2010MNRAS.402.2657C

We investigate the effect of two important, but of neglected, factors which can affect the detectability of Hi 21-cm absorption in MgII absorption systems. The effect of line-of-sight geometry on the coverage of the background radio flux and any possible correlation between the 21-cm line strength and the rest-frame equivalent width of the MgII 2796 Å line, as is seen in the case of damped Lyman α absorption systems (DLAs). Regarding the former, while the observed detection rate at small angular diameter distance ratios (DA_abs/DA_QSO> 0.8) is a near certainty (P > 0.9), for an unbiased sample, where either a detection or a non-detection is equally likely, at DA_abs/DA_QSO≥ 0.8 the observed detection rate has only a probability of P ≲ 10^–15 of occurring by chance. This ≳8σ significance suggests that the mix of DA_abs/DA_QSOvalues at z_abs≲ 1 is correlated with the mix of detections and non-detections at low redshift, while the exclusively high values of the ratio (DA_abs/DA_QSO∼ 1) at z_abs≳ 1 contribute to the low detection rates at high redshift.

In DLAs, the correlation between the 21-cm line strength and the MgII equivalent width (W^λ2796_r) is dominated by the velocity spread of the 21-cm line. This has recently been shown not to hold for MgII systems in general. However, we do find the significance of the correlation to increase when the MgII absorbers with Mgi 2852 Å equivalent widths of W^λ2852_r> 0.5 Å are added to the DLA sample. This turns out to be a subset of the parameter space where MgII absorbers and DLAs overlap and the fraction of MgII absorbers known to be DLAs rises to 50 per cent (Turnshek & Nestor 2006). We therefore suggest that the width of the 21-cm line is correlated with W^λ2796_r for all systems likely to be DLAs and note a correlation between W^λ2852_r (Mgi) and N_Hi, which is not apparent for the singly ionized lines. Furthermore, the 21-cm detection rate at DA_abs/DA_QSO< 0.8 rises to ≳90 per cent for absorbers with W^λ2852_r> 0.5 Å and large values of DA_abs/DA_QSO may explain why the absorbers which have similar values of W^λ2796_r to the detections remain undetected. We do, however, also find the neutral hydrogen column densities of the non-detections to be significantly lower than those of the detections, which could also contribute to their weak absorption. Applying the correlation to yield column densities for the MgII absorbers in which this is unmeasured, we find no evidence of a cosmological evolution in the neutral hydrogen column density in the absorbers searched for in 21-cm.