We report results from a Giant Metrewave Radio Telescope search for "associated" redshifted HI 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 3σ optical depth detection limits of ≈ 0.01 at a velocity resolution of ≈ 30 km s −1 . A single tentative absorption detection was obtained at z ≈ 3.530 towards TXS 0604+728. If confirmed, this would be the highest redshift at which HI 21 cm absorption has ever been detected.Including 29 CJF sources with searches for redshifted HI 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 ≈ 3σ significance) that the strength of HI 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 HI 21 cm absorption in a uniformly selected AGN sample. However, the two-sample test also finds that the HI 21 cm absorption strength is higher in AGNs with low ultraviolet or radio luminosities, at ≈ 3.4σ 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 21 cm opacities in high-redshift, high-luminosity active galactic nuclei.

We report the first detections of associated Hi 21 cm absorption in Gigahertz-peakedspectrum (GPS) sources at high redshifts, z > 1, using the Giant Metrewave Radio Telescope (GMRT). Our GMRT search for associated Hi 21 cm absorption in a sample of 12 GPS sources yielded two new detections of absorption, towards TXS 1200+045 at z = 1.226 and TXS 1245−197 at z = 1.275, and five non-detections. These are only the sixth and seventh detections of associated Hi 21 cm absorption in active galactic nuclei (AGNs) at z > 1. Both Hi 21 cm absorption profiles are wide, with velocity spans between nulls of ≈ 600 km s −1 (TXS 1200+045) and ≈ 1100 km s −1 (TXS 1245−197). In both absorbers, the large velocity spread of the absorption and its blueshift from the AGN, suggests that it arises in outflowing neutral gas, perhaps driven by the radio jets to high velocities. We derive mass outflow rates ofṀ ≈ 32 M ⊙ yr −1 (TXS 1200+045) andṀ ≈ 18 M ⊙ yr −1 (TXS 1245−197), comparable to the mass outflow rates seen earlier in low-redshift active galactic nuclei.