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

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