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 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 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 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 (α_ 150 MHz^ 1.4 GHz, S_ν∝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 3σ upper limit of OH column density to be 2.27×10^14T_ ex/10 K ×1/f_ c cm^-2. By stacking the OH spectra for 7 associated H I absorbers, we get a 3σ upper limit of 3.47×10^14 T_ ex/10 K ×1/f_ c cm^-2 on OH column density and 1.78×10^-7 on [OH]/[H I] ratio.