%0 Generic %1 Kanekar2011 %A Kanekar, Nissim %A Braun, Robert %A Roy, Nirupam %D 2011 %K 21cm cnm column density wnm %T An HI column density threshold for cold gas formation in the Galaxy %U http://arxiv.org/abs/1107.0744 %X 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 $N_HI = 2 10^20$ per cm$^2$; sightlines with lower HI column densities have high spin temperatures (median $T_s 1800$ K), indicating low fractions of the cold neutral medium (CNM), while sightlines with $N_HI 2 10^20$ per cm$^2$ have low spin temperatures (median $T_s 240$ 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.

@misc{Kanekar2011, 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. }, added-at = {2011-07-06T08:13:31.000+0200}, author = {Kanekar, Nissim and Braun, Robert and Roy, Nirupam}, biburl = {https://www.bibsonomy.org/bibtex/2b9f19785a8e2cb66cdb657ac274a544a/miki}, description = {[1107.0744] An HI column density threshold for cold gas formation in the Galaxy}, interhash = {32cc80e70b4c25994e1826ac499a5065}, intrahash = {b9f19785a8e2cb66cdb657ac274a544a}, keywords = {21cm cnm column density wnm}, note = {cite arxiv:1107.0744Comment: 5 pages, three figures; accepted for publication in Astrophysical Journal Letters}, timestamp = {2011-07-06T08:13:31.000+0200}, title = {An HI column density threshold for cold gas formation in the Galaxy}, url = {http://arxiv.org/abs/1107.0744}, year = 2011 }