# Atomic and Molecular Absorption in Redshifted Radio Sources

## TL;DR

This study conducted a radio survey for neutral hydrogen and hydroxyl absorption in redshifted sources, finding no detections but revealing correlations with optical and radio properties, emphasizing the role of photo-ionisation.

## Contribution

It provides new insights into the factors affecting neutral gas detection in radio sources, especially the impact of ultra-violet luminosity and sight-line obscuration.

## Key findings

- No absorption detections in the six sources surveyed.
- Correlation between absorption strength and optical/infrared colour.
- Critical ionising photon rate confirmed to ionise all neutral gas.

## Abstract

We report on a survey for associated HI 21-cm and OH 18-cm absorption with the Giant Metrewave Radio Telescope at redshifts z = 0.2-0.4. Although the low redshift selection ensures that our targets are below the critical ultra-violet luminosity, which is hypothesised to ionise all of the neutral gas in the host galaxy, we do not obtain any detections in the six sources searched. Analysing these in context of the previous surveys, in addition to the anti-correlation with the ultra-violet luminosity (ionising photon rate), we find a correlation between the strength of the absorption and the blue -- near-infrared colour, as well as the radio-band turnover frequency. We believe that these are due to the photo-ionisation of the neutral gas, an obscured sight-line being more conducive to the presence of cold gas and the compact radio emission being better intercepted by the absorbing gas, maximising the flux coverage, respectively. Regarding the photo-ionisation, the compilation of the previous surveys increases the significance of the critical ionising photon rate, above which all of the gas in the host galaxy is hypothesised to be ionised, to >5 sigma. This reaffirms that this is an ubiquitous effect, which has profound implications for the detection of neutral gas in these objects with the Square Kilometre Array.