Milky Way Disk-Halo Transition in HI: Properties of the Cloud Population

TL;DR

This study uses 21cm HI observations to analyze the properties and distribution of disk-halo clouds in the Milky Way, revealing their connection to large-scale star formation and galactic structure.

Contribution

First detailed comparison of HI cloud populations in two Galactic quadrants, linking cloud properties to galactic features like the bar and spiral arms.

Findings

Similar cloud properties in both quadrants suggest a common origin.

Higher cloud density and larger scale height at the Galactic bar tip.

Cloud distribution correlates with large-scale star formation activity.

Abstract

Using 21cm HI observations from the Parkes Radio Telescope's Galactic All-Sky Survey, we measure 255 HI clouds in the lower Galactic halo that are located near the tangent points at 16.9 < l < 35.3 degrees and |b| < 20 degrees. The clouds have a median mass of 700 Msun and a median distance from the Galactic plane of 660 pc. This first Galactic quadrant (QI) region is symmetric to a region of the fourth quadrant (QIV) studied previously using the same data set and measurement criteria. The properties of the individual clouds in the two quadrants are quite similar suggesting that they belong to the same population, and both populations have a line of sight cloud-cloud velocity dispersion of sigma_cc ~ 16 km/s. However, there are three times as many disk-halo clouds at the QI tangent points and their scale height, at h=800 pc, is twice as large as in QIV. Thus the observed line of sight random cloud motions are not connected to the cloud scale height or its variation around the Galaxy. The surface density of clouds is nearly constant over the QI tangent point region but is peaked near R~4 kpc in QIV. We ascribe all of these differences to the coincidental location of the QI region at the tip of the Milky Way's bar, where it merges with a major spiral arm. The QIV tangent point region, in contrast, covers only a segment of a minor spiral arm. The disk-halo HI cloud population is thus likely tied to and driven by large-scale star formation processes, possibly through the mechanism of supershells and feedback.