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

We present a detailed high spectral resolution (R similar to 40 000) study of five high-z damped Lyman alpha 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 z(abs) = 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 z(abs) = 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 similar to 10 times less than what was reported previously. The gas temperature in this DLA is estimated to be in the range of 5000-8000 K, 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 greater than or similar to 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 similar to 33 per cent of the systems with C ii* detections.