Milky way disk-halo transition in hi: Properties of the cloud population

Abstract

Using 21 cm H i observations from the Parkes Radio Telescope’s Galactic All-Sky Survey, we measure 255 H i clouds in the lower Galactic halo that are located near the tangent points at 16. ◦9 l 35. ◦3 and |b| 20◦. The clouds have a median mass of 700 M 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 (LOS) cloud–cloud velocity dispersion of σcc ≈ 16 km s−1. 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 LOS 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 H i cloud population is thus likely tied to and driven by large-scale star formation processes, possibly through the mechanism of supershells and feedback. Show more