The Distribution of Disk-Halo HI Clouds in the Inner Milky Way

TL;DR

This study compares the properties and distribution of disk-halo HI clouds in different regions of the Milky Way, revealing asymmetries likely related to spiral arm star formation and feedback processes.

Contribution

It provides the first detailed comparison of HI cloud properties and distributions in symmetric Galactic regions, highlighting major asymmetries and their possible origins.

Findings

Number of clouds in QI is three times that in QIV.

Cloud scale height is twice as large in QI.

Distribution of clouds is more uniform in QI.

Abstract

Using data from the Galactic All-Sky Survey, we have compared the properties and distribution of HI clouds in the disk-halo transition at the tangent points in mirror-symmetric regions of the first quadrant (QI) and fourth quadrant (QIV) of the Milky Way. Individual clouds are found to have identical properties in the two quadrants. However, there are 3 times as many clouds in QI as in QIV, their scale height is twice as large, and their radial distribution is more uniform. We attribute these major asymmetries to the formation of the clouds in the spiral arms of the Galaxy, and suggest that the clouds are related to star formation in the form of gas that has been lifted from the disk by superbubbles and stellar feedback, and fragments of shells that are falling back to the plane.