Cold Atomic Gas in the CGPS and Beyond

TL;DR

This paper discusses the discovery and analysis of cold hydrogen clouds in the Milky Way using the Canadian Galactic Plane Survey, revealing their widespread presence and properties, and exploring their role in star formation.

Contribution

It introduces new observations of cold HI clouds through HI self-absorption, expanding understanding of the transition between diffuse gas and star-forming molecular clouds.

Findings

Widespread HI self-absorption clouds identified in the Milky Way.

Detailed analysis of the spatial structure and properties of cold HI clouds.

Correlation between cold HI clouds and molecular gas regions.

Abstract

The Canadian Galactic Plane Survey has opened new vistas on the Milky Way, including cold hydrogen clouds that bridge a critical gap between the classical diffuse interstellar medium and the gravitationally bound molecular clouds that can form stars. The CGPS and its fellow IGPS surveys revealed these transitional clouds to be surprisingly widespread as HI self-absorption (HISA) shadows against the Galactic HI emission background. The richness of the IGPS data allows detailed examination of HISA cloud spatial structure, gas properties, Galactic distribution, and correspondence with molecular gas, all of which can constrain models of cold HI clouds in the evolving interstellar medium. Augmenting the landmark IGPS effort are new and upcoming surveys with the Arecibo 305m and Australian SKA Pathfinder telescopes.