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

TL;DR

This study investigates an intermediate velocity HI cloud, IVC 86-36, revealing its potential extragalactic origin, low metallicity, and interaction with the Galactic disk through detailed HI and dust emission analysis.

Contribution

The paper provides the first detailed analysis of IVC 86-36's properties, including its distance, morphology, and low metallicity, suggesting an external origin outside the Galactic disk.

Findings

IVC 86-36 shows a head-tail structure and filamentary streamers.

The cloud's metallicity is less than 0.2 times solar.

The cloud likely originated from the Galactic halo or Magellanic system.

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.