On the HI column density - radio source size anti-correlation in compact radio sources

TL;DR

This study re-evaluates the observed anti-correlation between HI column density and radio source size in compact sources, attributing it mainly to geometric effects and optical depth variations rather than an intrinsic relationship.

Contribution

It clarifies that the anti-correlation arises from geometry and optical depth assumptions, and models the absorber-source interaction considering two-component radio sources.

Findings

The anti-correlation is driven by optical depth and geometry, not an intrinsic property.

Models of two-component radio sources reproduce the observed trend.

Absorber sizes of 100-1000 pc match the peak 21-cm detection rates.

Abstract

Existing studies of the atomic hydrogen gas content in distant galaxies, through the absorption of the 21-cm line, often infer that the total column density is anti-correlated with the linear extent of the background radio source. We investigate this interpretation, by dissecting the various parameters from which the column density is derived, and find that the relationship is driven primarily by the observed optical depth, which, for a given absorber size, is anti-correlated with the linear size. Therefore, the inferred anti-correlation is merely the consequence of geometry, in conjunction with the assumption of a common spin temperature/covering factor ratio for each member of the sample, an assumption for which there is scant observational justification. While geometry can explain the observed correlation, many radio sources comprise two radio lobes and so we model the projected area of a two component emitter intercepted by a foreground absorber. From this, the observed optical depth/linear size relationship is best reproduced through models which approximate either of the two Fanaroff & Riley classifications, although the observed scatter in the sample cannot be duplicated using a single deprojected radio source size. Furthermore, the trend is best reproduced using an absorber of diameter ~100 - 1000 pc, which is also the range of values of linear sizes at which the 21-cm detection rate peaks. This may indicate that this is the characteristic linear size of the absorbing gas structure.