A Giant Metrewave Radio Telescope survey for associated H I 21 cm absorption in the Caltech-Jodrell flat-spectrum sample

We report a Giant Metrewave Radio Telescope survey for associated H I 21-cm absorption from 50 active galactic nuclei (AGNs), at z approximate to 0.04-3.01, selected from the Caltech-Jodrell Bank flat-spectrum (CJF) sample. Clean spectra were obtained towards 40 sources, yielding two new absorption detections, at z = 0.229 towards TXS 0003+380 and z = 0.333 towards TXS 1456+375, besides confirming an earlier detection, at z = 1.277 towards TXS 1543+480. There are 92 CJF sources, at 0.01 less than or similar to z less than or similar to 3.6, with searches for associated H I 21-cm absorption, by far the largest uniformly selected AGN sample with searches for such absorption. We find weak (approximate to 2 sigma) evidence for a lower detection rate of H i 21-cm absorption at high redshifts, with detection rates of 28(_8)(+10) per cent and 7(_4)(+6) per cent in the low-z (z < z(med)) and high-z (z > z (med)) sub-samples, respectively. We use two-sample tests to find that the strength of the H I 21-cm absorption in the AGNs of our sample depends on both redshift and AGN luminosity, with a lower detection rate and weaker absorption at high redshifts and high ultraviolet/radio AGN luminosities. Unfortunately, the luminosity bias in our sample, with high-luminosity AGNs arising at high redshifts, implies that it is not currently possible to identify whether redshift evolution or AGN luminosity is the primary cause of the weaker absorption in high-z, high-luminosity AGNs. We find that the strength of H I 21-cm absorption does not depend on AGN colour, suggesting that dust extinction is not the main cause of reddening in the CJF sample.