Issue |
A&A
Volume 659, March 2022
|
|
---|---|---|
Article Number | A185 | |
Number of page(s) | 14 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/202142550 | |
Published online | 25 March 2022 |
Redshift evolution of the H I detection rate in radio-loud active galactic nuclei
1
Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands
2
ASTRON, the Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
3
Joint Institute for VLBI ERIC, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
e-mail: murthy@jive.eu
4
National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Pune University, Pune 411007, India
Received:
29
October
2021
Accepted:
30
December
2021
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.
Key words: galaxies: active / radio lines: galaxies / galaxies: ISM
© ESO 2022
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