摘要：We present a search for associated H i 21 cm absorption in a sample of 29 radio-loud active galactic nuclei (AGNs) at 0.7 < z < 1, carried out with the upgraded Giant Metrewave Radio Telescope. We detect H i 21 cm absorption against none of our target AGNs, obtaining 3σ upper limits to the optical depth of .1% per 50 km s−1 channel. The radio luminosity of our sources is lower than that of most AGNs searched for H i 21 cm absorption at similar redshifts in the literature, and, for all targets except two, the UV luminosity is below the threshold 1023 W Hz−1 , above which the H i in the AGN environment has been suggested to be completely ionised. We stacked the H i 21 cm spectra to obtain a more stringent limit of ≈0.17% per 50 km s−1 channel on the average H i 21 cm optical depth of the sample. The sample is dominated by extended radio sources, 24 of which are extended on scales of tens of kiloparsecs. Including similar extended sources at 0.7 < z < 1.0 from the literature, and comparing with a low-z sample of extended radio sources, we find statistically significant (≈3σ) evidence that the strength of H i 21 cm absorption towards extended radio sources is weaker at 0.7 < z < 1.0 than at z < 0.25, with a lower detection rate of H i 21 cm absorption at 0.7 < z < 1.0. Redshift evolution in the physical conditions of H i is the likely cause of the weaker associated H i 21 cm absorption at high redshifts, due to either a low H i column density or a high spin temperature in high-z AGN environments.
关键词：galaxies: active;radio lines: galaxies;galaxies: ISM
