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

TL;DR

This study identifies a specific HI column density threshold in the Galaxy, below which cold gas formation is suppressed, linked to self-shielding effects and distinguishing different types of gas absorbers.

Contribution

It reveals a critical HI column density threshold for cold gas formation in the Galaxy, connecting it to self-shielding and absorber classifications.

Findings

Threshold at N_HI = 2 x 10^{20} cm^{-2} for cold gas formation.

Sightlines below threshold have high spin temperatures (~1800 K).

Sightlines above threshold have low spin temperatures (~240 K).

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.