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

Abstract

We conducted a HI 21cm absorption study of a sample of 147 nearby (z < 0.1) low-power radio sources with $10\,\mathrm{mJy} < S_{1.4\,\mathrm{GHz}} < 30\,\mathrm{mJy}$ and $\log(P_{1.4\,\mathrm{GHz}}/\mathrm{W\,Hz^{-1}}) = 20.5-23.7$, using the Five-hundred-meter Aperture Spherical radio Telescope. By investigating the origin and kinematics of HI 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 $\sim10.2^{+3.1}_{-2.0}\%$. 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 HI outflows. In our sample, compact sources show a higher HI detection rate than extended sources. Contrary to expectations from higher-power samples, MIR-bright sources at low-power radio do not exhibit a higher HI 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.

Figures (8)

• Figure 1: Left: distribution of radio power vs. redshift for our sample and the sample of 2017AA...604A..43M. Right: histograms of the radio power for our sample and the sample of 2017AA...604A..43M.
• Figure 2: Comparison of the detection rate between our sample and that of 2017AA...604A..43M, and the overall detection rates of the two samples. The detection rates of each sample are divided into four bins based on the radio power. The four bins are (1) $\text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})<23$; (2) $23\leqslant \text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})<24$; (3) $24\leqslant \text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})<25$; (4) $\text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})\geqslant25$.
• Figure 3: (a) Full width measured at 20% of the intensity ($W_{20}$) of the $\text{H\,{\sc i}}$ profiles vs. the radio power of the sources. (b) Line centroid offset with respect to the systemic velocity vs. the radio power of the sources. The fine dashed lines in blue show the interval of $\pm 100\ \text{km s}^{-1}$. (c) Integrated optical depth vs. the radio power of the sources. (d) Peak optical depth vs. the radio power of the sources.
• Figure 4: The number of detected absorbers, redshifted absorbers and blueshifted absorbers in our sample, and the WSRT sample 2017AA...604A..43M, together with the fractions of redshift/blueshift rates among the detected absorbers. Left: as a function of radio power bins; Middle: according to the WISE color-color classification; Right: according to the BPT diagram classification.
• Figure 5: Sources are classified according to the extension of their radio continuum. (a) Full width measured at 20% of the intensity ($W_{20}$) of the $\text{H\,{\sc i}}$ profiles vs. the radio power of the sources. (b) Line centroid offset with respect to the systemic velocity vs. the radio power of the sources. The fine dashed lines in blue show the interval of $\pm 100\ \text{km s}^{-1}$.