A study of low-metallicity DLAs at high redshift and C ii* as a probe of their physical conditions★

Abstract

We present a detailed high spectral resolution (R $\\sim$ 40000) study of five\nhigh-z damped Lyman $\\alpha$ systems (DLAs) and one sub-DLA detected along four\nQSO sightlines. Four of these DLAs are very metal-poor with [Fe/H] $\\le$ $-$2.\nOne of them, at z$_{abs}$ = 4.20287 towards J0953$-$0504, is the most\nmetal-poor DLA at z $>$ 4 known till date. This system shows no enhancement of\nC over Fe and O, and standard Population II star yields can explain its\nrelative abundance pattern. The DLA at z$_{abs}$ = 2.34006 towards J0035$-$0918\nhas been claimed to be the most carbon-enhanced metal-poor DLA. However, we\nshow that thermal broadening is dominant in this system and, when this effect\nis taken into account, the measured carbon enhancement ([C/Fe] = 0.45 $\\pm$\n0.19) becomes $\\sim$ 10 times less than what was reported previously. The gas\ntemperature in this DLA is estimated to be in the range of 5000 $-$ 8000 K,\nconsistent with a warm neutral medium phase. From photoionization modelling of\ntwo of the DLAs showing C II* absorption, we find that the metagalactic\nbackground radiation alone is not sufficient to explain the observed C II*\ncooling rate, and local heating sources, probably produced by in-situ star\nformation, are needed. Cosmic ray heating is found to contribute $\\gtrsim$ 60%\nto the total heating in these systems. Using a sample of metal-poor DLAs with C\nII* measurements, we conclude that the cosmic ray ionization rate is equal to\nor greater than that seen in the Milky Way in $\\sim$ 33% of the systems with C\nII* detections.\n