FAST survey of H I and OH absorption towards extragalactic radio sources

Neutral atomic hydrogen and molecular gas in the host galaxies of radio active galactic nuclei (AGN) 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 (FAST) towards 40 radio sources of low-intermediate radio luminosity ( ∼ 10 23 -10 26 W Hz −1 at 1.4 GHz), red mid-infrared color (W2[4.6 μ m] − W3[12 μ m] > 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 towards 8 sources, 5 of which are new detections including 4 in the redshift range 0.25 to 0.35. Our detection rates are consistent with our previous results with dependence on the star-formation history of the host galaxy reflected in the mid-infrared \textit{WISE} W2 − W3 colors and the compactness of the radio source. We find no significant dependence of detection rates on radio luminosity or redshift. We also find that H I column densities are anti-correlated with the low-frequency spectral indices ( α1.4GHz150MHz , Sν∝ν−α ). We do not have any detection from 23 sources with good data at OH observing frequencies. However, by stacking the spectra we estimate the 3 σ upper limit of OH column density to be 2.27 × 10 14 Tex /10 K × 1/ fc cm −2 . By stacking the OH spectra for 7 associated H I absorbers, we get a 3 σ upper limit of 3.47 × 10 14 Tex /10 K × 1/ fc cm −2 on OH column density and 1.78 × 10 −7 on [OH]/[H I] ratio.