Atomic Hydrogen in the Milky Way: A Stepping Stone in the Evolution of Galaxies

TL;DR

This paper reviews recent surveys of atomic hydrogen in the Milky Way, highlighting its physical properties, structure, and role in galactic evolution, with key findings on the cold and warm HI phases and their environmental dependence.

Contribution

It provides a comprehensive overview of new HI surveys and insights into the physical state, structure, and environmental dependence of atomic hydrogen in the Milky Way.

Findings

Cold neutral medium constitutes up to 40% of HI mass.

Cold HI extends to at least 25 kpc from the galactic center.

Approximately 40% of HI is warm, shaped by feedback events.

Abstract

Atomic hydrogen (HI) is a critical stepping stone in the gas evolution cycle of the interstellar medium (ISM) of the Milky Way. Hi traces both the cold, premolecular state before star formation and the warm, diffuse ISM before and after star formation. This review describes new, sensitive HI absorption and emission surveys, which, together with high angular and spectral resolution Hi emission data, have revealed the physical properties of HI, its structure, and its association with magnetic fields. We give an overview of the HI phases and discuss how Hi properties depend on the environment and what its structure can tell us about feedback in the ISM. Key findings include the following: - The mass fraction of the cold neutral medium is $\lesssim 40$\% on average, increasing with $A_V$ due to the increase of mean gas density. - The cold disk extends to at least $R\sim 25$ kpc. - Approximately 40% of the HI is warm, with structural characteristics that derive from feedback events. - Cold HI is highly filamentary, whereas warm HI is more smoothly distributed. We summarize future observational and simulation opportunities that can be used to unravel the 3D structure of the atomic ISM and the effects of heating and cooling on HI properties.