2014MNRAS.440..307D

We present a detailed high spectral resolution (R ∼ 40 000) study of five high-z damped Lyman α systems (DLAs) and one sub- DLA detected along four QSO sightlines. Four of these DLAs are very metal poor with [Fe/H] ≤ -2. One of them, at zabs = 4.202 87 towards J0953-0504, is the most metal-poor DLA at z > 4 known till date. This system shows no enhancement of C over Fe and O, and standard Population II star yields can explain its relative abundance pattern. The DLA at zabs = 2.340 06 towards J0035-0918 has been claimed to be the most carbon-enhanced metal-poor DLA. However, we show that thermal broadening is dominant in this system and, when this effect is taken into account, the measured carbon enhancement ([C/Fe] = 0.45±0.19) becomes ∼ 10 times less than what was reported previously. The gas temperature in this DLA is estimated to be in the range of 5000-8000K, consistent with a warm neutral medium phase. From photoionization modelling of two of the DLAs showing C ii* absorption, we find that the metagalactic background radiation alone is not sufficient to explain the observed C ii* cooling rate, and local heating sources, probably produced by in situ star formation, are needed. Cosmic ray heating is found to contribute ≳ 60 per cent to the total heating in these systems. Using a sample of metal-poor DLAs with C ii* measurements, we conclude that the cosmic ray ionization rate is equal to or greater than that seen in the Milky Way in ∼ 33 per cent of the systems with C ii* detections.