# Ionisation of the atomic gas in redshifted radio sources

## TL;DR

This study investigates the absence of HI 21-cm absorption in high-redshift radio sources, confirming photo-ionisation as the main cause and highlighting the role of dust and source compactness in gas detection.

## Contribution

It provides new observational evidence linking ionising photon rates, dust shielding, and source properties to the detectability of 21-cm absorption in radio sources.

## Key findings

- Non-detection of 21-cm absorption is due to photo-ionisation.
- Strong correlation between absorption strength and reddening.
- Anti-correlation between detection rate and turnover frequency.

## Abstract

We report the results of a survey for HI 21-cm absorption at z < 0.4 in a new sample of radio sources with the Giant Metrewave Radio Telescope. Of the 11 sources for which there are good data, we obtain zero detections, where four are expected upon accounting for the ionising photon rates and sensitivity. Adding these to the previously published values, we confirm that the non-detection of 21-cm absorption in active sources at high redshift is due to photo-ionisation of the gas rather than excitation by 21-cm photons (significant at 6.09 sigma and 2.90 sigma, respectively). We also confirm a strong correlation between the absorption strength and the reddening of the source, suggesting that dust plays a significant role in shielding the gas from the ambient ultra-violet field. An anti-correlation between the 21-cm detection rate and the radio turnover frequency is also found, which runs contrary to what is expected on the basis that the higher the turnover frequency, the more compact the source. It is, however, consistent with the hypothesis that the turnover frequency is related to the electron density, supported by a correlation between the turnover frequency and ionising photon rate.