# H i Absorption in Low-power Radio AGNs Detected by FAST

Qingzheng Yu (余清正) Department of Astronomy, Xiamen University, Xiamen, Fujian
361005, People’s Republic of China;
fangt<EMAIL_ADDRESS>Taotao Fang (方陶陶) Department of
Astronomy, Xiamen University, Xiamen, Fujian 361005, People’s Republic of
China; fangt<EMAIL_ADDRESS>Junfeng Wang (王俊峰)
Department of Astronomy, Xiamen University, Xiamen, Fujian 361005, People’s
Republic of China; fangt<EMAIL_ADDRESS>Jianfeng Wu
(武剑锋) Department of Astronomy, Xiamen University, Xiamen, Fujian 361005,
People’s Republic of China; fangt<EMAIL_ADDRESS>
(Received November 19, 2022; Revised May 30, 2023; Accepted June 1, 2023)

###### Abstract

We report the discovery of three H i absorbers toward low-power radio active
galactic nuclei (AGNs) in a pilot H i absorption survey with the Five-hundred-
meter Aperture Spherical radio Telescope (FAST). Compared to past studies,
FAST observations have explored lower radio powers by $\sim$0.4 dex and
detected these weakest absorbers at given redshifts. By comparing the gas
properties and kinematics of sources along radio powers, we aim to explore the
interplay between AGN and the surrounding interstellar medium (ISM). Compared
to brighter sources at similar redshifts, our observations suggest a slightly
lower detection rate of H i absorption lines ($\sim$$11.5\%$) in low-power
radio AGNs with $\text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})=21.8-23.7$.
The low-power sources with $\text{log}(P_{\text{1.4 GHz}}/\text{W
Hz}^{-1})<23$ have a lower detection rate of $\sim$$6.7\%$. Due to the
incompleteness of the sample, these detection rates may represent the lower
limits. The selection of more extended sources and dilution by H i emission at
lower redshifts may contribute to the lower detection rate of H i absorption
lines. These detected absorbers present relatively narrow line widths and
comparable column densities consistent with previous observations. One
absorber has a symmetric profile with a large velocity offset, while the other
two show asymmetric profiles that can be decomposed into multiple components,
suggesting various possibilities of gas origins and kinematics. These H i
absorbers may have connections with rotating disks, gas outflows, galactic gas
clouds, gas fueling of the AGN, and jet-ISM interactions, which will be
further investigated with the upcoming systematic survey and spatially
resolved observations.

Active galactic nuclei (16); Radio spectroscopy (1359); Neutral hydrogen
clouds (1099);

††facilities: FAST††software: Astropy (Astropy Collaboration et al., 2013),
Scipy (Virtanen et al., 2020), LMFIT (Newville et al., 2014).

July 25, 2023

##  1 Introduction

The associated H i 21 cm absorption provides an important method to study the
physical conditions of active galactic nuclei (AGNs) and their interplay with
the interstellar medium (ISM) of host galaxies (for a review see Morganti &
Oosterloo, 2018). Previous observations of H i absorption lines in radio AGNs
show that the H i gas can be traced in regularly rotating large-scale disks or
circumnuclear disks (Gallimore et al., 1999; Struve & Conway, 2010; Morganti
et al., 2011). Kinematics of the H i absorption lines also have been used to
reveal infalling clouds correlated to the feeding of the supermassive black
hole (SMBH) (Morganti et al., 2009; Maccagni et al., 2014; Tremblay et al.,
2016), as well as outflows or galactic gas clouds indicating the interactions
between jets and surrounding ISM (Conway, 1999; Morganti et al., 2013; Aditya
& Kanekar, 2018; Schulz et al., 2021). Observations of H i absorption lines in
different types of AGNs can provide information on the evolution of AGNs and
host galaxies, e.g., some recent studies suggest the H i absorption is tightly
correlated to galaxy mergers (Dutta et al., 2018, 2019; Dutta & Srianand,
2022).

Although H i absorption has been studied for decades and more than 100 H i
absorbers have been detected at $z<1$ (Morganti & Oosterloo, 2018), our
understanding is still limited on the H i absorption and related evolution
processes in various classes of AGN, especially the faint low-power radio
AGNs. Due to the sensitivity limits, most H i 21 cm absorption surveys are
limited to bright sources with $S_{\rm 1.4GHz}>$ 50 mJy (Gallimore et al.,
1999; Geréb et al., 2015; Aditya, 2019). A recent survey carried out with the
Westerbork Synthesis Radio Telescope (WSRT) has expanded the sample to weaker
sources with $S_{\rm 1.4GHz}>$ 30 mJy (Maccagni et al., 2017) in the redshift
range $0.02<z<0.25$, which increases detections of H i absorption in low-power
radio AGNs ($\text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})<24$). This survey
has found a detection rate of $\sim$27% in the entire sample, and the
detection rate remains similar across the range of radio powers. Their results
indicate AGNs at the low-power range mostly show narrow absorption features,
which traced gas in rotating disks, while the broad H i absorption lines
(e.g., fast H i outflows) are only found in high-power radio AGNs or mergers.
For the low-power radio AGNs, the number of observed sources with
$\text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})<23$ is smaller than that of
brighter sources with $\text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})>25$ by
a factor of $\sim$4 (Maccagni et al., 2017; Murthy et al., 2021). It is
unclear whether these previous results reveal the nature of H i absorption in
low-power sources or have a bias due to the flux-limited sample. Furthermore,
larger samples and more observations of low-power radio sources are needed to
help understand what drives the observational differences of the H i
absorption properties between low-power and high-power sources. In particular,
comparisons among sources of all radio powers can provide critical information
to study the interplay between the radio activity of AGN and the surrounding
ISM. Thus, a systematic search of H i absorption lines in faint AGNs will help
understand the possible variations of detection rate, gas properties, and
kinematics along radio powers. In addition, further study with the faint AGN
sample will help explore the connection between the radio power of the AGN and
H i outflows/fueling. On the other hand, detailed observations with high
spatial resolution can provide information on the origins and impacts of H i
absorbing gas in low-power AGNs, which may help in investigating various
underlying physical processes of the H i absorptions and AGN activity between
low-power and high-power sources.

Constructed as the most sensitive single-dish radio telescope, the Five-
hundred-meter Aperture Spherical radio Telescope (FAST; Nan, 2006; Nan et al.,
2011; Jiang et al., 2020) has made significant progress in probing the
faintest H i gas in our Milky Way (Hong et al., 2022) and the nearby universe
(Zhu et al., 2021; Xu et al., 2022). FAST has also successfully carried out a
blind search and targeted observations of H i 21 cm absorption lines (Zhang et
al., 2021; Hu et al., 2023; Jing et al., 2023). Recently, Yu et al. (2022)
discovered a new H i absorber in an interacting galaxy pair observed by FAST.
The newly detected H i absorption line has a very faint radio continuum as
background (Yu et al., 2022), which shows the ability of FAST to search the H
i absorption toward the faint sources. Hence, we are carrying out a systematic
survey of H i absorption lines in low-power radio AGNs. In this paper, we
present our results from pilot observations of the survey.

This paper is organized as follows. In Section 2, we introduce the selection
of the low-power radio AGN sample, observations with FAST, and data reduction.
The main results are presented in Section 3, which includes the detection rate
of the H i absorption lines and properties of individual detections. In
Section 4, we further discuss the results of this study. Finally, we summarize
the main results in Section 5. Throughout this paper, we adopt $H_{0}=70$ km
s-1 Mpc-1, $\Omega_{M}=0.3$, and $\Omega_{\Lambda}=0.7$.

##  2 Observations and Data Reduction

The three H i absorbers were newly discovered through our pilot FAST PI
program (PT2021_0067 PI: Q. Yu). To investigate the H i absorption properties
of low-power radio AGNs, we selected the targets by cross-matching the NRAO
Very Large Array (VLA) Sky Survey (NVSS) catalog (Condon et al., 1998) and the
Sloan Digital Sky Survey Data Release 16 (SDSS DR16) catalog of galaxies
(Ahumada et al., 2020). We selected all sources that have radio flux of 10 mJy
$<S_{\rm 1.4GHz}<$ 30 mJy to expand the sample of radio AGNs presented in
Maccagni et al. (2017). We restricted the redshift of sources to $z<0.1$ to
avoid strong radio frequency interference (RFI). We crossmatched the selected
sources with the second data release of HI-MaNGA (Stark et al., 2021) and
Arecibo Legacy Fast Arecibo L-Band Feed Array Survey (ALFALFA) catalogs
(Haynes et al., 2018) to exclude the sources that already have H i 21 cm
emission line detections at S/N $>$ 5\. With the above constraints, we have
compiled a sample of 159 low-power radio AGNs. In our pilot observations, 29
sources in the radio power range of $\text{log }(P_{\text{1.4 GHz}}/\text{W
Hz}^{-1})=21.8-23.7$ were observed in 2021 August to 2022 May with the FAST
19-beam receiver using the ON-OFF mode. The OFF-target position was set to
11$\farcm$5 to avoid contamination of radio continuum sources. With a beam
size of $\sim$2$\farcm$9, the 19-beam receiver was configured with the wide-
band spectrometer (Spec(W)) to cover a band of 1.05$-$1.45 GHz with a spectral
resolution of $\sim$1.6 $\text{km s}^{-1}$. The integration time varies in the
range of 3$-$21 minutes for each source based on the flux of the radio
continuum at 1.4 GHz (Condon et al., 1998). Our observations are designed to
reach a depth comparable to those in previous surveys (Geréb et al., 2015;
Maccagni et al., 2017), which are sensitive enough to detect absorption lines
with a peak optical depth of $\tau_{\text{p}eak}\sim 0.08$.

Data were reduced with astropy (Astropy Collaboration et al., 2013) and scipy
(Virtanen et al., 2020). We first calibrated the spectrum with the injected
noise signal to derive the antenna temperature (Jiang et al., 2020). After
checking the consistency of polarization XX and YY for each spectrum, we
combined the two polarizations. Then the bandpass subtraction was applied with
calibrated ON-target and OFF-target spectra. The temperature of each spectrum
was converted to flux density based on the gain factor of different beams on
the receiver (see Table 5 of Jiang et al. 2020). We subtracted the baseline of
the spectrum by fitting a sinusoidal plus a polynomial function. RFI was
flagged manually during the subtraction process, and we excluded the RFI-
contaminated data in later works of spectral line fitting and measurement. The
velocity of the final spectrum was Doppler corrected and converted to the
barycentric frame. After data reduction, our observations achieved a median
rms noise of $\sim$0.63 mJy measured at the velocity resolution of $\sim$1.6
$\text{km s}^{-1}$. For the detections, we have rebinned the spectrum to
improve the S/N, and the final rms noise is $\sim$0.31$-$0.40 mJy at the
velocity resolution of $\sim$10$-$20 $\text{km s}^{-1}$ (Table 1). For the
non-detections, the median rms noise is 0.19 mJy at the velocity resolution of
$\sim$16 $\text{km s}^{-1}$. Although the longest integration time of
individual sources is less than a half hour, the average noise level of our
observations is lower than previous surveys (e.g., Geréb et al., 2015;
Maccagni et al., 2017) by a factor of 4 at similar spectral resolutions.

Figure 1: Comparison of the radio power and redshift between our observed
sources and that of Maccagni et al. (2017). Figure 2: Distributions of the
radio power of our sample and that of Maccagni et al. (2017).

In the following analysis, we also include an H i absorber in an interacting
galaxy pair J1558$+$2759 previously discovered by FAST (Yu et al., 2022) to
investigate the gas properties and origins in Section 4. This H i absorption
line has a low-power radio AGN as a background continuum source similar to
these three absorbers. Therefore, we discuss the H i gas properties using the
new FAST observations and previous data in Yu et al. (2022). However,
considering the different observing setups between the new and previous
observations, we only include the 29 sources in the pilot observations to
discuss the detection rate of H i absorption lines (Section 3).

Figure 3: Comparison of the detection rate between our sample and that of
Maccagni et al. (2017). The detection rates of each sample are divided into 4
bins based on the radio power. The 4 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) $\text{log}(P_{\text{1.4 GHz}}/\text{W
Hz}^{-1})\geqslant 25$.

In Figure 1, we plot the distributions of our observed sources and sources
detected by Maccagni et al. (2017) in the radio power-redshift space for
comparison. Compared to sources in Maccagni et al. (2017), the radio power of
our sample is systematically lower than that of AGNs at similar redshifts, and
the detected H i absorbers in our sample are among the weakest of detections
at given redshifts. The blue dashed lines marked the region of
$\text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})<23$ and $z<0.1$, which has
been less explored in previous surveys. In the radio power range of
$\text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})<23$, our observations have
increased the number of observed sources by a factor of 2. As shown in Figure
2, we present the histograms of the radio power range for our sample and that
of Maccagni et al. (2017). The radio power of our sample distributes in the
range of $\text{log }(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})=21.8-23.7$, and the
median value of our sample ($\text{log }(P_{\text{1.4 GHz}}/\text{W
Hz}^{-1})=22.9$) is lower than that of Maccagni et al. (2017) ($\text{log
}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})=24.2$). Briefly, through observations
of our sample, we aim to explore the H i absorption line in the faint radio
AGN population, which has been less investigated in previous observations.
Compared with previous observations (Geréb et al., 2015; Maccagni et al.,
2017), our FAST observations on average have explored lower radio powers by
$\sim$0.4 dex at given redshifts (Figure 1).

##  3 Results

The pilot observations presented in this work yield the detection of new H i
absorption lines in three galaxies with S/N $>4.5$. The detection of new H i
absorbers suggests our observations can reach the sensitivity limit of
absorption lines with the peak optical depth of $\tau_{\text{p}eak}\sim 0.04$.
Excluding three sources severely contaminated by the RFI, our pilot
observations detected H i absorption lines in three out of 26 sources,
suggesting a detection rate of H i absorption lines in low-power radio AGNs
that is $\sim$$11.5^{+9.2}_{-3.7}\%$ in the radio power range of $\text{log
}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})=21.8-23.7\ $. The detection rate is
relatively lower than that of previous surveys (Geréb et al., 2015; Maccagni
et al., 2017), but the difference is not significant ($<2\sigma$).

To investigate possible variations of the detection rate with the radio power,
we plot the detection rate of our sample and that of Maccagni et al. (2017)
along the radio power in Figure 3. The sources of each sample were divided
into 4 bins based on radio powers, and the detection rate was calculated for
each radio power bin. Since the sizes of subsamples in each bin are different
and some are relatively small, we estimate the 1$\sigma$ error bars based on
the binomial statistics of Cameron (2011). The results of Maccagni et al.
(2017) suggest the detection rate is similar along the entire range of radio
powers in their survey. In contrast, our sample shows a lower detection rate
in the radio power range of $\text{log}(P_{\text{1.4 GHz}}/\text{W
Hz}^{-1})<23$. As shown in Figure 3, the detection rate of sources with
$\text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})<23$ from Maccagni et al.
(2017) is higher than the detection rate of our sources
($\sim$$6.7^{+12.5}_{-2.2}\%$) by a factor of $\sim$4.6. Although the
difference between these two subsamples is not significant ($<3\sigma$)
considering the error bars, it is worth further investigating with larger
samples to confirm whether the low-power radio AGNs with
$\text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})<23$ have a lower detection
rate of H i absorption. In the radio power range of
$23\leqslant\text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})<24$, the detection
rate of our sample is consistent with that of Maccagni et al. (2017) within
the error bars. Our results indicate that the low-power sources with
$\text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})<23$ may have a lower
detection rate of H i absorption, and the detection rate increases and remains
similar as the radio power elevating.

Figure 4: The upper panels show SDSS composite images of each source. The FAST
beam size ($\sim$2$\farcm$9) is much larger than the size of these optical
images. The lower panels show the corresponding H i spectra of J1332$+$2634,
J1512$+$3050, and J1628$+$2529\. The velocity resolution of J1332$+$2634 and
J1512$+$3050 is $\sim$$10\ \text{km s}^{-1}$, while the spectrum of
J1628$+$2529 is rebinned to $\sim$$20\ \text{km s}^{-1}$. The red lines in
each panel show the profiles of Gaussian fits. On the bottom of each panel,
the residuals of the fit are plotted with the 1$\sigma$ noise values presented
with shaded regions.

As shown in Figure 4, two H i absorbers present multi-peak profiles, and each
peak is identified with an S/N $>3$. Therefore, we have performed a
multicomponent Gaussian fit to measure the properties of each absorption line.
In the multicomponent Gaussian models, we decided the number of components by
requiring the reduced $\chi^{2}$ to be closest to unity. We have checked the
Akaike information criterion (AIC) between a single Gaussian fit and a multi-
Gaussian fit, and the AIC suggests the multi-Gaussian model can better fit the
data. The $\chi^{2}$, degree of freedom (d.o.f.), and AIC of both single-
component and multicomponent Gaussian fits are listed in Table 2. The results
of multicomponent Gaussian fit revealed the asymmetric shape of the line
profiles, indicating the absorbing gas may have multiple velocity components.
However, these components still need to be confirmed with higher spectral
resolution and S/N. We summarize the measured parameters and the results of
multicomponent Gaussian fit in Table 1 and Table 3, respectively. The line
width $W_{20}$ is measured at 20% of peak flux density for each Gaussian fit
profile, and the line centroids of each absorption system are measured at the
center point of $W_{20}$. The column density is calculated with the following
equation:

${N_{\rm H\,{\textsc{i}}}}=1.823\times 10^{18}\ {T_{s}}\int\tau(v){\rm
d}v\text{ cm}^{-2},$ (1)

where ${T_{s}}$ represents the spin temperature in Kelvin, and $v$ is the
Doppler-corrected velocity in kilometer per second. The optical depth
$\tau(v)$ is derived by:

$\tau(v)=-\text{ln}\left(1+\frac{{S_{\rm H\,{\textsc{i}}}}}{c_{f}\
S_{\text{1.4 GHz}}}\right),$ (2)

where ${S_{\rm H\,{\textsc{i}}}}$ is the line flux of the H i absorption,
$S_{\text{1.4 GHz}}$ is the continuum flux at 1.4 GHz, and ${c_{f}}$ is the
covering factor of the gas. In this work, we assume the spin temperature
${T_{s}}$ = 100 K and the covering factor ${c_{f}}=1$ for each source, which
has been commonly adopted in previous studies (e.g., Geréb et al., 2015;
Maccagni et al., 2017; Chowdhury et al., 2020).

Table 1: Absorption line properties

Source name | R.A. | Decl. | $t_{\text{ON}}$ | $z$ | $S_{\text{1.4\ GHz}}$ | log($P_{\text{1.4\ GHz}}$) | rms | $\Delta v$ | $W_{20}$ | $v_{\text{centroid}}$ | $\tau_{\text{peak}}$ | $\tau_{\text{int}}$ | ${N_{\rm H\,{\textsc{i}}}}$
---|---|---|---|---|---|---|---|---|---|---|---|---|---
| deg. | deg. | (min) |  | (mJy) | (W $\text{Hz}^{-1}$) | (mJy) | ($\text{km s}^{-1}$) | ($\text{km s}^{-1}$) | ($\text{km s}^{-1}$) |  | ($\text{km s}^{-1}$) | ($10^{20}\text{cm}^{-2}$)
J1332$+$2634 | 203.19009 | 26.58038 | 5 | 0.04701 | 21.1 | 23.04 | 0.31 | 10 | 106.9 |  $-$26.8 | 0.091 $\pm$ 0.017 | 4.91 $\pm$ 0.67 | 8.95 $\pm$ 1.22
J1512$+$3050 | 228.02324 | 30.83984 | 6.5 | 0.08963 | 27.8 | 23.74 | 0.40 | 10 | 172.7 | 93.0 | 0.107 $\pm$ 0.019 | 8.81 $\pm$ 0.81 | 16.06 $\pm$ 1.48
J1558$+$2759aaThe detailed data and measurements of this source are adopted from Yu et al. (2022). | 239.75275 | 27.98537 | 13 | 0.05140 | 20.0 | 23.10 | 0.44 | 1.6 | 72.3 | 125.3 | 0.490 $\pm$ 0.050 | 14.50 $\pm$ 0.32 | 47.30 $\pm$ 1.01
J1628$+$2529 | 247.19221 | 25.49472 | 3.3 | 0.04010 | 25.2 | 22.98 | 0.38 | 20 | 131.6 |  $-$195.9 | 0.072 $\pm$ 0.017 | 6.46 $\pm$ 0.97 | 11.78 $\pm$ 1.77

Note. — The columns are (1) the source name of the AGN; (2) R.A. in degrees;
(3) Decl. in degrees; (4) integration time; (5) redshift from SDSS
spectroscopy (Abolfathi et al., 2018); (6) radio continuum flux at 1.4 GHz
from NVSS (Condon et al., 1998); (7) radio power at 1.4 GHz; (8) 1$\sigma$
noise measured at re-binned velocity resolution; (9) re-binned velocity
resolution; (10) line width measured at 20% of the peak flux; (11) line
centroid of the absorption; (12) peak optical depth; (13) integrated optical
depth; (14) estimated H i column density assuming $T_{s}=100$ K, $c_{f}=1$.

Table 2: Comparison of single and multiple Gaussian fits for the multi-peak H
i absorbers

Source name | $\chi^{2}_{\rm single}$ | $\rm d.o.f._{single}$ | $\rm AIC_{single}$ | $\chi^{2}_{\rm multi}$ | $\rm d.o.f._{multi}$ | $\rm AIC_{multi}$
---|---|---|---|---|---|---
J1332$+$2634 | 270.3 | 237 | 34.5 | 247.0 | 231 | 24.9
J1512$+$3050 | 136.0 | 100 | 34.6 | 98.7 | 91 | 19.6

Note. — The columns are (1) the source name of the AGN; (2) $\chi^{2}$ of the
single-component Gaussian fit; (3) the degree of freedom (d.o.f.) of the
single-component Gaussian fit; (4) Akaike information criterion (AIC) of the
single-component Gaussian fit; (5) $\chi^{2}$ of the multi-component Gaussian
fit; (6) d.o.f. of the multi-component Gaussian fit; (7) AIC of the multi-
component Gaussian fit.

###  3.1 J1332+2634

The source J1332$+$2634 is an isolated galaxy with a spectroscopic redshift of
$z=0.04701\pm 0.00002$ (Ahumada et al., 2020). The host galaxy is a star-
forming galaxy with a low stellar mass of
$\text{log}(M_{\star}/M_{\odot})=9.7$ and star formation rate (SFR) of 3.1
$M_{\odot}\ \text{yr}^{-1}$ (Salim et al., 2016). J1332$+$2634 is classified
as a Seyfert 2 AGN (Toba et al., 2014) and has a flux of 21.1 mJy at 1.4 GHz
(Condon et al., 1998). The corresponding radio power at 1.4 GHz is
$\text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})=23.04$.

The H i absorption line profile of J1332$+$2634 is shown in Figure 4, with the
velocity Doppler corrected based on the optical redshift of the host galaxy.
The detected H i absorption line presents a multi-peak profile, implying
unsettled kinematics of the absorbing gas. Fitted with a multicomponent
Gaussian function, the line profile is decomposed into three velocity
components centered at $-$72.0 $\text{km s}^{-1}$, $-$20.5 $\text{km s}^{-1}$,
and 10.0 $\text{km s}^{-1}$, respectively. Despite the shallower blueshifted
wing component, the velocity of the absorption system is in good agreement
with the optical redshift of the host galaxy, suggesting the dominant
components are likely tracing the circumnuclear rotating disk (Gallimore et
al., 1999). The blueshifted wing centered at $-$72.0 $\text{km s}^{-1}$ has a
narrow profile with an FWHM of 13.5 $\text{km s}^{-1}$. Adopting a 21.1 mJy
continuum flux density at 1.4 GHz (Condon et al., 1998), we calculate the peak
optical depth $\tau_{\text{peak}}=0.091\pm 0.017$ and the integrated optical
depth $\tau_{\text{int}}=4.91\pm 0.67$ $\text{km s}^{-1}$. Based on Equation
(1), the column density of this absorption system is $\sim 8.95\times 10^{20}\
\text{cm}^{-2}$.

###  3.2 J1512+3050

The source J1512$+$3050 is an isolated galaxy with the spectroscopic redshift
of $z=0.08963\pm 0.00002$ (Ahumada et al., 2020). The host galaxy is an
elliptical galaxy with a high stellar mass of
$\text{log}(M_{\star}/M_{\odot})=11.7$ (Salim et al., 2016). Classified as a
LINER-type AGN (Toba et al., 2014), J1512$+$3050 has a 1.4 GHz continuum flux
of 27.8 mJy (Condon et al., 1998), corresponding to a radio power at 1.4 GHz
of $\text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})=23.74$.

As shown in Figure 4, the H i absorption line profile of J1512$+$3050 is
Doppler corrected based on the optical redshift of the host galaxy. The
multiple-peak profile of the H i absorption line suggests the H i gas is
kinematically unsettled. A multicomponent Gaussian fit yields the peak optical
depth $\tau_{\text{peak}}=0.107\pm 0.019$ and the integrated optical depth
$\tau_{\text{int}}=8.81\pm 0.81$ $\text{km s}^{-1}$. The column density
${N_{\rm H\,{\textsc{i}}}}$ of this absorption system based on Equation (1) is
$\sim 1.61\times 10^{21}\ \text{cm}^{-2}$. The line profile is decomposed into
one broad component centered at 74.9 $\text{km s}^{-1}$, and three narrow
Gaussian components centered at 49.7 $\text{km s}^{-1}$, 120.2 $\text{km
s}^{-1}$, and 167.5 $\text{km s}^{-1}$, respectively. The fitting parameters
of each Gaussian component are listed in Table 3. The redshifted wings may
suggest the H i gas is clumpy and falling toward the AGN.

###  3.3 J1628+2529

The source J1628$+$2529 is a member galaxy in a galaxy group with a
spectroscopic redshift of $z=0.04010\pm 0.00001$ (Ahumada et al., 2020). The
host galaxy is an elliptical galaxy with a high stellar mass of
$\text{log}(M_{\star}/M_{\odot})=11.1$ (Salim et al., 2016). Classified as a
LINER-type AGN (Jeong et al., 2013), J1628$+$2529 has a 1.4 GHz continuum flux
of 25.2 mJy (Condon et al., 1998), corresponding to a radio power at 1.4 GHz
of $\text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})=22.98$.

As shown in Figure 4, the H i absorption line profile of J1628$+$2529 is
Doppler corrected and rebinned to a velocity resolution of 20 $\text{km
s}^{-1}$, and the zero-point is corrected to the optical redshift of the host
galaxy. Since this H i absorption line shows a single-peak profile, we apply a
single-component Gaussian fit to measure the peak optical depth
$\tau_{\text{peak}}=0.072\pm 0.017$ and the integrated optical depth
$\tau_{\text{int}}=6.46\pm 0.97$ $\text{km s}^{-1}$. Based on Equation (1),
the column density ${N_{\rm H\,{\textsc{i}}}}$ of this absorption system is
$\sim 1.18\times 10^{21}\ \text{cm}^{-2}$. The line centroid is blueshifted
$\sim-196\ \text{km s}^{-1}$ with respect to the systemic velocity of the host
galaxy. Although the line width of this absorber is relatively narrow with a
blueshifted centroid, it is likely the associated absorption of the AGN,
considering the high column density. Since the velocity of the absorption line
is significantly blueshifted, the H i absorption with high column density may
suggest the case of gas outflowing from the AGN.

Table 3: Gaussian fit parameters of the multi-peak H i absorbers

Source name | FWHM | $v_{\text{centroid}}$ | $\rm S/N_{peak}$ | $\tau_{\text{peak}}$ | $\tau_{\text{int}}$ | ${N_{\rm H\,{\textsc{i}}}}$
---|---|---|---|---|---|---
| ($\text{km s}^{-1}$) | ($\text{km s}^{-1}$) |  |  | ($\text{km s}^{-1}$) | ($10^{20}\text{cm}^{-2}$)
J1332$+$2634 | 22.5 | 10.0 | 5.9 | 0.075 | 1.79 $\pm$ 0.47 | 3.26 $\pm$ 0.85
| 21.0 | -20.5 | 5.9 | 0.085 | 1.89 $\pm$ 0.47 | 3.44 $\pm$ 0.85
| 13.5 | -72.0 | 3.0 | 0.046 | 0.67 $\pm$ 0.35 | 1.22 $\pm$ 0.63
J1512$+$3050 | 21.1 | 49.7 | 7.1 | 0.091 | 2.32 $\pm$ 0.58 | 4.23 $\pm$ 1.06
| 153.6 | 74.9 | - | 0.047 | 7.68 $\pm$ 1.36 | 14.00 $\pm$ 2.48
| 6.9 | 120.2 | 4.2 | 0.046 | 0.72 $\pm$ 0.41 | 1.31 $\pm$ 0.74
| 17.7 | 167.5 | 3.0 | 0.041 | 0.81 $\pm$ 0.57 | 1.48 $\pm$ 1.04

Note. — The columns are (1) the source name of the AGN; (2) full width at half
maximum; (3) line centroid of the absorption component; (4) the S/N of
identified peak in the spectrum; (5) peak optical depth; (6) integrated
optical depth; (7) estimated H i column density assuming $T_{s}=100$ K,
$c_{f}=1$.

##  4 Discussions

###  4.1 Detection Rate

The pilot observations of H i absorption lines suggest a detection rate of
$\sim$$11.5^{+9.2}_{-3.7}\%$ in the sample of 26 low-power radio sources, and
sources with $\text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})<23$ present a
lower detection rate of $\sim$$6.7^{+12.5}_{-2.2}\%$. Considering the
uncertainties, the detection rate of H i absorption in lower-power sources is
relatively lower than that of previous targeted surveys through direct
comparisons (van Gorkom et al., 1989; Emonts et al., 2010; Chandola et al.,
2011; Geréb et al., 2015; Maccagni et al., 2017). However, a fair comparison
of detection rates between surveys requires comparable detection limits and
considerations of selection bias.

Since the representative sample observed by WSRT (Geréb et al., 2014, 2015;
Maccagni et al., 2017) has overlapped regions with our sample in the radio
power-redshift space (see Figure 1), the comparison between our sample and the
WSRT sample under similar detection limits is reasonable and helpful to
investigate the detection rate of low-power sources. In Figure 5(C), we
presented the upper limits of peak optical depth ($\tau_{\text{peak}}$) for
non-detections in both our sample and the WSRT sample (Maccagni et al., 2017).
The upper limits of $\tau_{\text{peak}}$ were estimated with 3$\sigma$ rms
noise (Table A1) at the velocity resolution of $\sim$16 $\text{km s}^{-1}$
based on Equation (2). Our observations achieved a median $\tau_{\text{peak}}$
upper limit of $\sim$0.04, which is similar to that of the WSRT observations
($\sim$0.04, Maccagni et al., 2017). Thus, the detection limit of FAST
observations is similar to that of Maccagni et al. (2017).

Apart from the detection limit, selection effects can also significantly
impact the detection rate of H i absorption lines. Previous studies have
revealed that a large fraction of early-type galaxies harbors amounts of
detectable H i gas (Oosterloo et al., 2010; Serra et al., 2012). Therefore,
the elimination of gas-rich sources in the AGN sample will decrease the
detection probability of H i absorption and reduce a large fraction of the
parent population. In the selection of our parent sample, we excluded sources
that already have H i emission detections (S/N$>5$) to reduce redundant
observations, because FAST observations focus on searching H i absorption
lines to build up an H i absorption sample of faint radio AGNs. After
crossmatching with HI-MaNGA DR2 and ALFALFA catalogs, we excluded 19 out of
180 sources in the parent sample, corresponding to approximately two to three
potentially excluded sources in the pilot observations. The rest sources of
the parent sample were not observed by either HI-MaNGA or ALFALFA, and the
sample still contains a certain amount of gas-rich ($\sim$35%) galaxies based
on SDSS spectra. The potential exclusion of two to three H i-rich sources
could reduce the chance of detecting new H i absorption lines in the sample,
which may underestimate the detection rate. Therefore, the H i absorption
detection rate of $11.5^{+9.2}_{-3.7}\%$ can be considered as the lower limit
for our sample. For sources with $\text{log}(P_{\text{1.4 GHz}}/\text{W
Hz}^{-1})<23$, the H i absorption detection rate of $6.7^{+12.5}_{-2.2}\%$
could also represent the lower limit.

For our sample observed by FAST, the H i emission and the radio morphology of
sources may reduce the likelihood of detecting H i absorption. Since the H i
emission becomes more detectable at a lower redshift, the lower detection rate
of H i absorption may be partly due to the dilution by H i emission (Allison
et al., 2014; Curran & Duchesne, 2018). In the selection of the parent sample,
most of the excluded gas-rich sources distribute at lower redshifts
($z<0.03$), and their potential H i absorptions could be diluted by H i
emission. In addition, previous studies have found the detection rate of H i
absorption in compact sources is higher than that in extended sources (e.g.,
Maccagni et al., 2017). We followed Geréb et al. (2015) and Maccagni et al.
(2017) to inspect our sources, and the classification results suggest our
sample has a large fraction of extended radio sources ($\sim$65%, Table A1),
which may also lead to the observed lower detection rate. Our sample has nine
compact sources ($\sim$35%) with two detected absorptions, and one detection
out of 17 extended sources ($\sim$65%). The detection rate of the compact
sources in our sample ($22.2^{+18.3}_{-8.1}$%) is consistent with that of
Maccagni et al. (2017) within error bars. Considering the uncertainties, the
detection rate of the extended sources in our sample ($5.9^{+11.3}_{-1.9}$%)
is consistent with that of Maccagni et al. (2017). Therefore, the lower
detection rate of H i absorption in FAST observations may be due to the
dilution by H i emission at lower redshift and the selection of more extended
sources.

Thus, as a reasonable comparison of detection rates between our sample and the
WSRT sample, our observations suggest a lower detection rate of H i absorption
lines in the low-power radio sources. Due to the incompleteness of the sample,
these detection rates may represent the lower limits. The selection of more
extended sources and dilution by H i emission at lower redshift may contribute
to the lower detection rate of H i absorption lines. One caveat of this result
is the fact that the detection rate could be affected by low number
statistics, which will be further investigated with the complete survey of the
parent sample.

###  4.2 Kinematics and the Origin of the Absorbing Gas

To investigate the origin of H i 21 cm absorption, many studies have used high
spatial resolution interferometry observations to explore the detailed
kinematics and structures of the absorption. These observations have found
evidence of various probabilities that can form H i absorption in radio AGNs:
rotating gas disks (Gallimore et al., 1999; Morganti et al., 2008; Struve &
Conway, 2010), high-velocity clouds (HVCs; Conway, 1999; Struve & Conway,
2012), gas outflows (Morganti et al., 2013; Schulz et al., 2021), and gas
falling toward the supermassive black hole (Morganti et al., 2009; Maccagni et
al., 2014; Tremblay et al., 2016). Based on these observations, the origin and
related mechanisms of the H i absorption can be discussed by analyzing the
line profiles and kinematics of the absorbing gas. Alternatively, since the
absorption line traces the foreground gas along the line of sight of the
continuum source, the morphology and structure of the radio continuum can also
contribute to the observed absorption profiles (Murthy et al., 2021).

Figure 5: (A) Line width measured at 20% of the intensity ($W_{20}$) of the H
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)
Peak optical depth vs. the radio power of the sources. The interacting sources
of Maccagni et al. (2017) are marked with orange diamonds. (D) Integrated
optical depth vs. the radio power of the sources.

Figure 5(A) shows the $W_{20}$ of the H i absorption lines versus the radio
power of the sources. Maccagni et al. (2017) has found that broad lines
($W_{20}>400\ \text{km s}^{-1}$) were only detected in the interacting
galaxies at low radio power ($\text{log}(P_{\text{1.4 GHz}}/\text{W
Hz}^{-1})<24$). The line widths of the new H i absorbers detected by FAST are
relatively narrow and consistent with the results of Maccagni et al. (2017).
One of our sources is discovered in a merging galaxy pair (Yu et al., 2022),
and comparable narrow lines also have been detected in the interacting sources
(Maccagni et al., 2017; Dutta et al., 2019). In Figure 5(B), we plot the
centroid shift of the line with respect to the galaxy’s systemic velocity
versus the radio power of the sources. The fine dashed lines in blue show the
interval of $\pm 100\ \text{km s}^{-1}$. Maccagni et al. (2017) suggests the
majority of the lines detected in low-power sources are centered at the
systemic velocity ($|\Delta v|<100\ \text{km s}^{-1}$), while two of four in
our FAST detected sources are offset relative to the systemic velocity. For
the source J1512$+$3050, although the systemic centroid of the absorbing gas
is $\sim$93 $\text{km s}^{-1}$, it still has two narrow components redshifted
more than 100 $\text{km s}^{-1}$. Our detections have a high fraction of
sources with large velocity offset ($|\Delta v|>100\ \text{km s}^{-1}$), which
may suggest various kinematics and possible connections to the inflows and
outflows of the gas.

Previous observations suggest the H i 21cm absorption originated from rotating
disks generally have symmetric profiles and line widths of a few hundred
$\text{km s}^{-1}$, centered at or close to the systemic velocity (Gallimore
et al., 1999; Murthy et al., 2021). The absorption lines associated with gas
outflows usually show broad and asymmetric profiles, and the line centroid is
blueshifted from the systemic velocity (Morganti et al., 2013; Schulz et al.,
2021). Relatively narrow redshifted profiles may originate from gas clouds
falling into the SMBH (Morganti et al., 2009; Maccagni et al., 2014).
Alternatively, HVCs along the line of sight can also be observed as narrow H i
absorption lines (Conway, 1999; Araya et al., 2010). Besides, the structures
(e.g., core, jets, or lobes) of the background continuum source may partially
align with the H i gas, which can result in the multiple velocity components
and asymmetries observed in the absorption profiles (Peck & Taylor, 2001;
Struve et al., 2010; Murthy et al., 2021). In particular, for the radio AGN
with the typical two-jetted structure, since one jet is often behind the H i
gas, the jet-cloud alignment can produce asymmetric or velocity-shifted
absorption profiles even if the H i disk and radio continuum both have
symmetric morphologies.

The H i absorption profile of J1332$+$2634 shows a symmetric shape of the main
component centered at the systemic velocity of the host galaxy, which
indicates that a rotating disk very likely contributes the majority of
absorption. However, J1332$+$2634 presents a shallower narrow wing centered at
$-$72.0 $\text{km s}^{-1}$, with a column density of $\sim$$1.22\times
10^{20}\ \text{cm}^{-2}$. Since confirmed H i outflows generally present broad
profiles with large blueshifted velocity, the narrow wing of absorption in
J1332$+$2634 likely traces an intermediate velocity cloud (IVC). Additionally,
with spatially unresolved observations, we cannot rule out the scenario that
the observed wing component or asymmetric absorption profile may be due to
structures of the radio continuum, so the foreground H i gas could distribute
in a regular rotating disk.

The absorption profile of J1512$+$3050 presents an asymmetric multi-peak
shape, redshifted with respect to the systemic velocity of the host galaxy.
These two shallower redshifted components show narrow profiles with column
densities of $\sim$$1.31\times 10^{20}\ \text{cm}^{-2}$ and $\sim$$1.48\times
10^{20}\ \text{cm}^{-2}$, respectively. The redshifted wings may suggest gas
clouds falling onto the SMBH, which is correlated to the feeding and
triggering of AGN. Alternatively, clumpy HVCs along the line of sight may also
produce the presented absorption profile. Considering the possible partial
alignment between the continuum structures (e.g., lobes) and the H i cloud,
the warped H i could also present such features (Murthy et al., 2021).

The absorption profile of J1628$+$2529 shows a narrow blueshifted shape, and
the offset relative to systemic velocity is $-$195.9 $\text{km s}^{-1}$.
Although the line width of the absorption is relatively narrow compared to the
broad lines revealed by previous observations (e.g., Morganti et al., 2013;
Schulz et al., 2021), gas outflows from the AGN could be the possible origin.
In particular, observations at higher redshift have discovered a narrow
($\sim$40 $\text{km s}^{-1}$) blueshifted H i line that traced the outflows
driven by the radio jet (Aditya et al., 2017). As shown in Figure 5(B), low-
power radio sources ($\text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})<24$)
with large blueshifted velocity offsets are relatively rare compared to high-
power radio sources ($\text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})>24$).
Especially, for sources with $\text{log}(P_{\text{1.4 GHz}}/\text{W
Hz}^{-1})<23$, J1628$+$2529 is the only detection of sources with large
blueshifted velocity offsets ($\Delta v<-100\ \text{km s}^{-1}$). If the H i
absorption line of J1628$+$2529 is confirmed to be outflows, it could be the
lowest radio power source with outflows. Assuming the absorbing gas to be
outflows, we can estimate the mass outflow rate following Heckman (2002):

$\dot{M}_{\rm H\,{\textsc{i}}}\sim 30\frac{r_{\star}}{\rm kpc}\frac{{N_{\rm
H\,{\textsc{i}}}}}{\rm 10^{21}\ cm^{-2}}\frac{v}{\rm 300\ km\
s^{-1}}\frac{\Omega}{4\pi}\ M_{\odot}\ {\rm yr^{-1}},$ (3)

where the $r_{\star}$ is the radius of the outflow gas, ${N_{\rm
H\,{\textsc{i}}}}$ is the estimated column density assuming $T_{s}=100$ K,
$c_{f}=1$, $v$ is the velocity of the outflow gas, and $\Omega$ is the solid
angle of the gas assumed to be $\pi$. Since neither FAST nor VLA resolves the
compact continuum source, we assume a typical upper limit radius of
$r_{\star}=1$ kpc considering outflows generally appear to be limited to the
central kiloparsec scale (Morganti & Oosterloo, 2018). Therefore, we estimate
the mass outflow rate to be $\sim$5.8 $M_{\odot}\ {\rm yr^{-1}}$, with
${N_{\rm H\,{\textsc{i}}}}=1.18\times 10^{21}\ \text{cm}^{-2}$ and
$r_{\star}=1$ kpc. Alternatively, HVCs away from the nucleus could also result
in the blueshifted absorption feature. Besides, for a typical two-jetted radio
AGN, the jet-cloud alignment may give rise to the significantly blueshifted
profile when one jet is behind the rotating H i disk.

Similarly, the absorption profile of J1558$+$2759 is narrow and redshifted,
which can be contributed either by gas fueling or galactic gas clouds.
Observations of H i absorptions in mergers have detected significant systemic
velocity redshift, which suggests the signature of gas infall (Dutta et al.,
2019). Since J1558$+$2759 is an interacting source, gas driven by the merging
process could infall toward the AGN, which may connect with the gas fueling
onto the SMBH (Srianand et al., 2015). On the other hand, HVCs or a large-
scale gas disk along the line of sight can also be observed as redshifted
absorption lines.

Since FAST cannot resolve these sources, spatially resolved H i observations
by interferometry will help understand the kinematics and origin of the
absorbing gas, as well as possible jet-ISM interactions.

###  4.3 Uncertainties of the Systemic Velocity

To understand the origin of the absorbing gas, the important information of
gas kinematics depends on the comparison between the systemic velocity of the
galaxy and the velocity of H i gas. As revealed by previous studies (e.g.,
Morganti et al., 2001), the uncertainties of the redshift of the galaxy could
bias the derived systemic velocity and further affect our understanding of the
origin of the gas.

For our detected sources, the systemic velocities are derived by optical
redshift from SDSS spectroscopy. The 1$\sigma$ error of the redshift is $\sim
0.00002$, which corresponds to 6 $\text{km s}^{-1}$. Considering the spectral
resolution of the profile, the line centroid of J1332$+$2634 is $-26.8\pm
11.7$ $\text{km s}^{-1}$, which indicates the velocity offset of H i
absorption line is not significant with respect to the systemic velocity. The
source J1512$+$3050 has a velocity offset of $93.0\pm 11.7$ $\text{km
s}^{-1}$, which suggests the H i absorption line is significantly redshifted
as discussed above. The line centroid of J1628$+$2529 is $-195.9\pm 20.9$
$\text{km s}^{-1}$, indicating the H i absorbing gas is significantly
blueshifted relative to the systemic velocity. For the interacting source
J1558$+$2759, an older study claimed a spectroscopic redshift of $z=0.05174\pm
0.00027$ (Kim et al., 1995), which corresponds to a larger error of 81
$\text{km s}^{-1}$. If the older redshift is adopted to derive the systemic
velocity, the velocity offset of the H i absorption line becomes $23.4\pm
81.0$ $\text{km s}^{-1}$. The old observations with large uncertainties could
be limited by the previous instruments, while the SDSS dataset contains
spatially resolved MaNGA observations, which can alleviate uncertainties from
motions of emitting gas. Thus, the redshift from SDSS is likely more accurate
to derive the real systemic velocity, and the source J1558$+$2759 is
significantly redshifted with respect to the systemic velocity.

###  4.4 Optical Depth

In Figure 5(C) and (D), we show a comparison of the peak optical depth and the
integrated optical depth of our sources with that of Maccagni et al. (2017).
The peak optical depths of our sources are consistent with previous
observations, except for the interacting source J1558$+$2759, which presents
remarkably high $\tau_{\text{peak}}$. The high $\tau_{\text{peak}}$ value of
J1558$+$2759 is close to that of H i absorbers detected at higher redshift
(Chowdhury et al., 2020). We have marked the interacting sources of Maccagni
et al. (2017) with orange to check whether all the interacting sources show
higher $\tau_{\text{peak}}$ values. The mean $\tau_{\text{peak}}$ value of the
interacting sources ($0.14\pm 0.03$) is slightly higher than that of
noninteracting sources ($0.08\pm 0.01$). In addition, a recent survey of H i
absorption lines in the galaxy merger sample (Dutta et al., 2018, 2019) also
reveals a few sources with higher peak optical depth
($\tau_{\text{peak}}>0.4$). This provides a hint that galaxy interactions and
mergers may have contributions to the high value of $\tau_{\text{peak}}$.
Observations and simulations suggest galaxy interactions can induce tidal
torque to trigger the infall of diffuse ionized halo gas toward the central
region of the galaxy, which increases the density of the ionized gas and makes
the cooling more efficient (Braine & Combes, 1993; Moreno et al., 2019). In
this scenario, the ionized gas will turn into atomic gas, and the external
pressure (e.g., shocks) induced by interactions will accelerate the transition
from atomic to molecular gas (Barnes, 2004; Kaneko et al., 2017), which would
create a dense environment. Indeed, previous observations of H i absorption
lines have detected significant high ${N_{\rm H\,{\textsc{i}}}}$ in mergers
(Dutta et al., 2019), which may result in the high $\tau_{\text{peak}}$ of the
interacting sources. For the integrated optical depth, the results of our
detections are consistent with those of Maccagni et al. (2017).

##  5 Summary

We present pilot observations of H i absorption search toward low-power radio
AGNs by FAST. With the high sensitivity of FAST, we discovered three new H i
absorbers at a very weak flux level. Our pilot observations suggest the
detection rate of H i absorption lines in our sample is
$\sim$$11.5^{+9.2}_{-3.7}\%$. Considering the uncertainties, the detection
rate is slightly lower than that of brighter sources, but the difference is
not significant ($<2\sigma$). Our results indicate that the low-power sources
with $\text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})<23$ may have a lower
detection rate of H i absorption, and the detection rate increases and remains
similar as the radio power elevating. Due to the incompleteness of the sample,
these detection rates may represent the lower limits. The selection of more
extended sources and dilution by H i emission at lower redshift may contribute
to the lower detection rate of H i absorption lines. These detected absorbers
present relatively narrow line widths and comparable column densities as
revealed by previous observations of low-power radio AGNs. One absorber has a
symmetric profile with a large velocity offset, while the other two absorbers
show asymmetric profiles that can be decomposed into multiple components,
suggesting various possibilities of gas kinematics and origins. These
absorbers may have connections with rotating disks, gas outflows, galactic gas
clouds, gas fueling of the AGN, and jet-ISM interactions, which needs to be
confirmed or ruled out by spatially resolved observations. The upcoming
systematic survey and high spatial resolution observations of the H i
absorbers in low-power AGNs will shed new light on our understanding of
multiple evolution processes of the faint radio AGN population.

We thank the anonymous referee for the careful reading and helpful comments
that improved the paper. Q.Y. appreciates the helpful suggestions by Dr.
Mouyuan Sun, and observing support from Dr. Bo Zhang. This work is supported
by the National Key R&D Program of China under No. 2017YFA0402600, and the
National Natural Science Foundation of China under Nos. 11890692, 12133008,
12221003. Junfeng Wang acknowledges support by the NSFC grants Nos. U1831205
and 12033004. We acknowledge the science research grants from the China Manned
Space Project with Nos. CMS-CSST-2021-A04, CMS-CSST-2021-A05, CMS-
CSST-2021-A06, CMS-CSST-2021-B02. This work made use of the data from FAST
(Five-hundred-meter Aperture Spherical radio Telescope). FAST is a Chinese
national mega-science facility, operated by National Astronomical
Observatories, Chinese Academy of Sciences.

##  Appendix A Summary Table of Non-detections

We summarized the ancillary information of the H i absorption non-detections
in Table A1.

Table A1:  H i Absorption Non-detections

Source name | R.A. | Decl. | $t_{\text{ON}}$ | $z$ | $S_{\text{1.4\ GHz}}$ | log($P_{\text{1.4\ GHz}}$) | rms | $\tau_{\text{peak}}$ | Radio type
---|---|---|---|---|---|---|---|---|---
| deg. | deg. | (min) |  | (mJy) | (W $\text{Hz}^{-1}$) | (mJy) |  |
J0312$-$0004 | 48.01039 |  $-$0.07847 | 12.3 | 0.03762 | 14.7 | 22.68 | 0.19 |  $<$0.040 | E
J0322$-$0000 | 50.63664 |  $-$0.00121 | 12.3 | 0.02175 | 17.2 | 22.27 | 0.18 |  $<$0.032 | E
J0352$-$0620 | 58.05244 |  $-$6.34894 | 10.7 | 0.03295 | 12.5 | 22.50 | 0.22 |  $<$0.044 | E
J0406$-$0506 | 61.68081 |  $-$5.11472 | 12.3 | 0.06600 | 14.2 | 23.18 | 0.21 |  $<$0.045 | E
J0735$+$4212 | 113.98199 | 42.20336 | 12.3 | 0.08822 | 18.5 | 23.56 | 0.20 |  $<$0.033 | C
J0746$+$3029 | 116.63530 | 30.49075 | 12.3 | 0.05704 | 16.3 | 23.10 | 0.20 |  $<$0.038 | E
J0839$+$2308 | 129.96150 | 23.14335 | 21.3 | 0.02520 | 12.3 | 22.25 | 0.15 |  $<$0.037 | C
J0850$+$5522 | 132.59502 | 55.37874 | 5.0 | 0.03076 | 20.0 | 22.64 | 0.30 |  $<$0.046 | E
J0903$+$4026 | 135.75897 | 40.43399 | 12.3 | 0.02877 | 15.5 | 22.47 | 0.18 |  $<$0.029 | C
J0920$+$0102 | 140.00904 | 1.03830 | 21.3 | 0.01703 | 10.7 | 21.84 | 0.17 |  $<$0.052 | E
J1022$+$3634 | 155.58076 | 36.58304 | 5.3 | 0.02592 | 13.4 | 22.31 | 0.24 |  $<$0.055 | E
J1106$+$4602 | 166.65570 | 46.03878 | 6.2 | 0.02501 | 16.3 | 22.37 | 0.23 |  $<$0.043 | E
J1120$+$5049 | 170.16647 | 50.82729 | 5.0 | 0.02762 | 24.8 | 22.64 | 0.40 |  $<$0.050 | E
J1124$+$4708 | 171.24463 | 47.14351 | 6.2 | 0.05372 | 10.5 | 22.86 | 0.23 |  $<$0.068 | C
J1140$+$4632 | 175.19730 | 46.54049 | 12.3 | 0.05359 | 14.9 | 23.01 | 0.18 |  $<$0.037 | E
J1153$+$5241 | 178.37742 | 52.68945 | 2.5 | 0.07164 | 23.0 | 23.46 | 0.45 |  $<$0.060 | E
J1340$+$2629 | 205.23577 | 26.48672 | 5.0 | 0.07502 | 26.0 | 23.55 | 0.25 |  $<$0.029 | C
J1423$+$4015 | 215.96472 | 40.25883 | 21.3 | 0.08220 | 12.1 | 23.31 | 0.15 |  $<$0.038 | E
J1445$+$5134 | 221.43798 | 51.58082 | 16.0 | 0.02963 | 11.5 | 22.37 | 0.19 |  $<$0.051 | C
J1539$+$4438 | 234.87364 | 44.64845 | 6.5 | 0.07299 | 20.6 | 23.43 | 0.32 |  $<$0.048 | C
J1544$+$4330 | 236.21348 | 43.51406 | 21.3 | 0.03664 | 11.4 | 22.55 | 0.14 |  $<$0.038 | E
J1624$+$2507 | 246.09814 | 25.13012 | 16.0 | 0.09603 | 12.6 | 23.47 | 0.21 |  $<$0.051 | E
J2128$+$0017 | 322.04858 | 0.29988 | 21.3 | 0.05250 | 15.2 | 23.00 | 0.18 |  $<$0.036 | E

Note. — The columns are (1) the source name of the AGN; (2) R.A. in degrees;
(3) Decl. in degrees; (4) integration time; (5) redshift from SDSS
spectroscopy (Abolfathi et al., 2018); (6) radio continuum flux at 1.4 GHz
from NVSS (Condon et al., 1998); (7) radio power at 1.4 GHz; (8) 1$\sigma$
noise measured at the velocity resolution of 16 $\text{km s}^{-1}$; (9) upper
limit of the peak optical depth estimated with 3$\sigma$ rms; (10) radio
morphology classification as compact (C) and extended (E).

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