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

Abstract

We present a comprehensive analysis of the spin temperature/covering factor degeneracy, T/f, in damped Lyman-alpha absorption systems. By normalising the upper limits and including these via a survival analysis, there is, as previously claimed, an apparent increase in T/f with redshift at z > 1. However, when we account for the geometry effects of an expanding Universe, neglected by the previous studies, this increase in T/f at z > 1 is preceded by a decrease at z < 1. Using high resolution radio images of the background continuum sources, we can transform the T/f degeneracy to T/d^2, where d is the projected linear size of the absorber. Again, there is no overall increase with redshift, although a dip at z ~ 2 persists. Furthermore, we find d^2/T to follow a similar variation with redshift as the star formation rate. This suggests that, although the total hydrogen column density shows little relation to the SFR, the fraction of the cold neutral medium may. Therefore, further efforts to link the neutral gas with the star formation history should also consider the cool component of the gas.