Cold Neutral Gas in a z=4.2 Damped Lyman-alpha System: The Fuel for Star Formation

Abstract

We discuss interstellar temperature determinations using the excitation equilibrium of the ^2P levels of Si II and C II. We show how observations of the ^2P_3/2 fine structure levels of Si II and C II (which have significantly different excitation energies, corresponding to ~413 and 92 K, respectively) can be used to limit gas kinetic temperatures. We apply this method to the z=4.224 damped Lyman-alpha system toward the quasar PSS1443+27. The lack of significant absorption out of the SiII ^2P_3/2 level and the presence of very strong C II ^2P_3/2 provides an upper limit to the temperature of the C II*-bearing gas in this system. Assuming a solar Si/C ratio, the observations imply a 2-sigma limit T<954 K for this absorber; a super-solar Si/C ratio gives stricter limits, T<524 K. The observations suggest the presence of a cold neutral medium; such cold gas may serve as the fuel for star formation in this young galaxy.

Accepted to ApJ Letters; 13 pages, including figures and tables

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• The most metal-poor damped Lyman alpha systems: An insight into dwarf galaxies at high redshift
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• Investigating Nearby Star-Forming Galaxies in the Ultraviolet with HST/COS Spectroscopy. I: Spectral Analysis and Interstellar Abundance Determinations
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• Probing the Physical Conditions of Atomic Gas at High Redshift
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