A Giant Metrewave Radio Telescope search for associated HI 21cm absorption in high-redshift flat-spectrum sources

TL;DR

This study used the GMRT to search for HI 21cm absorption in high-redshift AGNs, finding weaker absorption at higher redshifts and in more luminous sources, indicating possible evolution or luminosity effects.

Contribution

First statistically significant evidence of redshift evolution in HI 21cm absorption strength in a uniformly selected AGN sample, highlighting the impact of luminosity and redshift.

Findings

Weak HI 21cm absorption in high-redshift AGNs

Potential detection of highest-redshift HI 21cm absorption

Absorption strength correlates with luminosity and redshift

Abstract

We report results from a Giant Metrewave Radio Telescope search for "associated" redshifted HI 21cm absorption from 24 active galactic nuclei (AGNs), at $1.1 < z < 3.6$, selected from the Caltech-Jodrell Bank Flat-spectrum (CJF) sample. 22 out of 23 sources with usable data showed no evidence of absorption, with typical $3\sigma$ optical depth detection limits of $\approx 0.01$ at a velocity resolution of $\approx 30$~km~s$^{-1}$. A single tentative absorption detection was obtained at $z \approx 3.530$ towards TXS0604+728. If confirmed, this would be the highest redshift at which HI 21cm absorption has ever been detected. Including 29 CJF sources with searches for redshifted HI 21cm absorption in the literature, mostly at $z < 1$, we construct a sample of 52 uniformly-selected flat-spectrum sources. A Peto-Prentice two-sample test for censored data finds (at $\approx 3\sigma$ significance) that the strength of HI 21cm absorption is weaker in the high-$z$ sample than in the low-$z$ sample, this is the first statistically significant evidence for redshift evolution in the strength of HI 21cm absorption in a uniformly selected AGN sample. However, the two-sample test also finds that the HI 21cm absorption strength is higher in AGNs with low ultraviolet or radio luminosities, at $\approx 3.4 \sigma$ significance. The fact that the higher-luminosity AGNs of the sample typically lie at high redshifts implies that it is currently not possible to break the degeneracy between AGN luminosity and redshift evolution as the primary cause of the low HI 21cm opacities in high-redshift, high-luminosity active galactic nuclei.