On the detectability of HI 21-cm in MgII absorption system

TL;DR

This study examines factors influencing the detection of HI 21-cm absorption in MgII systems, focusing on line-of-sight geometry and correlations with MgII properties, revealing redshift-dependent detection rates and no evidence of evolution in hydrogen column density.

Contribution

It highlights the impact of geometric and spectral factors on detection rates and clarifies the relationship between 21-cm line strength and MgII equivalent width, especially in DLAs.

Findings

Detection rate depends on angular diameter distance ratio.

Correlation between 21-cm line strength and MgII equivalent width increases with MgI strength.

No evidence of cosmological evolution in neutral hydrogen column density.

Abstract

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.