Journal
ASTROPHYSICAL JOURNAL LETTERS
Volume 737, Issue 2, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.1088/2041-8205/737/2/L33
Keywords
galaxies: high-redshift; ISM: clouds; quasars: absorption lines
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Funding
- Netherlands Foundation for Scientific Research (NWO)
- Department of Science and Technology
- Associated Universities, Inc.
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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.
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