A FAST Survey of H I Absorption in Low-power Radio Sources

Abstract We conducted a H i 21 cm absorption study of a sample of 147 nearby ( z < 0.1) low-power radio sources with 10 mJy < S 1.4 GHz < 30 mJy and log ( P 1.4 GHz / W Hz − 1 ) = 20.5 – 23.7 , using the Five-hundred-meter Aperture Spherical radio Telescope. By investigating the origin and kinematics of H i absorbing gas, we aim to study the interplay between the active galactic nucleus (AGN) and its surrounding interstellar medium. Our observations detect 12 new absorbers, combining results from the pilot survey (three absorbers out of 26 sources), yielding a detection rate of ∼ 10 . 2 − 2.0 + 3.1 % . The detection rate in our sample is lower than in higher-power samples, which is likely due to emission dilution and the dominance of extended sources, indicating a gas-rich and star-forming-dominated population in low-power sources. Among new detections, most line profiles are narrow and show velocities close to systemic ones, consistent with rotating disks, while four show disturbed kinematics indicative of inflows or outflows. The fraction of outflow candidates rises with radio power, while the fraction of inflow ones remains constant, suggesting the effect of radio emission on driving H i outflows. In our sample, compact sources show a higher H i detection rate than extended sources. Contrary to expectations from higher-power samples, MIR-bright sources at low-power radio do not exhibit a higher H i detection rate or more disturbed kinematics. In low-power radio sources, blueshifted absorption occurs only in Seyferts and low-ionization nuclear emitting regions, indicating the connection between atomic outflows and the ionization state of AGN.