Redshift evolution of the HI detection rate in radio-loud active galactic nuclei

TL;DR

This study investigates the evolution of HI 21 cm absorption in radio-loud AGNs at redshifts 0.7 to 1, finding weaker absorption signals at higher redshifts likely due to changing physical conditions of HI gas.

Contribution

First to compare HI 21 cm absorption in extended radio sources at intermediate redshifts with low-redshift samples, revealing redshift evolution in HI properties.

Findings

No HI 21 cm absorption detected in the sample.

Statistically significant weaker HI absorption at 0.7<z<1 compared to z<0.25.

Redshift evolution likely causes changes in HI column density or spin temperature.

Abstract

We present a search for associated HI 21 cm absorption in a sample of 29 radio-loud active galactic nuclei (AGNs) at $0.7 < z < 1$, carried out with the upgraded Giant Metrewave Radio Telescope. We detect HI 21 cm absorption against none of our target AGNs, obtaining $3\sigma$ upper limits to the optical depth of $\lesssim$ 1% per 50 km s$^{-1}$ channel. The radio luminosity of our sources is lower than that of most AGNs searched for HI 21 cm absorption at similar redshifts in the literature, and, for all targets except two, the UV luminosity is below the threshold $10^{23}$ W Hz$^{-1}$, above which the HI in the AGN environment has been suggested to be completely ionised. We stacked the HI spectra to obtain a more stringent limit of $\approx 0.17$% per 50 km s$^{-1}$ channel on the average HI 21 cm optical depth of the sample. The sample is dominated by extended radio sources, 24 of which are extended on scales of tens of kiloparsecs. Including similar extended sources at $0.7 < z < 1.0$ from the literature, and comparing with a low-$z$ sample of extended radio sources, we find statistically significant ($\approx 3\sigma$) evidence that the strength of HI 21 cm absorption towards extended radio sources is weaker at $0.7<z<1.0$ than at $z < 0.25$, with a lower detection rate of HI 21 cm absorption at $0.7 < z < 1.0$. Redshift evolution in the physical conditions of HI is the likely cause of the weaker associated HI 21 cm absorption at high redshifts, due to either a low HI column density or a high spin temperature in high-$z$ AGN environments.