New HI views of the Galaxy and the Magellanic Clouds

TL;DR

This paper reviews recent advances in HI observations of the Milky Way and nearby galaxies, highlighting how new radio telescopes like the SKA will enhance understanding of the thermal structure and distribution of atomic hydrogen in different galactic environments.

Contribution

It provides an overview of recent observational results and discusses future prospects for studying HI thermal phases beyond the Local Group with upcoming radio telescopes.

Findings

The cold HI disk of the Milky Way is extended and flared.

The CNM is colder and more abundant at higher optical extinctions.

Comparison with simulations shows good agreement on CNM fraction dependence on metallicity.

Abstract

Atomic hydrogen (HI) is a vital player in the star-formation process in galaxies. It is the raw fuel for making molecules, an important shielding agent against interstellar radiation, and a buffer that soaks up the energy and momentum of stellar feedback. While for many years detailed studies of the HI thermal structure have been possible only in the Milky Way, the SKA pathfinders are expanding our view beyond the Solar neighborhood allowing for crucial tests of the HI heating and cooling processes under a wide range of physical conditions. This overview article highlights a few recent results and emphasizes areas where future observations can make large contributions. We show that the cold HI disk of the Milky Way is extended and flared, yet appears spatially coupled to the molecular gas. The cold neutral medium (CNM) in the Milky Way is colder and more abundant at higher optical extinctions due to more intense cooling and shielding. A comparison between the Milky Way, the Small Magellanic Cloud, the Large Magellanic Cloud and NGC 6822 shows a good agreement with predictions from recent numerical simulations on how the CNM fraction depends on metalliciity. The fraction and spatial distribution of the thermally unstable HI remain as open questions and observationally have not been studied beyond the Milky Way. The excitation temperature of the warm neutral medium (WNM) is not well understood. Observations suggest higher WNM temperatures than what is seen in numerical simulations. The SKA, once operational, will take the HI studies into a new era of dense HI absorption grids for the Milky Way and nearby galaxies, and the ability to study the HI thermal structure beyond the Local Group.