HI Clouds in the Lower Halo: I. The Galactic All-Sky Survey Pilot Region

TL;DR

This study reports the detection and analysis of over 400 neutral hydrogen clouds in the lower halo of the Milky Way, revealing their properties, distribution, and possible origins related to star formation and stellar feedback.

Contribution

First detailed characterization of HI clouds in the lower Galactic halo using GASS pilot data, linking their properties to spiral arms and star formation processes.

Findings

Over 400 HI clouds detected with median size <1 degree

Clouds have a median HI mass of 630 solar masses

Clouds are concentrated at a Galactocentric radius of 3.8 kpc

Abstract

We have detected over 400 HI clouds in the lower halo of the Galaxy within the pilot region of the Galactic All-Sky Survey (GASS), a region of the fourth quadrant that spans 18 degrees in longitude, 40 degrees in latitude and is centered on the Galactic equator. These clouds have a median peak brightness temperature of 0.6 K, a median velocity width of 12.8 km/s, and angular sizes <1 degree. The motion of these clouds is dominated by Galactic rotation with a random cloud-to-cloud velocity dispersion of 18 km/s. A sample of clouds likely to be near tangent points was analyzed in detail. These clouds have radii on the order of 30 pc and a median HI mass of 630 Msun. The population has a vertical scale height of 400 pc and is concentrated in Galactocentric radius, peaking at R=3.8 kpc. This confined structure suggests that the clouds are linked to spiral features, while morphological evidence that many clouds are aligned with loops and filaments is suggestive of a relationship with star formation. The clouds might result from supernovae and stellar winds in the form of fragmenting shells and gas that has been pushed into the halo rather than from a galactic fountain.