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arXiv · 1711.09529

An Intermediate Velocity HI Cloud Falling to the Galactic Disk; Possible Evidence for Low Metallicity HI Gas Originated Outside the Galactic Disk

Yasuo Fukui·Masako Koga·Shohei Maruyama·Takahiro Hayakawa·Ryuji Okamoto·Hiroaki Yamamoto·Kengo Tachihar·Robin Shelton·Manami Sasaki
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Abstract

We found that an intermediate velocity cloud (IVC) IVC 86-36 in HI 21 cm emission shows a head-tail distribution toward the Galactic plane with marked parallel filamentary streamers, which is extended over 40 degrees 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 opticalspectroscopy. There is a bridge feature in velocity between the IVC and the local ISM 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 7X103(d/1kpc)2Msol. 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 Hi and the Planck/IRAS dust emission indicates that the dust emission of IVC 86-36 is not detectable in spite of its HI column density of 2X10^20 cm-2, indicating low metalicity of IVC 86-36 by a factor of ~< 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.

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Yasuo Fukui, Masako Koga, Shohei Maruyama, Takahiro Hayakawa, Ryuji Okamoto, Hiroaki Yamamoto, Kengo Tachihar, Robin Shelton, Manami Sasaki. 2018-10-05. An Intermediate Velocity HI Cloud Falling to the Galactic Disk; Possible Evidence for Low Metallicity HI Gas Originated Outside the Galactic Disk. https://doi.org/10.1093/pasj%2Fpsy120

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