A metallicity-spin temperature relation in damped Lyα systems

We report evidence for an anti-correlation between spin temperature T _s and metallicity [Z/H], detected at 3.6σ significance in a sample of 26 damped Lyα absorbers (DLAs) at redshifts 0.09 < z < 3.45. The anti-correlation is detected at 3σ significance in a sub-sample of 20 DLAs with measured covering factors, implying that it does not stem from low covering factors. We obtain T_s = (-0.68 ±0.17) × [Z/H] + (2.13 ±0.21) from a linear regression analysis. Our results indicate that the high T_s values found in DLAs do not arise from differences between the optical and radio sightlines, but are likely to reflect the underlying gas temperature distribution. The trend between T_s and [Z/H] can be explained by the larger number of radiation pathways for gas cooling in galaxies with high metal abundances, resulting in a high cold gas fraction, and hence, a low spin temperature. Conversely, low-metallicity galaxies have fewer cooling routes, yielding a larger warm gas fraction and a high T_s . Most DLAs at z > 1.7 have low metallicities, [Z/H] <-1, implying that the H I in high-z DLAs is predominantly warm. The anti-correlation between T_s and [Z/H] is consistent with the presence of a mass-metallicity relation in DLAs, suggested by the tight correlation between DLA metallicity and the kinematic widths of metal lines. Most high-z DLAs are likely to arise in galaxies with low masses (M _vir < 10^10.5 M _⊙), low metallicities ([Z/H] <-1), and low cold gas fractions.