An intermediate-velocity H I cloud falling to the galactic disk; Possible evidence for low-metallicity H I gas originating outside the galactic disk

Abstract

We found that an intermediate-velocity cloud (IVC), IVC 86-36, in H I 21 cm emission shows a head-tail distribution toward the Galactic plane with marked parallel filamentary streamers, which is extended over 40° in the sky. The distance of IVC 86-36 is constrained to be less than ∼3 kpc from absorption of a background star as determined from optical spectroscopy. There is a bridge feature in velocity between the IVC and the local interstellar medium with velocity separation of ∼50 km s^-1, which may indicate dynamical interaction of the IVC with the disk. If the interaction is correct, the distance estimate d of the IVC ranges from 200 pc to 3 kpc, and the mass of the IVC head is estimated to be $7\times 10^3(d/1\:\mbox{kpc})^2\, M_{\odot }$ . The IVC shares similar properties to the Smith Cloud located at 12 kpc, including the head-tail distribution, streamers, and bridge feature, while the mass of the IVC is less than ∼0.1 of the Smith Cloud. A comparison between the H I and the Planck/IRAS dust emission indicates that the dust emission of IVC 86-36 is not detectable in spite of its H I column density of 2 × 10²⁰ cm^-2, indicating a low metallicity for IVC 86-36, reduced by a factor of $\lesssim 0.2$ as compared with the solar neighbor. We conclude that IVC 86-38 is an infalling cloud which likely originated in the low-metallicity environment of the Galactic halo or the Magellanic system.