A possible connection between the spin temperature of damped Lyman α absorption systems and star formation history

Abstract

We present a comprehensive analysis of the spin temperature/covering factor degeneracy, Tspin/f, in damped Lyman α absorption systems. By normalizing the upper limits and including these via a survival analysis, there is, as previously claimed, an apparent increase in Tspin/f with redshift at zabs ≳ 1. However, when we account for the geometry effects of an expanding Universe, neglected by the previous studies, this increase in Tspin at zabs ≳ 1 is preceded by a decrease at zabs ≲ 1. Using high resolution radio images of the background continuum sources, we can transform the Tspin/f degeneracy to |$T_{\rm spin}/d_{\rm abs}^{\,\,\,2}$|⁠, where dabs is the projected linear size of the absorber. Again, there is no overall increase with redshift, although a dip at zabs ≈ 2 persists. Furthermore, we find |$d_{\rm abs}^{\,\,\,2}/T_{\rm spin}$| to follow a similar variation with redshift as the star formation rate, ψ*. This suggests that, although the total hydrogen column density, |$N_{\rm H\,\small {I}}$|⁠, shows little relation to ψ*, the fraction of the cold neutral medium, |$\int \!\tau _{\rm obs}\,\text{d}v/N_{\rm H\,\small {I}}$|⁠, may. Therefore, further efforts to link the neutral gas with the star formation history should also consider the cool component of the gas.