FAST Survey of H i and OH Absorption toward Extragalactic Radio Sources

Neutral atomic hydrogen and molecular gas in the host galaxies of radio active galactic nuclei can be traced using H i 21 cm and OH-1667 MHz absorption lines to understand the fueling and feedback processes. We present the results of an H i and OH absorption survey with the Five-hundred-meter Aperture Spherical radio Telescope toward 40 radio sources of low-intermediate radio luminosity (∼10²³-10²⁶ W Hz⁻¹ at 1.4 GHz), red mid-infrared color (W2[4.6 μm]-W3[12 μm] > 2.5 mag), and redshift up to 0.35. From 13 sources with good data at H i observing frequencies, we report the detection of H i absorption toward eight sources, five of which are new detections, including four in the redshift range 0.25-0.35. Our detection rates are consistent with our previous results, with the dependence on the star formation history of the host galaxies reflected in the mid-infrared Wide-field Infrared Survey Explorer W2−W3 colors and the compactness of the radio sources. We find no significant dependence of the detection rates on radio luminosity or redshift. We also find that the H i column densities are anticorrelated with the low-frequency spectral indices ( α 150 MHz 1.4 GHz , S _ν ∝ ν ^−α ). We do not have any detections from 23 sources with good data at OH observing frequencies. However, by stacking the spectra, we estimate the 3σ upper limit of the OH column density to be 2.27 × 10¹⁴ T _ex/10 K × 1/f _c cm⁻². By stacking the OH spectra for seven associated H i absorbers, we get a 3σ upper limit of 3.47 × 10¹⁴ T _ex/10 K × 1/f _c cm⁻² on the OH column density and 1.78 × 10⁻⁷ on the [OH]/[H i] ratio.