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arXiv · 1107.0744

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

Nissim Kanekar·Robert Braun·Nirupam Roy
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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-$α$ absorber; this indicates a physical difference between damped and sub-damped Lyman-$α$ systems, with the latter class of absorbers containing predominantly warm gas.

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Nissim Kanekar, Robert Braun, Nirupam Roy. 2011-07-22. An HI column density threshold for cold gas formation in the Galaxy. https://doi.org/10.1088/2041-8205%2F737%2F2%2Fl33

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