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

Abstract

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 ($\sim$10${}^{23}$-10${}^{26}$ W Hz${}^{-1}$ at 1.4 GHz), red mid-infrared color (W2[4.6 $\hat{I}¼$m]$-$W3[12 $\hat{I}¼$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 ($\hat{I}{\pm }_{150MHz}^{1.4GHz}$, $S_{\hat{I}}½\propto \hat{I}{½}^{-\hat{I}\pm }$). 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$\ddot{I}ƒ$ upper limit of OH column density to be 2.27$\times$10${}^{14}$$T_{\mathrm{ex}}$/10 K $\times$1/$f_{\mathrm{c}}$ cm${}^{-2}$. By stacking the OH spectra for 7 associated H I absorbers, we get a 3$\ddot{I}ƒ$ upper limit of 3.47$\times$10${}^{14}$ $T_{\mathrm{ex}}$/10 K $\times$1/$f_{\mathrm{c}}$ cm${}^{-2}$ on OH column density and 1.78$\times$10${}^{-7}$ on [OH]/[H I] ratio.

20 pages, 8 figures, accepted for publication in The Astrophysical Journal (ApJ)