# Further Observational Evidence for a Critical Ionising Luminosity in Active Galaxies

## TL;DR

This study investigates the absence of HI 21-cm absorption in high-redshift active galaxies, supporting the idea of a critical ionising photon rate that ionizes all neutral gas, with implications for galaxy evolution.

## Contribution

It provides observational evidence for a critical ionising luminosity threshold in active galaxies at z > 2.6, reinforcing the link between UV luminosity and neutral gas ionisation.

## Key findings

- Zero detections in 16 out of 25 sources surveyed.
- Expected about four detections based on lower-redshift data.
- Significant support (~7 sigma) for a critical ionising photon rate of logQ=56.

## Abstract

We report the results of a survey for HI 21-cm absorption at redshifts of z > 2.6 in a new sample of radio sources with the Green Bank and Giant Metrewave Radio Telescopes. From a total of 25 targets, we report zero detections in the 16 for which optical depth limits could be obtained. Based upon the detection rate for z > 0.1 associated absorption, we would expect approximately four detections. Of the 11 which have previously not been searched, there is sufficient source-frame optical/ultra-violet photometry to determine the ionising photon rate for four. Adding these to the literature, the hypothesis that there is a critical rate of logQ = 56 ionising photons per second is now significant at ~7 sigma. This reaffirms our assertion that searching z > 3 active galaxies for which optical redshifts are available selects sources in which the ultra-violet luminosity is sufficient to ionise all of the neutral gas in the host galaxy.