Properties of extra-planar HI clouds in the outer part of the Milky Way

TL;DR

This study characterizes extra-planar HI clouds in the Milky Way's outer regions, revealing their physical properties, small-scale structures, and stability, contributing to understanding the Galactic halo gas dynamics.

Contribution

It provides detailed observational analysis of HI clouds in the Milky Way's halo, including their temperature, density, and structure, which was previously less well characterized.

Findings

HI clouds have median temperature of 620 K

Most clouds are surrounded by warmer HI envelopes

Cores show narrow line widths (~3 km/s) and are in pressure equilibrium

Abstract

There is mounting evidence for an extra-planar gas layer around the Milky Way disk, similar to the anomalous HI gas detected in a few other galaxies. As much as 10% of the gas may be in this phase. We analyze HI clouds located in the disk-halo interface outside the solar circle to probe the properties of the extra-planar HI gas, which is following Galactic rotation. We use the Leiden/Argentine/Bonn (LAB) 21-cm line survey to search for HI clouds which take part in the rotation of the Galactic plane, but are located above the disk layer. Selected regions are mapped with the Effelsberg 100-m telescope. Two of the HI halo clouds are studied in detail for their small scale structure using the Westerbork Synthesis Radio Telescope (WSRT). Data from the 100m telescope allow for the parameterization of 25 distinct HI halo clouds at Galactocentric radii 10 kpc <R<15 kpc and heights 1 kpc <z<5 kpc. The clouds have a median temperature of 620 K, column densities of NH~10E19 cm^-2, and most of them are surrounded by an extended envelope of warmer HI gas. Interferometer observations for two selected regions resolve the HI clouds into several arc-minute sized cores. These cores show narrow line widths (FWHM ~3 km/s), they have volume densities of n > 1.3 cm^-3, masses up to 24 M_{sol}, and are on average in pressure equilibrium with the surrounding envelopes. Pressures and densities fall within the expectations from theoretical phase diagrams (P vs <n_{H}>). The HI cores tend to be unstable if one assumes a thermally bistable medium, but are in better agreement with models that predict thermal fragmentation driven by a turbulent flow.