Tracing dense and diffuse neutral hydrogen in the halo of the Milky Way

TL;DR

This study combines two HI surveys of the Milky Way halo, using machine learning to distinguish dense and diffuse neutral hydrogen, revealing that diffuse gas is more prevalent and likely contributes significantly to the halo's mass.

Contribution

It introduces a machine learning method to separate dense and diffuse HI populations across different surveys, highlighting the importance of diffuse gas in the halo.

Findings

Diffuse gas surrounds dense high velocity clouds.

Diffuse gas has broader line widths (~30 km/s) and lower brightness temperature (~0.2 K).

Most surveys miss the diffuse component, which may contain as much mass as dense gas.

Abstract

We have combined observations of Galactic high-velocity HI from two surveys: a very sensitive survey from the Green Bank 140ft Telescope with limited sky coverage, and the less sensitive but complete Galactic All Sky Survey from the 64m Parkes Radio Telescope. The two surveys preferentially detect different forms of neutral gas due to their sensitivity. We adopt a machine learning approach to divide our data into two populations that separate across a range in column density: 1) a narrow line-width population typical of the majority of bright high velocity cloud components, and 2) a fainter, broad line-width population that aligns well with that of the population found in the Green Bank survey. We refer to these populations as dense and diffuse gas respectively, and find that diffuse gas is typically located at the edges and in the tails of high velocity clouds, surrounding dense components in the core. A fit to the average spectrum of each type of gas in the Galactic All Sky Survey data reveals the dense population to have a typical line width of ~20 km/s and brightness temperature of ~0.3 K, while the diffuse population has a typical line width of ~30 km/s and a brightness temperature of ~0.2 K. Our results confirm that most surveys of high velocity gas in the Milky Way halo are missing the majority of the ubiquitous diffuse gas, and that this gas is likely to contribute at least as much mass as the dense gas.