On the detectability of HI 21-cm in MgII absorption system
  We investigate the effect of two important, but oft neglected, factors which
can affect the detectability of HI 21-cm absorption in MgII absorption systems:
The effect of line-of-sight geometry of 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 line. Regarding the former, while the
observed detection rate at small angular diameter distance ratios is a near
certainty, for an unbiased sample, where either a detection or a non-detection
are equally likely, at ratios > 0.8 the observed detection rate has an 8 sigma
significance, suggesting that the mix of ratios values at z < 1 is correlated
with the mix of detections and non-detections at low redshift, while the
exclusively high values of the ratio at z > 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 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 equivalent widths of >0.5 A are added to the
DLA sample. Large values of the angular diameter distance ratio may explain why
the absorbers which have similar equivalent widths to the detections remain
undetected. We do, however, also find the neutral hydrogen column densities of
the non-detections to be significantly lower. Applying the 21-cm line
strength/equivalent width 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.
