AN H I COLUMN DENSITY THRESHOLD FOR COLD GAS FORMATION IN THE GALAXY

We report the discovery of a threshold in the H I column density of Galactic gas clouds below which the formation of the cold phase of H I is inhibited. This threshold is at N-H (I) = 2 x 10(20) cm(-2); sight lines with lower H I column densities have high spin temperatures (median T-s similar to 1800 K), indicating low fractions of the cold neutral medium (CNM), while sight lines with N-H (I) >= 2 x 10(20) cm(-2) have low spin temperatures (median T-s similar to 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 H I column densities. The threshold is similar to the defining column density of a damped Ly alpha absorber; this indicates a physical difference between damped and sub-damped Ly alpha systems, with the latter class of absorbers containing predominantly warm gas.