A metallicity-spin temperature relation in damped Lyman-$\alpha$ systems

Abstract

We report evidence for an anti-correlation between spin temperature $T_s$ and metallicity [Z/H], detected at $3.6\sigma$ significance in a sample of 26 damped Lyman-$\alpha$ absorbers (DLAs) at redshifts $0.09<z<3.45$. The anti-correlation is detected at $3\sigma$ significance in a sub-sample of 20 DLAs with measured covering factors, implying that it does not stem from low covering factors. We obtain $T_s=(-0.68\pm 0.17)\times [Z/H]+(2.13\pm 0.21)$ 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 HI 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 ($M_{\mathrm{vir}}<10^{10.5}{M}_{\odot }$), low metallicities ([Z/H]$<-1$, and low cold gas fractions.

5 pages, 2 figures; accepted for publication in Astrophysical Journal Letters

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