A Global View of Molecule-Forming Clouds in the Galaxy

TL;DR

This study maps cold atomic hydrogen in the Milky Way and compares it with CO emission to understand molecular cloud formation, revealing many HISA features may contain CO-dark H2 rather than CO.

Contribution

It provides the first large-scale, high-resolution comparison of HISA and CO, suggesting the presence of CO-dark H2 in cold clouds.

Findings

Most inner-Galaxy HISA overlaps with CO, but many are chance superpositions.

Many HISA features are CO-free, indicating CO-dark H2 presence.

Standard models cannot explain cold HI without molecules, implying CO-dark H2.

Abstract

We have mapped cold atomic gas in 21cm line HI self-absorption (HISA) at arcminute resolution over more than 90% of the Milky Way's disk. To probe the formation of H2 clouds, we have compared our HISA distribution with CO J=1-0 line emission. Few HISA features in the outer Galaxy have CO at the same position and velocity, while most inner-Galaxy HISA has overlapping CO. But many apparent inner-Galaxy HISA-CO associations can be explained as chance superpositions, so most inner-Galaxy HISA may also be CO-free. Since standard equilibrium cloud models cannot explain the very cold HI in many HISA features without molecules being present, these clouds may instead have significant CO-dark H2.