A metallicity-spin temperature relation in damped Lyα systems

Abstract

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 Ts = (-0.68 ±0.17) × [Z/H] + (2.13 ±0.21) from a linear regression analysis. Our results indicate that the high Ts 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 Ts 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 Ts . Most DLAs at z > 1.7 have low metallicities, [Z/H] < 1010.5 M ⊙), low metallicities ([Z/H]