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 _ { \sc HI } } = 2 \times 10 ^ { 20 } per cm ^ { 2 } ; sightlines with lower H i 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 _ { \sc HI } } \geq 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 H i 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 .