A Giant Metrewave Radio Telescope search for associated H Ι 21 cm absorption in high-redshiftflat-spectrum sources

We report results from a Giant Metrewave Radio Telescope search for `associated' redshifted H (Iota) 21 cm 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 approximate to 0.01 at a velocity resolution of approximate to 30 km s(-1). A single tentative absorption detection was obtained at z approximate to 3.530 towards TXS 0604+728. If confirmed, this would be the highest redshift at which H (Iota) 21 cm absorption has ever been detected. Including 29 CJF sources with searches for redshifted H (Iota) 21 cm 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 approximate to 3 sigma significance) that the strength of H (Iota) 21 cm 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 H (Iota) 21 cm absorption in a uniformly selected AGN sample. However, the two-sample test also finds that the H (Iota) 21 cm absorption strength is higher in AGNs with low ultraviolet or radio luminosities, at approximate to 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 H (Iota) 21 cm opacities in high-redshift, high-luminosity AGNs.