An HI column density threshold for cold gas formation in the Galaxy

Abstract

We report the discovery of a threshold in the HI column density of Galactic gas clouds below which the formation of the cold phase of HI is inhibited. This threshold is at $N_{HI}=2\times 10^{20}$ per cm${}^2$; sightlines with lower HI column densities have high spin temperatures (median $T_s\sim 1800$ K), indicating low fractions of the cold neutral medium (CNM), while sightlines with $N_{HI}\ge 2\times 10^{20}$ per cm${}^2$ have low spin temperatures (median $T_s\sim 240$ K), implying high CNM fractions. The threshold for CNM formation is likely to arise due to inefficient self-shielding against ultraviolet photons at lower HI column densities. The threshold is similar to the defining column density of a damped Lyman-$\alpha$ absorber; this indicates a physical difference between damped and sub-damped Lyman-$\alpha$ systems, with the latter class of absorbers containing predominantly warm gas.

5 pages, three figures; Astrophysical Journal Letters, in press. Final version, with updated references

References in corpus (4)

• A Wide-field High Resolution HI Mosaic of Messier 31: I. Opaque Atomic Gas and Star Formation Rate Density
• Do Damped and Sub-damped Lyman-alpha Absorbers Arise in Galaxies of Different Masses?
• On the Absence of High Metallicity-High Column Density Damped Lyman Alpha Systems: Molecule Formation in a Two-Phase Interstellar Medium
• The thermally-unstable warm neutral medium: key for modeling the interstellar medium

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