An H I 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 NHI=2×10²⁰ per cm^-2; sightlines with lower HI column densities have high spin temperatures (median Ts∼1800 K), indicating low fractions of the cold neutral medium (CNM), while sightlines with NHI≥2×10²⁰ per cm^-2 have low spin temperatures (median Ts∼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-α absorber; this indicates a physical difference between damped and sub-damped Lyman-α systems, with the latter class of absorbers containing predominantly warm gas.