摘要

摘要： The Smith Cloud is a gaseous high-velocity cloud (HVC) in an advanced state of accretion, only 2.9 kpc below the Galactic plane and due to impact the disk in 27 Myr. It is unique among HVCs in having a known distance (12.4+/-1.3 kpc) and a well-constrained 3D velocity (296 km/s), but its origin has long remained a mystery. Here we present the first absorption-line measurements of its metallicity, using HST/COS UV spectra of three AGN lying behind the Cloud together with Green Bank Telescope 21 cm spectra of the same directions. Using Voigt-profile fitting of the S II 1250, 1253, 1259 triplet together with ionization corrections derived from photoionization modeling, we derive the sulfur abundance in each direction; a weighted average of the three measurements gives [S/H]=-0.28+/-0.14, or 0.53+0.21-0.15 solar metallicity. The finding that the Smith Cloud is metal-enriched lends support to scenarios where it represents recycled Galactic material rather than the remnant of a dwarf galaxy or accreting intergalactic gas. The metallicity and trajectory of the Cloud are both indicative of an origin in the outer disk. However, its large mass and prograde kinematics remain to be fully explained. If the cloud has accreted cooling gas from the corona during its fountain trajectory, as predicted in recent theoretical work, its current mass would be higher than its launch mass, alleviating the mass concern.