FASHI 21 cm absorption galaxies
Zhang et al.

FASHI: A blind survey of 21 cm absorption galaxies with FAST

Chuan-Peng Zhang

0000-0002-4428-3183]Chuan-Peng Zhang
National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
Guizhou Radio Astronomical Observatory, Guizhou University, Guiyang 550000, China

Ming Zhu
Peng Jiang
National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
Guizhou Radio Astronomical Observatory, Guizhou University, Guiyang 550000, China

Cheng Cheng
Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing 100101, China

Jin-Long Xu
Nai-Ping Yu
Xiao-Lan Liu
Bo Zhang
National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
Guizhou Radio Astronomical Observatory, Guizhou University, Guiyang 550000, China

The FAST All Sky H I survey (FASHI) is broader in frequency band and deeper in detection sensitivity than most of previous surveys. FASHI is designed to cover the entire sky observable by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Based on the FASHI data, we perform a blind survey of 21 cm absorption galaxies at redshift $z<0.09$ over an area of about 10000 square degrees. We detected 51 absorbers, including 21 previously known and 30 new ones, with 8 sources having no optical spectroscopic redshift. The probability of occurrence for absorbers in all galaxies is 1/1078. The radio flux densities of the FASHI absorbers are mainly concentrated in the range of $S_{\rm 1.4GHz}=10\sim100$ mJy, but even as low as $2.6\pm0.4$ mJy. The number of redshifted absorbers is slightly higher than the number of blueshifted absorbers. Such results would provide some important clues for future flux-selected absorber surveys. Therefore, FAST has significantly improved the capabilities and performance for absorption observations and provided a true blind survey of 21 cm absorption galaxies for such studies.

§ INTRODUCTION

Studies using the 21 cm emission line have played a key role in understanding the kinematics, structure, and evolution of galaxies in general <cit.>. However, the 21 cm absorption line is a more powerful tracer than the emission line on all scales, from the parsec scales near the central black hole to the tens of kpc structures tracing interactions and mergers of galaxies [Morganti & Oosterloo, 2018]. This is because the intensity of a absorption line depends on the intensity of the background continuum source, independent of redshift. Furthermore, absorption observations can be used to study the gas near an active nucleus out to much higher redshifts than is possible using emission, since the strong radio continuum emission is often associated with the central activity <cit.>. Therefore, absorption observations, especially at high redshifts, are much better suited to address the evolution of the properties of galaxies and related issues than emission line studies.

absorption observations have been made with all available radio telescopes, from single dish to interferometer <cit.>, but a number of limitations have affected absorption observations in the past [Morganti & Oosterloo, 2018]. One is the limited velocity range that can be covered by an observation while maintaining good spectral resolution. Another major limitation has been the limited coverage and sensitivity at lower frequencies, making it difficult to study high redshift objects. The third is the current lack of a “blind" survey of a very large volume of the Universe. Blind surveys could largely remove the effects of many of the selection biases that plague most studies today and sometimes make it difficult to draw firm conclusions. Darling et al., 2011 conducted a blind pilot survey for 21 cm absorption lines in a 517 deg$^2$ section of the ALFALFA survey at $z<0.058$, but found no new absorption line systems. Hu et al., 2023 presented a blind search for the extragalactic 21 cm absorption lines with the Commensal Radio Astronomy FasT Survey (CRAFTS), but only two new and three known absorbers were detected. Therefore, further high-sensitivity, broadband, large-scale, and unbiased absorption surveys are needed to overcome the above difficulty.

Locations for 51 H I absorption galaxies detected by FASHI over an area of about 10000 square degrees. The circles indicate their positions in the equatorial coordinate system. The background image is the all-sky 21-cm radio continuum of Stockert 25 m [Reich, 1982] and Villa Elisa 30 m [Reich & Reich, 1986, Reich et al., 2001]. The white dashed rectangles indicate roughly the area currently covered by FAST.

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) All Sky survey (FASHI) aims to cover the entire sky visible to FAST, between declinations of $-14^\circ$ and $+66^\circ$, within a frequency range of $1.0-1.5$ GHz [Nan et al., 2011, Jiang et al., 2019, Jiang et al., 2020]. The survey currently has a typical map rms of $\sim$0.76 $\mjyb$ with a velocity resolution of 6.4 $\kms$ for line [Zhang et al., 2024], while the Arecibo Legacy Fast ALFA Survey (ALFALFA) has a lower detection sensitivity of $\sim$1.86 $\mjyb$ after smoothing to a velocity resolution of 10 $\kms$ [Haynes et al., 2018]. This suggests that the FASHI could detect more absorbers than the ALFALFA. Furthermore, the Ultra-Wide Bandwidth (UWB) receiver on FAST can simultaneously cover a frequency range of 500-3300 MHz, making FAST a powerful instrument for observing the high redshift absorbers in extragalactic objects [Zhang et al., 2023]. Therefore, FAST with 19-beam and UWB receivers could be used to carry out such surveys mentioned above.

In this paper we mainly report a blind survey of 21 cm absorption galaxies with FAST over an area of about 10000 square degrees (see Figure <ref>). We detected 51 absorber, including 21 previously known and 30 new ones. Currently, there are about 160 known absorber in the literature <cit.>. FAST has significantly improved the capabilities and performance for absorption observations compared to previous observations. Section <ref> shows the FAST observations and source identification. Section <ref> presents the characterization of the detected absorbers and lists their parameters. Section <ref> discusses the features of detected absorbers. Section <ref> is a summary. Throughout this paper, we assume a $\Lambda$CDM cosmology with $H_{0}$ = 75 $\kms$ Mpc$^{-1}$, $\Omega_{\rm M} = 0.3$, and $\Omega_{\rm \Lambda} = 0.7$.

§ OBSERVATIONS AND DATA PROCESSING

§.§ Observations

The 21 cm absorption spectral data come from the FASHI project [Zhang et al., 2024]. The FAST 19-beam receiver was employed to efficiently cover the FAST sky. The aperture of FAST is 500 m, while the effective aperture is approximately 300 m, resulting in a beam size of $\sim$2.9$'$ at 1.4 GHz. The full band of 500 MHz in the spectral line backend has a frequency coverage of 1000 to 1500 MHz with a total of 64k channels. The corresponding frequency resolution is 7.63 kHz or 1.6 $\kms$ at 1.4 GHz. The pointing error is less than or equal to $15''$. To calibrate intensity, a high-noise signal with an amplitude of $\sim$11 K was injected into the entire FASHI observation for a period of 32 seconds. The factor of degrees per flux unit (DPFU) is relevant for 19 beams, with a value of $\rm DPFU=13\sim17$ K Jy$^{-1}$ at a frequency of 1400 MHz, as detailed in [Nan et al., 2011, Jiang et al., 2019, Jiang et al., 2020]. The FASHI
project is predominantly performed using the drift scan mode. Throughout the survey, the telescope's azimuth arm was positioned on the meridian at a pre-assigned J2000 declination, with a spacing of $21\dotmin65$ between primary drift centers. The feed array was rotated by $23\dotdeg4$ to enable super-Nyquist sampling of Earth-rotation drift-scan tracks of individual beams in the declination of J2000 coordinates, with sampling rates smaller than half the FWHM.

§.§ Data reduction

Currently, we only use the datasets in the frequency range of above 1305.5 MHz (or $z<0.09$) due to serious RFI and relatively low sensitivity at low frequency ranges <cit.>. The FASHI data are reduced using the FAST spectral data reduction pipeline,  [Jing et al., 2024]. The  pipeline combines data reduction packages, including antenna temperature correction, baseline correction, RFI mitigation, standing wave correction, gridding, flux correction, and fit cube generation. In baseline correction, we utilized the Asymmetrically Reweighted Penalized Least Squares algorithm <cit.> to adjust the entire 500 MHz spectral baseline. Several frequency periods ($\sim$1 MHz, $\sim$2 MHz, and $\sim$0.04 MHz) of standing waves are inhabited in the FASHI spectral data. The spectral quality can be significantly improved if the standing wave is completely corrected. Standing waves are predominantly fitted and subtracted using the SW package  in the  pipeline [Jing et al., 2024]. The data have been smoothed to a spectral resolution of 6.4 $\kms$ per channel and the fit cubes have been gridded to a pixel scale of 1 arcmin. A heliocentric velocity correction has been applied to the data due to the Doppler effect. A detailed observational setup and data reduction procedure is presented in the FASHI paper by Zhang et al., 2024.

§.§ Source finding

The process of extracting sources for FASHI is comprised of two steps, involving both automated and interactive procedures. The application utilized for identifying sources within data cubes is designated as the Source Finding Application () in version 2[<https://github.com/SoFiA-Admin/SoFiA-2>]. The  source finding algorithm involves the smoothing of the data on various spatial and spectral scales specified by the user, along with the measurement of the noise level in each smoothing iteration. Subsequently, a user-defined flux threshold is applied relative to the noise in order to retain all pixels with statistically significant flux density. The fully automated and reliable source finding tool, , is crucial for the scientific success of surveys <cit.>.

In the  setup, the detection threshold is 4.5$\sigma$ ($\sigma$ means the local noise level within the bounding box of the source in native units of the data cube; see the  Cookbook); the smoothing kernels are  and  respectively; in the linking parameters, the minimum size of the sources in the spatial dimension is 5 pixels/channels each in XY and Z space, while the maximum size is 50 pixels in XY space, but not limited in Z space; furthermore, we set the  to false, so that artifacts such as RFI and bad baselines occasionally remain in the spectral data. Based on the above setup, we found that the source candidates found by  contain some false signals. Therefore, a manual interactive source extraction has to be done, judging by the source moment maps, spectral profile, signal-to-noise ratio, and so on.

The absorption source finding procedure is similar to the emission source extraction in FASHI [Zhang et al., 2024]. The only difference is to let  be , so that to search for negative signals, such as absorption lines.

The candidates found by  include fake sources due to RFI in FAST observation <cit.>. A manual source extraction is needed to judge the source moment maps, spectral profile, signal to noise ratio, coordinate, flux density, optical counterparts and radio continuum sources, and then discard fake sources.

As for FASHI ID 20240000003, 20240000024, and 20240000022, the flux densities of the absorption peaks are not below zero, although they exhibit clear absorption characteristics. They thus cannot be extracted using the aforementioned method. In fact, the three sources were extracted from the FASHI emission sources by manual checking [Zhang et al., 2024].

In the end, we discovered and conformed 51 absorber, including 21 previously known and 30 new ones (see Figures <ref> and <ref>). Detailed information on these sources is presented in the following sections.

H I absorption galaxy FASHI 135036.30+334216.5 or ID 20240000427. Left: H I absorption spectrum. The redshift measured by FAST (in red) and an optical spectroscopic redshift (in green) are presented in the panel. The red curve line is the fit to the spectrum, and the green dashed line is the optical spectroscopic redshift. Middle: Blue circle indicates the position of source with a beam size of $\sim$2.9$'$. The background shows 1.4 GHz radio continuum distribution. The bright source within a square is the radio counterpart. Right: The background shows DSS image. The bright source within a square is the DSS counterpart. The other 50 H I absorption galaxies are shown in the online version of Figure <ref>.

§.§ Line fitting

Gaussian fitting has been widely employed to derive absorption line properties, such as the width of the profile, and to ascertain the presence of multiple components <cit.>. When multiple peaked profiles occur, as is the case in our absorption sample, Gaussian fitting methods have the disadvantage of necessitating a prior assumption regarding the physical conditions of the gas, namely the number of components to be fitted. The busy-function fitting method, as described in Westmeier et al., 2014, was employed in this study. This method has been successfully utilized in Geréb et al., 2015.

In the context of line fitting, if there is only one absorption line present, it is possible to utilise the busy-function fitting method to fit the absorption line directly. In the case that an emission line exists, but the absorption line is not obscured by a nearby emission line, the emission line will be masked first, and then the absorption line will be fitted (e.g., FASHI 20240000196, 20240000264, 20240000051, 20240000013). In the case that the absorption line is inaccessible to the emission line, the absorption line is masked, and then the emission line is fitted. Subsequently, the original line is employed to subtract the emission, after which the absorption line can be fitted (e.g., FASHI 20240000005, 20240000472, 20240000016).

§ DETECTION RESULTS AND ANALYSIS

§.§ Locations of FASHI absorbers

The FASHI data has already covered over an area of about 10000 square degrees within $0^h\lesssim{\rm RA}\lesssim24^h$,  $-14^\circ\lesssim{\rm Dec}\lesssim64^\circ$ with a typical spectral detection sensitivity of $\sim$1.50 mJy for a velocity resolution of $\sim$6.4 $\kms$ at 1.4 GHz. Because the FASHI project can only operate in schedule-filler mode, parts of the above sky areas are not fully covered. In Figure <ref> we show the locations of 51 absorption sources already detected by FAST at $z<0.09$. This suggests that the current detection rate of the absorbers is $5.1\times10^{-3}$ sources per square degree, while the FASHI emission sources have a detection rate of about 5.5 sources per square degree [Zhang et al., 2024]. We can further estimate that the probability of occurrence for the absorbers in all galaxies is 1/1078. At the current detection sensitivity, the final number of absorbers detected by FASHI would be about 100 if FAST were able to cover 20000 square degrees at $z<0.09$.

The ALFALFA survey used the seven-beam Arecibo $L$-band Feed Array to perform a drift-scan survey, covering an area of about 7000 square degrees over a redshift range of $z<0.06$ and cataloging 31502 emission galaxies. Wu et al., 2015 predicted that the whole ALFALFA could eventually produce about 25 absorbers. Their probability of occurrence is about 1/1260 in all galaxies. This number is slightly lower than that found by FASHI.

Additionally, from Figure <ref> we can see that the sources are mostly located at $30^\circ\lesssim{\rm Dec}\lesssim60^\circ$. This is because current FASHI observations mainly focus on such sky area. It is expected that the FAST observation will be able to detect more sources at $-15^\circ\lesssim{\rm Dec}\lesssim30^\circ$. With increasing detection sensitivity and area, the FASHI absorpter sample could be expected to be much larger than the current number.

Coordinates and counterparts of FASHI absorption galaxies

[1]       [2]      [3]       [4]      [5]     [6]      [7]

FASHI ID     FASHI     RA     Dec     NVSS     OC     Galaxy

J2000     deg     deg     J2000     J2000

20240000480     J030428.71-115315.1    46.1196    -11.8875     J030429-115422     J030429.63-115422.4     IRAS03021-1205

20240000482     J050451.78-101501.7    76.2157    -10.2505     J050453-101451     J050453.06-101452.5     IRAS05025-1018

20240000405     J141310.09-031140.8    213.2921    -3.1947     J141314-031227     J141314.84-031227.2     NGC5506

20240000476     J105833.00-023559.1    164.6375    -2.5997     J105835-023552     J105835.37-023551.9     PGC156086

20240000399     J031558.84-022532.1    48.9952    -2.4256     J031600-022539     J031600.78-022538.2     NGC1266

20240000093     J152121.51+042039.1    230.3396    4.3442     J152122+042033     J152122.54+042030.1     PGC054794

20240000483     J160329.87+171133.9    240.8745    17.1927     J160332+171158     J160332.09+171155.3     NGC6034

20240000003     J081937.14+210636.1    124.9048    21.1100     J081937+210653     J081937.95+210651.6     UGC04332

20240000408     J153455.59+233013.2    233.7316    23.5037     J153457+233011     J153457.38+233012.0     IC1127 IC4553 Arp220

20240000409     J131501.12+243651.4    198.7546    24.6143     J131503+243707     J131503.52+243707.7     IC0860

20240000116     J005922.61+270337.1    14.8442    27.0603     J005924+270332     J005924.43+270332.3     IC0064 UGC00613

20240000122     J012406.73+295238.5    21.0281    29.8774     J012407+295246     J012407.50+295244.0     LEDA1881594

20240060788     J005747.27+302108.9    14.4469    30.3525     J005748+302114     J005748.89+302108.8     NGC0315

20240000140     J135215.88+312631.6    208.0662    31.4421     J135217+312646     J135217.82+312646.6     UGC08782 3C293

20240000155     J074735.82+313625.2    116.8992    31.6070     J074742+313757     J074736.14+313659.6     PGC1955807

20240000014     J053117.26+315418.4    82.8219    31.9051     J053118+315412     J053118.79+315412.3     NVSSJ053118+315412

20240000159     J011932.90+321101.9    19.8871    32.1839     J011935+321050     J011935.00+321050.1     4C31.04

20240000027     J230222.13+321111.5    345.5922    32.1865     J230221+321125     J230222.47+321120.6     PGC070309

20240000026     J225442.56+321324.2    343.6773    32.2234     J225445+321248     J225444.97+321249.2     PGC069981

20240000129     J122512.74+321353.4    186.3031    32.2315     J122513+321401     J122513.11+321401.6     PGC1987158

20240000141     J090733.90+325729.4    136.8912    32.9582     J090734+325722     J090734.87+325723.1     PGC165523

20240000427     J135036.30+334216.5    207.6513    33.7046     J135036+334218     J135036.02+334217.6     NGC5318

20240000196     J120547.43+335016.9    181.4476    33.8380     J120547+335024     J120547.67+335022.1     PGC083530

20240000174     J150803.69+342339.2    227.0154    34.3942     J150805+342323     J150805.08+342315.9     PGC054033 IRAS15060+3434

20240000243     J103327.36+360550.9    158.3640    36.0975     J103328+360606     J103328.56+360559.5     PGC2074966

20240000024     J143235.35+361806.7    218.1473    36.3019     J143239+361808     J143239.90+361808.2     NGC5675

20240000037     J150033.79+364829.3    225.1408    36.8081     J150034+364844     J150034.58+364845.0     2MASXJ150034+364845

20240000039     J160542.81+371049.2    241.4284    37.1803     J160543+371046     J160543.21+371044.7     PGC084723

20240000036     J051419.90+371721.2    78.5829    37.2892     J051421+371714     J051421.10+371714.0     NVSSJ051421+371714

20240000500     J095055.65+375800.8    147.7319    37.9669     J095058+375758     J095058.71+375758.7     PGC090952

20240000022     J132849.73+383419.9    202.2072    38.5722     J132852+383438     J132852.11+383439.5     UGC08471

20240000443     J031646.12+395955.4    49.1921    39.9987     J031646+400011     J031646.70+400013.0     UGC02625 IC311

20240000264     J123943.52+401459.7    189.9313    40.2499     J123945+401450     J123945.16+401448.9     PGC089547

20240000051     J071908.19+403355.0    109.7841    40.5653     J071908+403343     J071908.66+403341.5     LAMOSTJ071908.70+403341.7

20240000445     J032421.15+404714.8    51.0881    40.7874     J032422+404719     J032422.86+404719.2     IRAS03211+4036 UGC2715

20240000501     J031946.81+413034.4    49.9450    41.5096     J031948+413042     J031948.26+413041.4     3C84 NGC1275

20240000013     J075134.30+425242.1    117.8929    42.8784     J075135+425249     J075135.64+425248.3     UGC04056

20240000454     J044226.82+440615.0    70.6117    44.1042     J044228+440638     J044228.49+440638.5     NVSSJ044228+440638

20240000059     J033337.92+445011.9    53.4080    44.8366     J033337+444941     J033337.90+444949.0     NVSSJ033337+444941

20240000507     J060801.90+450814.2    92.0079    45.1373     J060803+450825     J060803.44+450825.0     LEDA5061018

20240000078     J032614.32+492357.3    51.5597    49.3993     J032615+492357     J032616.00+492359.0     NVSSJ032615+492357

20240000512     J024600.14+511458.1    41.5006    51.2495     J024600+511515     J024600.54+511514.0     IRAS02425+5102

20240000007     J044433.30+525213.5    71.1388    52.8704     J044434+525224     J044434.76+525224.8     2MASXJ04443470+5252243

20240000083     J030630.87+541208.7    46.6286    54.2024     J030632+541158     J030632.74+541158.5     NVSSJ030632+541158

20240000005     J043636.25+542201.5    69.1510    54.3671     J043638+542159     J043638.61+542159.3     2MASXJ04363857+5421598

20240000023     J134440.84+555306.2    206.1702    55.8850     J134442+555313     J134442.09+555312.8     UGC08696 Mrk273

20240000513     J125614.58+565222.9    194.0608    56.8730     J125614+565223     J125614.17+565225.1     UGC08058 Mrk231

20240000515     J085519.79+575155.5    133.8324    57.8654     J085521+575143     J085519.04+575140.7     SDSSJ085519.04+575140.7

20240000472     J162636.67+580849.8    246.6528    58.1472     J162637+580911     J162637.74+580904.3     SDSSJ162637.74+580904.2

20240000016     J112832.57+583323.0    172.1357    58.5564     J112832+583346     J112831.99+583346.8     NGC3690

20240000471     J114852.43+592501.9    177.2185    59.4172     J114850+592457     J114850.36+592456.4     NGC3894

Notes. The FASHI IDs in bold indicate newly discovered absorber.

Physical parameters of FASHI absorption galaxies

[1]       [2]      [3]       [4]      [5]     [6]      [7]      [8]     [9]     [10]

FASHI ID     $V_{\rm heli}$     $z_{\rm HI}$     $z_{\rm OC}$       $W_{50}$     $S_{\rm HI}$     $S_{\rm 1.4GHz}$      ${\rm log}L_{\rm 1.4GHz}$      $\tau_{\rm HI}$     $N_{\rm HI}$

$\kms$                         $\kms$      mJy     mJy     W Hz$^{-1}$         10$^{21}$cm$^{-2}$

20240000480    6960.0    0.02322    0.03077    $ 48.7 \pm  4.8 $   $-19.03 \pm  2.56 $   $  29.4 \pm   1.3 $   $ 22.75 $   $ 1.042 \pm 0.260 $   $  9.26 \pm  2.31 $

20240000482    11837.5    0.03949    0.04032    $176.2 \pm  7.8 $   $-34.20 \pm  1.19 $   $1482.4 \pm  44.5 $   $ 24.69 $   $ 0.023 \pm 0.001 $   $  0.75 \pm  0.03 $

20240000405    1809.4    0.00604    0.00607    $ 72.3 \pm  5.8 $   $-32.97 \pm  1.96 $   $ 338.8 \pm  10.2 $   $ 22.38 $   $ 0.102 \pm 0.007 $   $  1.35 \pm  0.09 $

20240000476    10888.2    0.03632    0.03622    $115.9 \pm  9.0 $   $-16.25 \pm  1.33 $   $ 205.4 \pm   6.2 $   $ 23.74 $   $ 0.082 \pm 0.007 $   $  1.74 \pm  0.16 $

20240000399    2172.1    0.00725    0.00732    $ 88.6 \pm  8.3 $   $ -8.33 \pm  0.60 $   $ 115.6 \pm   3.5 $   $ 22.08 $   $ 0.075 \pm 0.006 $   $  1.21 \pm  0.10 $

20240000093    15660.3    0.05224    0.05216    $ 25.5 \pm  3.6 $   $-20.20 \pm  1.88 $   $ 407.3 \pm  13.4 $   $ 24.36 $   $ 0.051 \pm 0.005 $   $  0.24 \pm  0.02 $

20240000483    10224.0    0.03410    0.03402    $ 25.4 \pm  4.3 $   $-28.35 \pm  2.21 $   $ 456.0 \pm  15.2 $   $ 24.03 $   $ 0.064 \pm 0.006 $   $  0.30 \pm  0.03 $

20240000003    5444.8    0.01816    0.01842    $158.1 \pm  9.6 $   $ -5.02 \pm  0.34 $   $  32.9 \pm   1.4 $   $ 22.34 $   $ 0.166 \pm 0.014 $   $  4.77 \pm  0.41 $

20240000408    5435.7    0.01813    0.01810    $334.9 \pm  4.2 $   $-50.51 \pm  0.66 $   $ 326.3 \pm   9.8 $   $ 23.32 $   $ 0.168 \pm 0.006 $   $ 10.27 \pm  0.37 $

20240000409    3903.1    0.01302    0.01291    $116.1 \pm 10.6 $   $-11.67 \pm  0.76 $   $  30.8 \pm   1.0 $   $ 22.00 $   $ 0.476 \pm 0.045 $   $ 10.08 \pm  0.94 $

20240000116    13960.5    0.04657    0.04583    $ 25.8 \pm  1.8 $   $-70.19 \pm  3.33 $   $ 112.6 \pm   4.0 $   $ 23.69 $   $ 0.977 \pm 0.098 $   $  4.59 \pm  0.46 $

20240000122    16656.6    0.05556        $129.0 \pm 10.0 $   $-11.05 \pm  0.64 $   $  27.1 \pm   0.9 $   $ 23.24 $   $ 0.524 \pm 0.046 $   $ 12.32 \pm  1.09 $

20240060788    5412.4    0.01805    0.01746    $ 10.2 \pm  1.6 $   $-114.01 \pm 24.82 $   $ 772.1 \pm  25.3 $   $ 23.67 $   $ 0.160 \pm 0.038 $   $  0.30 \pm  0.07 $

20240000140    13445.5    0.04485    0.04519    $ 57.8 \pm  2.1 $   $-257.61 \pm  4.20 $   $4085.4 \pm 144.0 $   $ 25.23 $   $ 0.065 \pm 0.003 $   $  0.69 \pm  0.03 $

20240000155    17767.5    0.05927    0.06055    $381.4 \pm 10.9 $   $ -6.61 \pm  0.27 $   $   7.3 \pm   0.5 $   $ 22.75 $   $ 2.352 \pm 0.761 $   $163.56 \pm 52.93 $

20240000014    19953.8    0.06656    0.06120    $ 87.0 \pm  4.1 $   $-22.58 \pm  1.38 $   $  39.8 \pm   1.3 $   $ 23.50 $   $ 0.838 \pm 0.091 $   $ 13.29 \pm  1.44 $

20240000159    18153.2    0.06055    0.05996    $ 12.9 \pm  1.8 $   $-68.26 \pm  4.13 $   $2635.2 \pm  79.1 $   $ 25.30 $   $ 0.026 \pm 0.002 $   $  0.06 \pm  0.00 $

20240000027    6411.2    0.02139    0.02128    $145.2 \pm 17.6 $   $ -6.78 \pm  1.63 $   $   2.6 \pm   0.4 $   $ 21.37 $   $ 2.303 \pm 1.000 $   $ 60.94 \pm 26.47 $

20240000026    6252.9    0.02086    0.02169    $ 58.0 \pm  9.1 $   $ -9.56 \pm  0.98 $   $  32.0 \pm   1.0 $   $ 22.48 $   $ 0.355 \pm 0.046 $   $  3.75 \pm  0.48 $

20240000129    17951.4    0.05988    0.05923    $ 61.3 \pm  8.7 $   $ -5.50 \pm  0.47 $   $  49.3 \pm   1.5 $   $ 23.56 $   $ 0.118 \pm 0.011 $   $  1.32 \pm  0.13 $

20240000141    14732.8    0.04914    0.04906    $ 78.3 \pm  4.6 $   $-23.02 \pm  0.92 $   $  46.8 \pm   1.5 $   $ 23.37 $   $ 0.677 \pm 0.050 $   $  9.67 \pm  0.71 $

20240000427    4256.6    0.01420    0.01434    $ 48.6 \pm  4.8 $   $-21.78 \pm  1.16 $   $ 101.1 \pm   3.1 $   $ 22.61 $   $ 0.243 \pm 0.017 $   $  2.15 \pm  0.15 $

20240000196    15762.2    0.05258    0.05388    $ 86.6 \pm 12.6 $   $ -5.21 \pm  0.49 $   $   8.5 \pm   0.5 $   $ 22.71 $   $ 0.949 \pm 0.175 $   $ 14.98 \pm  2.77 $

20240000174    13484.4    0.04498    0.04503    $117.7 \pm  3.2 $   $-52.47 \pm  1.24 $   $ 133.1 \pm   4.0 $   $ 23.74 $   $ 0.501 \pm 0.025 $   $ 10.76 \pm  0.53 $

20240000243    17503.2    0.05838    0.05876    $ 18.8 \pm  2.7 $   $-13.73 \pm  1.88 $   $ 105.4 \pm   3.2 $   $ 23.88 $   $ 0.140 \pm 0.021 $   $  0.48 \pm  0.07 $

20240000024    3947.6    0.01317    0.01325    $ 15.9 \pm  0.0 $   $ -9.90 \pm  0.00 $   $ 118.8 \pm   3.6 $   $ 22.61 $   $ 0.087 \pm 0.003 $   $  0.25 \pm  0.01 $

20240000037    19850.7    0.06621    0.06605    $ 61.6 \pm  3.9 $   $-16.20 \pm  2.01 $   $  57.3 \pm   1.8 $   $ 23.72 $   $ 0.332 \pm 0.051 $   $  3.74 \pm  0.57 $

20240000039    19906.3    0.06640    0.06649    $ 39.6 \pm  0.0 $   $ -8.68 \pm  1.47 $   $  28.4 \pm   1.2 $   $ 23.42 $   $ 0.365 \pm 0.077 $   $  2.64 \pm  0.56 $

20240000036    22626.6    0.07547        $ 19.2 \pm  2.2 $   $-17.23 \pm  1.80 $   $ 143.3 \pm   4.3 $   $ 24.24 $   $ 0.128 \pm 0.015 $   $  0.45 \pm  0.05 $

20240000500    12199.6    0.04069    0.04053    $ 31.8 \pm  1.8 $   $-50.84 \pm  1.99 $   $  66.4 \pm   2.0 $   $ 23.35 $   $ 1.451 \pm 0.161 $   $  8.41 \pm  0.94 $

20240000022    7837.8    0.02614    0.02649    $ 25.7 \pm  6.0 $   $-11.34 \pm  1.74 $   $  11.1 \pm   1.3 $   $ 22.19 $   $ 2.303 \pm 1.000 $   $ 10.78 \pm  4.68 $

20240000443    4276.0    0.01426    0.01419    $ 25.4 \pm  4.4 $   $-11.05 \pm  1.73 $   $  35.2 \pm   1.4 $   $ 22.14 $   $ 0.377 \pm 0.074 $   $  1.74 \pm  0.34 $

20240000264    15751.0    0.05254    0.05386    $294.1 \pm  0.0 $   $ -1.32 \pm  0.09 $   $  15.2 \pm   0.6 $   $ 22.96 $   $ 0.091 \pm 0.007 $   $  4.87 \pm  0.40 $

20240000051    19848.0    0.06621    0.06593    $101.6 \pm  3.7 $   $ -8.03 \pm  0.61 $   $  16.9 \pm   0.6 $   $ 23.19 $   $ 0.645 \pm 0.076 $   $ 11.95 \pm  1.40 $

20240000445    3842.8    0.01282    0.01299    $ 37.3 \pm  8.3 $   $-12.28 \pm  1.66 $   $  55.5 \pm   1.7 $   $ 22.26 $   $ 0.250 \pm 0.039 $   $  1.70 \pm  0.27 $

20240000501    8115.0    0.02707    0.01754    $ 10.9 \pm  2.4 $   $-2196.30 \pm 10.16 $   $22829.2 \pm 684.9 $   $ 25.14 $   $ 0.101 \pm 0.003 $   $  0.20 \pm  0.01 $

20240000013    9223.3    0.03077    0.03203    $127.9 \pm 17.1 $   $ -5.27 \pm  0.53 $   $   2.7 \pm   0.4 $   $ 21.75 $   $ 2.303 \pm 1.000 $   $ 53.67 \pm 23.31 $

20240000454    5362.5    0.01789        $179.3 \pm  8.7 $   $-23.78 \pm  0.92 $   $  54.9 \pm   1.7 $   $ 22.54 $   $ 0.568 \pm 0.038 $   $ 18.56 \pm  1.24 $

20240000059    19841.4    0.06618        $ 26.6 \pm  6.7 $   $ -6.61 \pm  1.29 $   $  10.3 \pm   0.5 $   $ 22.98 $   $ 1.027 \pm 0.359 $   $  4.98 \pm  1.74 $

20240000507    10214.9    0.03407    0.03566    $ 36.6 \pm  2.3 $   $-25.54 \pm  1.11 $   $  56.2 \pm   1.7 $   $ 23.16 $   $ 0.606 \pm 0.044 $   $  4.04 \pm  0.29 $

20240000078    23584.3    0.07867        $167.3 \pm  2.7 $   $-31.48 \pm  2.20 $   $  64.0 \pm   2.0 $   $ 23.93 $   $ 0.677 \pm 0.074 $   $ 20.64 \pm  2.26 $

20240000512    10463.7    0.03490    0.03476    $ 73.9 \pm  1.5 $   $-157.22 \pm  2.33 $   $ 223.5 \pm   6.7 $   $ 23.74 $   $ 1.216 \pm 0.079 $   $ 16.37 \pm  1.07 $

20240000007    9943.1    0.03317    0.03273    $ 95.5 \pm  6.5 $   $-12.66 \pm  1.09 $   $  77.2 \pm   2.4 $   $ 23.22 $   $ 0.179 \pm 0.018 $   $  3.12 \pm  0.31 $

20240000083    19527.9    0.06514        $ 35.8 \pm  4.1 $   $ -9.26 \pm  0.80 $   $  55.3 \pm   1.7 $   $ 23.70 $   $ 0.183 \pm 0.018 $   $  1.19 \pm  0.12 $

20240000005    5647.5    0.01884        $ 37.9 \pm  3.1 $   $-10.56 \pm  1.20 $   $  58.7 \pm   1.8 $   $ 22.61 $   $ 0.198 \pm 0.026 $   $  1.37 \pm  0.18 $

20240000023    11288.1    0.03765    0.03732    $521.8 \pm 11.9 $   $-12.57 \pm  0.58 $   $ 144.7 \pm   5.1 $   $ 23.61 $   $ 0.091 \pm 0.006 $   $  8.64 \pm  0.52 $

20240000513    12650.1    0.04220    0.04176    $188.0 \pm 11.5 $   $-16.77 \pm  0.73 $   $ 308.9 \pm  12.1 $   $ 24.04 $   $ 0.056 \pm 0.003 $   $  1.91 \pm  0.11 $

20240000515    7737.9    0.02581    0.02601    $ 10.4 \pm  0.0 $   $-101.22 \pm 11.53 $   $ 651.6 \pm  19.6 $   $ 23.94 $   $ 0.169 \pm 0.022 $   $  0.32 \pm  0.04 $

20240000472    4836.8    0.01613        $ 26.0 \pm  2.5 $   $-60.37 \pm  3.88 $   $ 532.9 \pm  18.3 $   $ 23.44 $   $ 0.120 \pm 0.009 $   $  0.57 \pm  0.04 $

20240000016    3140.3    0.01047    0.01044    $214.6 \pm  1.4 $   $-69.90 \pm  0.78 $   $ 677.1 \pm  25.4 $   $ 23.16 $   $ 0.109 \pm 0.005 $   $  4.26 \pm  0.18 $

20240000471    3292.4    0.01098    0.01081    $158.0 \pm  8.5 $   $-24.73 \pm  1.19 $   $ 481.4 \pm  14.4 $   $ 23.04 $   $ 0.053 \pm 0.003 $   $  1.52 \pm  0.09 $

Notes. The FASHI IDs in bold are first discovered in this work. For FASHI 20240000013, 20240000022, and 20240000027, $\mid S_{\rm HI}\mid$ is larger than $S_{\rm 1.4GHz}$, we assume $S_{\rm HI}/S_{\rm 1.4GHz}=-0.9$ for $\tau_{\rm HI}$ estimation. Further observations are needed to explain such a strange phenomenon.

§.§ Coordinate catalog

Table <ref> presents the source coordinates and counterparts of FASHI absorption galaxies. Each column is introduced as follows:

* Column 1: Index number for each FASHI source. This index number is unique to each FASHI source.

* Column 2: Centroid coordinate (J2000) for each FASHI source in the format of .

* Column 3-4: Right ascension (RA) and declination (Dec) in units of deg from the FASHI source centroid (J2000).

* Column 5: Centroid coordinate (J2000) or name for each NVSS radio  source in the format of  [Condon et al., 1998].

* Column 6: Centroid coordinate (J2000) for each optical counterpart in the format of .

* Column 7: Representatives of the corresponding galaxy counterparts.

§.§ Physical parameter catalog

Table <ref> presents the physical parameters of FASHI absorption galaxies. Each column is introduced as follows:

* Column 1: Index number for each FASHI source. This index number is unique to each FASHI source.

* Column 2: Heliocentric velocity of the source in units of $\kms$. All results are presented in this work using the optical heliocentric definition of velocity.

* Column 3: Redshift, $z_{\rm HI}$ of the 21 cm line, corresponding to the heliocentric velocity in Column 2.

* Column 4: Redshift, $z_{\rm OC}$ of the optical counterparts collected from the literature.

* Column 5: Velocity width of the absorption line profile, $W_{50}$ in $\kms$, measured at the level of 50% of the peak using the busy-function fitting method [Westmeier et al., 2014].

* Column 6: The depth of the absorption line, $S_{\rm HI}$ in mJy in negative value.

* Column 7: Integrated 1.4 GHz continuum flux density of radio source, $S_{\rm 1.4GHz}$ in mJy as provided by NVSS survey [Condon et al., 1998].

* Column 8: Assuming isotropic emission for a source, the radio luminosity (power) is calculated with the following relations [Yun et al., 2001]:

\begin{equation}
{\rm log}\frac{L_{\rm{1.4GHz}}}{\rm W\,Hz^{-1}} = 17.08+2{\rm log}\frac{D_{\rm L}}{\rm Mpc}+{\rm log}\frac{S_{\rm 1.4GHz}}{\rm mJy},
\label{eq:lumeq}
\end{equation}

where $D_{\rm L}$ is the cosmological luminosity distance in Mpc and was calculated with a calculator from Wright, 2006.

* Column 9: Optical depth $\tau_{\rm HI}$, estimated with equation <cit.>:

\begin{equation}
\tau_{\rm HI} \approx -{\rm ln}(1+\frac{S_{\rm HI}}{c_{\rm f}S_{\rm 1.4GHz}}),
\end{equation}

where we assume covering factor $c_{\rm f}=1$ indicating the fraction of background radio continuum intercepted by the foreground cloud, $S_{\rm HI}$ and $S_{\rm 1.4GHz}$ are listed in the above columns.

* Column 10: column density, $N_{\rm HI}$ can be estimated with equation <cit.>:

\begin{equation}
N_{\rm HI} = 1.823\times 10^{18} T_{\rm s}\int\tau_{\rm HI} {\rm d}V,
\end{equation}

where we assume spin temperature $T_{\rm s}=100\,$K for the gas [Morganti & Oosterloo, 2018].

§ DISCUSSION

§.§ Comparison with known detections

Two examples of FAST spectra superimposed on Arecibo spectra [Mirabel & Sanders, 1988, Springob et al., 2005]. The both show good agreement in velocity and flux.

In Figure <ref>, two examples of FAST spectra are superimposed on Arecibo spectra [Mirabel & Sanders, 1988, Springob et al., 2005]. The both show good agreement in velocity and flux. We also checked other previously observed absorbers in Section <ref>. We found that most of the absorbers have good agreement in flux, e.g., 20240000405, 20240000116, and 20240000014. In fact, we have made a detailed comparison between FASHI and ALFALFA survey data in Zhang et al., 2024. Comparisons of the measured rms, SNR, and integrated flux parameters between FASHI and ALFALFA sources suggest that FAST provides highly reliable data sets. However, there are also some absorbers that have large flux discrepancies between FAST and the other instruments. A likely reason is that the low spatial resolution observations may introduce large flux pollution from nearby objects or unknown RFI, and the high spatial resolution VLBI observations may miss some flux due to lack of large scale structure coverage.

§.§ Luminosity-redshift distribution

The rest-frame 1.4 GHz continuum luminosity versus redshift for the 21-cm absorbers. The red plus and blue dot represent the detected absorbers from Curran & Duchesne, 2018 and FASHI. The different lines indicate different 1.4 GHz continuum flux density.

The distribution between the depth of the absorption line ($S_{\rm HI}$) and the integrated 1.4 GHz continuum flux density ($S_{\rm 1.4GHz}$) for the detected absorbers by FASHI.

Figure <ref> shows the luminosity-redshift distribution, which includes 51 absorbers observed by FASHI and 123 sample compiled by Curran & Duchesne, 2018. A notable difference is that the measured radio luminosity for the FASHI absorbers is lower than that for the sample in Curran & Duchesne, 2018. Figure <ref> shows that the radio luminosities of the FASHI absorbers are mainly concentrated in the flux density range of $S_{\rm 1.4GHz}=10\sim100$ mJy, but even as low as $2.6\pm0.4$ mJy for FASHI 20240000027. The absorbers in the Curran & Duchesne, 2018 sample are located at $S_{\rm 1.4GHz}=100\sim10000$ mJy. The discrepancy is probably due to the selection effect. For example, the sample in Curran & Duchesne, 2018 come from the flux-selected sources, whereas the FASHI absorbers represent a completely blind and unbiased survey.

Figure <ref> shows the distribution between the depth of the absorption line ($S_{\rm HI}$) and the integrated 1.4 GHz continuum flux density ($S_{\rm 1.4GHz}$) for the absorbers detected by FASHI. We find that many detected absorbers have a relatively lower 1.4 GHz continuum flux density ($S_{\rm 1.4GHz}=10$ mJy) than previous flux-selected surveys, which usually have a selection threshold of $S_{\rm 1.4GHz}>50$ mJy <cit.>. Therefore, for flux-selected observations, it is recommended to lower the selection threshold to at least $S_{\rm 1.4GHz}=10$ mJy. Otherwise the survey would miss many targets, or the detection of absorption would be biased towards brighter sources.

§.§ Velocity distribution

The distribution of velocities for the principal absorption components with respect to the systemic velocity estimated from optical counterparts.

The morphology and kinematics of gas in radio galaxies are found to be very complex. can trace rotating disks, offset clouds, and complex morphological structures of unsettled gas, such as infall and outflow [Geréb et al., 2015]. gas associated with the circumnuclear disk/torus and rotating around the nucleus may appear both blue- and redshifted relative to the systemic velocity [Chandola et al., 2013]. Figure <ref> shows the velocity distribution for the main absorption components with respect to the systemic velocity estimated from the optical counterparts. We have a total of 43 samples in this figure, since other samples have no known spectroscopic redshift parameters. We find that the number of blue- and redshifted absorbers is 20 and 23, respectively. In the past, blueshifted absorption occurs relatively more often <cit.>. However, we found in this blind survey that the redshifted number is a little more than the blueshifted number. The relative velocity of the absorbing gas could be significantly blueshifted if it is due to gas which has been accelerated by interaction with the radio jet, while infalling material fuelling the central engine would appear redshifted. More recent studies <cit.> also show that if the line profiles show gas deviating from regular rotation, this gas is mostly seen as blueshifted absorption. The prevalence of blueshifted absorption is likely connected to AGN-driven outflows. In addition, a very narrow and high redshifted absorption is detected and likely tracing an infalling cloud at larger distance from the nucleus.

§.§ Line width distribution

Figure <ref> shows the distribution of line width for the 21 cm absorber measured by FASHI. We find that 64.7% of the sample has a relatively narrow line width with $W_{50}<100\,\kms$, while only 9.8% of the sample has a wide line width with $W_{50}>200\,\kms$. FASHI 20240000023 has the widest line width with $W_{50}=521.8\pm11.9\,\kms$. Taylor et al., 1999 utilized VLBI to demonstrate that the narrow component could be observed against both jets and the nucleus, while the broad component is visible exclusively against the nucleus. They proposed that the broad component represents a circumnuclear torus, while the narrow component is indicative of inward-flowing cold gas. Beswick et al., 2004 also found that the narrow absorption lines are often attributed to gas at a distance from the center of the galaxy, as they are often indicative of absorption by ambient gas. Furthermore, major merger systems typically exhibit broad absorption profiles, which trace perturbed disk-like structures <cit.>.

The distribution of line width for the 21 cm absorber measured by FASHI.

§.§ Several special absorbers

Geréb et al., 2015 presented an analysis of the absorption in 32 objects observed with the Westerbork Synthesis Radio Telescope (WSRT). The 32 absorption sample show a broad variety of widths, shapes, and kinematical properties, but they do not include a kind of special line profile. A common feature of such absorbers is the coexistence of an absorption line with an associated emission line. In the case of FASHI 20240000003, 20240000024, 20240000022, 20240000443, 20240000005, 20240000472, and 20240000016, the absorption line is situated in close proximity to the line centre of the emission line. In the case of FASHI 20240000196, 20240000264, and 20240000013, the absorption line is situated at the blueshifted wing of the emission profile. For FASHI 20240000027 and 20240000051, the absorption line is located at the redshifted wing of the emission line. The emission component may originate from the nearby interacting galaxy or the same galaxy as the absorbers. If they originate from the same galaxy, such coexisting absorption and emission lines could provide more physical information for us to understand the galaxy kinematics and evolution. Nevertheless, high spatial resolution observations, such as VLBI, are necessary to conduct such studies.

§ SUMMARY

The FAST All Sky H I survey (FASHI) is broader in frequency band and deeper in detection sensitivity than most of previous surveys. FASHI is designed to cover the entire sky observable by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Based on the FASHI data, we perform a blind survey of 21 cm absorption galaxies at redshift $z<0.09$ over an area of about 10000 square degrees. With  source finding algorithm, we extracted 51 absorbers, including 21 previously known and 30 new ones, with 8 sources having no optical spectroscopic redshift. The current detection rate of the absorbers is $5.1\times10^{-3}$ sources per square degree, while the FASHI emission sources have a detection rate of about 5.5 sources per square degree. We can further estimate that the probability of occurrence for the absorbers in all galaxies is 1/1078. At the current detection sensitivity, the final number of absorbers detected by FASHI would be about 100 if FAST were able to cover 20000 square degrees at $z<0.09$.

The radio luminosities of the FASHI absorbers are concentrated in the range of $S_{\rm 1.4GHz}=10\sim100$ mJy, even lower than 10 mJy for several absorbers. The number of redshifted absorbers is slightly higher than the number of blueshifted absorbers. We also find that 64.7% of the sample has a relatively narrow line width with $W_{50}<100\,\kms$, while only 9.8% of the sample has a wide line width with $W_{50}>200\,\kms$. FASHI 20240000023 has the widest line width with $W_{50}=521.8\pm11.9\,\kms$. Such results would provide some important clues for future flux-selected absorber surveys. We also detected some special absorbers. A common feature is the coexistence of an absorption line with an associated emission line, for example FASHI 20240000003, 20240000024, 20240000022, 20240000443, 20240000005, 20240000472, 20240000016, 20240000196, 20240000264, 20240000013, 20240000027 and 20240000051. The emission component may originate from the nearby interacting galaxy or the same galaxy as the absorbers. High spatial resolution observations, such as VLBI, are necessary to conduct such studies.

Therefore, FAST has significantly improved the capabilities and performance for absorption observations, providing a true blind survey of 21 cm absorption galaxies for such studies.

§ ACKNOWLEDGEMENTS

This work is supported by the National Key R&D Program of China (No. 2022YFA1602901), the West Light Foundation of the Chinese Academy of Sciences, the National Natural Science Foundation of China (Nos. 11803044, 11933003, 12173045, 12373001, and 11933011), and the science research grants from the China Manned Space Project (No. CMS-CSST-2021-A05). This work is also sponsored by the Chinese Academy of Sciences (CAS), through a grant to the CAS South America Center for Astronomy (CASSACA). FAST is a Chinese national mega-science facility, operated by the National Astronomical Observatories of Chinese Academy of Sciences (NAOC).

This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is funded by the National Aeronautics and Space Administration and operated by the California Institute of Technology.

[Aditya & Kanekar, 2018]
Aditya, J. N. H. S., & Kanekar, N. 2018, , 481, 1578,

[Alatalo et al., 2011]
Alatalo, K., Blitz, L., Young, L. M., et al. 2011, , 735, 88,

[Baan & Haschick, 1990]
Baan, W. A., & Haschick, A. 1990, , 364, 65, 10.1086/169385

[Baan & Haschick, 1981]
Baan, W. A., & Haschick, A. D. 1981, , 243, L143,

[Baan et al., 1978]
Baan, W. A., Haschick, A. D., & Greenfield, P. E. 1978, , 222, L7,

[Baan et al., 1982]
Baan, W. A., Wood, P. A. D., & Haschick, A. D. 1982, , 260, L49,

[Baek et al., 2015]
Baek, S.-J., Park, A., Ahn, Y.-J., & Choo, J. 2015, The Analyst, 140 1, 250

[Beswick et al., 2004]
Beswick, R. J., Peck, A. B., Taylor, G. B., & Giovannini, G. 2004,
, 352, 49, 10.1111/j.1365-2966.2004.07892.x

[Bicay & Giovanelli, 1986]
Bicay, M. D., & Giovanelli, R. 1986, , 91, 732, 10.1086/114054

[Chandola et al., 2013]
Chandola, Y., Gupta, N., & Saikia, D. J. 2013, , 429, 2380,

[Condon et al., 1998]
Condon, J. J., Cotton, W. D., Greisen, E. W., et al. 1998, , 115,
1693, 10.1086/300337

[Conway, 1996]
Conway, J. E. 1996, in Extragalactic Radio Sources, ed. R. D. Ekers,
C. Fanti, & L. Padrielli, Vol. 175, 92

[Courtois & Tully, 2015]
Courtois, H. M., & Tully, R. B. 2015, , 447, 1531,

[Curran & Duchesne, 2018]
Curran, S. J., & Duchesne, S. W. 2018, , 476, 3580,

[Curran et al., 2016]
Curran, S. J., Reeves, S. N., Allison, J. R., & Sadler, E. M. 2016,
, 459, 4136, 10.1093/mnras/stw943

[Darling et al., 2011]
Darling, J., Macdonald, E. P., Haynes, M. P., & Giovanelli, R. 2011,
, 742, 60, 10.1088/0004-637X/742/1/60

[de Vaucouleurs et al., 1976]
de Vaucouleurs, G., de Vaucouleurs, A., & Corwin, J. R. 1976, Second
reference catalogue of bright galaxies, 1976, 0

[De Young et al., 1973]
De Young, D. S., Roberts, M. S., & Saslaw, W. C. 1973, , 185, 809,

[Dickey, 1986]
Dickey, J. M. 1986, , 300, 190, 10.1086/163793

[Dickey, 1997]
—. 1997, , 113, 1939, 10.1086/118408

[Ferruit et al., 1997]
Ferruit, P., Adam, G., Binette, L., & Pécontal, E. 1997, , 2,
345, 10.1016/S1384-1076(97)00023-7

[Gallimore et al., 1999]
Gallimore, J. F., Baum, S. A., O'Dea, C. P., Pedlar, A., & Brinks,
E. 1999, , 524, 684, 10.1086/307853

[Geréb et al., 2015]
Geréb, K., Maccagni, F. M., Morganti, R., & Oosterloo, T. A. 2015,
, 575, A44, 10.1051/0004-6361/201424655

[Geréb et al., 2014]
Geréb, K., Morganti, R., & Oosterloo, T. A. 2014, , 569, A35,

[Giovanelli & Haynes, 2015]
Giovanelli, R., & Haynes, M. P. 2015, , 24, 1,

[Gupta et al., 2006]
Gupta, N., Salter, C. J., Saikia, D. J., Ghosh, T., & Jeyakumar, S.
2006, , 373, 972, 10.1111/j.1365-2966.2006.11064.x

[Haynes et al., 2011]
Haynes, M. P., Giovanelli, R., Martin, A. M., et al. 2011, , 142,
170, 10.1088/0004-6256/142/5/170

[Haynes et al., 2018]
Haynes, M. P., Giovanelli, R., Kent, B. R., et al. 2018, , 861, 49,

[Heckman et al., 1978]
Heckman, T. M., Balick, B., & Sullivan, W. T., I. 1978, , 224, 745,

[Hu et al., 2023]
Hu, W., Wang, Y., Li, Y., et al. 2023, , 675, A40,

[Iwasawa et al., 2011]
Iwasawa, K., Mazzarella, J. M., Surace, J. A., et al. 2011, , 528,
A137, 10.1051/0004-6361/201015872

[Jiang et al., 2019]
Jiang, P., Yue, Y., Gan, H., et al. 2019, Science China Physics,
Mechanics, and Astronomy, 62, 959502, 10.1007/s11433-018-9376-1

[Jiang et al., 2020]
Jiang, P., Tang, N.-Y., Hou, L.-G., et al. 2020, Research in Astronomy
and Astrophysics, 20, 064, 10.1088/1674-4527/20/5/64

[Jing et al., 2024]
Jing, Y., Wang, J., Xu, C., et al. 2024, Science China Physics,
Mechanics, and Astronomy, 67, 259514, 10.1007/s11433-023-2333-8

[Lavaux & Hudson, 2011]
Lavaux, G., & Hudson, M. J. 2011, , 416, 2840,

[Maccagni et al., 2017]
Maccagni, F. M., Morganti, R., Oosterloo, T. A., Geréb, K., &
Maddox, N. 2017, , 604, A43, 10.1051/0004-6361/201730563

[Mahony et al., 2013]
Mahony, E. K., Morganti, R., Emonts, B. H. C., Oosterloo, T. A., &
Tadhunter, C. 2013, , 435, L58, 10.1093/mnrasl/slt094

[Mirabel, 1983]
Mirabel, I. F. 1983, , 270, L35, 10.1086/184065

[Mirabel & Sanders, 1988]
Mirabel, I. F., & Sanders, D. B. 1988, , 335, 104,

[Morganti & Oosterloo, 2018]
Morganti, R., & Oosterloo, T. 2018, , 26, 4,

[Nagar et al., 2002]
Nagar, N. M., Oliva, E., Marconi, A., & Maiolino, R. 2002, , 391,
L21, 10.1051/0004-6361:20021039

[Nan et al., 2011]
Nan, R., Li, D., Jin, C., et al. 2011, International Journal of Modern
Physics D, 20, 989, 10.1142/S0218271811019335

[Nilson, 1973]
Nilson, P. 1973, Uppsala general catalogue of galaxies

[Peck & Taylor, 2000]
Peck, A., & Taylor, G. B. 2000, in EVN Symposium 2000, Proceedings of the
5th european VLBI Network Symposium, ed. J. E. Conway, A. G. Polatidis,
R. S. Booth, & Y. M. Pihlström, 119,

[Reich & Reich, 1986]
Reich, P., & Reich, W. 1986, , 63, 205

[Reich et al., 2001]
Reich, P., Testori, J. C., & Reich, W. 2001, , 376, 861,

[Reich, 1982]
Reich, W. 1982, , 48, 219

[Springob et al., 2005]
Springob, C. M., Haynes, M. P., Giovanelli, R., & Kent, B. R. 2005,
, 160, 149, 10.1086/431550

[Struve & Conway, 2012]
Struve, C., & Conway, J. E. 2012, , 546, A22,

[Taylor et al., 1999]
Taylor, G. B., O'Dea, C. P., Peck, A. B., & Koekemoer, A. M. 1999,
, 512, L27, 10.1086/311873

[Toba et al., 2014]
Toba, Y., Oyabu, S., Matsuhara, H., et al. 2014, , 788, 45,

[van Gorkom et al., 1989]
van Gorkom, J. H., Knapp, G. R., Ekers, R. D., et al. 1989, , 97,
708, 10.1086/115016

[Vermeulen et al., 2003]
Vermeulen, R. C., Pihlström, Y. M., Tschager, W., et al. 2003,
, 404, 861, 10.1051/0004-6361:20030468

[Wang et al., 2018]
Wang, L.-L., Luo, A. L., Shen, S.-Y., et al. 2018, , 474, 1873,

[Wenger et al., 2000]
Wenger, M., Ochsenbein, F., Egret, D., et al. 2000, , 143, 9,

[Westmeier et al., 2014]
Westmeier, T., Jurek, R., Obreschkow, D., Koribalski, B. S., &
Staveley-Smith, L. 2014, , 438, 1176, 10.1093/mnras/stt2266

[Westmeier et al., 2022]
Westmeier, T., Deg, N., Spekkens, K., et al. 2022, , 39, e058,

[Wolfe & Burbidge, 1975]
Wolfe, A. M., & Burbidge, G. R. 1975, , 200, 548,

[Wright, 2006]
Wright, E. L. 2006, , 118, 1711, 10.1086/510102

[Wu et al., 2015]
Wu, Z. Z., Haynes, M. P., Giovanelli, R., Zhu, M., & Chen, R. R.
2015, Acta Astronomica Sinica, 56, 112

[Yu et al., 2022]
Yu, N., Ho, L. C., Wang, J., & Li, H. 2022, , 261, 21,

[Yun et al., 2001]
Yun, M. S., Reddy, N. A., & Condon, J. J. 2001, , 554, 803,

[Zhang et al., 2021]
Zhang, B., Zhu, M., Wu, Z.-Z., et al. 2021, , 503, 5385,

[Zhang et al., 2022]
Zhang, C.-P., Xu, J.-L., Wang, J., et al. 2022, Research in Astronomy
and Astrophysics, 22, 025015, 10.1088/1674-4527/ac3f2d

[Zhang et al., 2023]
Zhang, C.-P., Jiang, P., Zhu, M., et al. 2023, Research in Astronomy
and Astrophysics, 23, 075016, 10.1088/1674-4527/acd58e

[Zhang et al., 2024]
Zhang, C.-P., Zhu, M., Jiang, P., et al. 2024, SCPMA, 67, 219511,

[Zwaan et al., 2015]
Zwaan, M. A., Liske, J., Péroux, C., et al. 2015, , 453,
1268, 10.1093/mnras/stv1717

§ NOTES ON INDIVIDUAL SOURCES

In this work, the optical spectroscopic redshift parameters are mostly obtained from the local galaxy sample Siena Galaxy Atlas[<https://www.legacysurvey.org/sga/sga2020/>] (SGA). Some other sample information comes from the SIMBAD Astronomical Database [Wenger et al., 2000] and the NASA/IPAC Extragalactic Database.

FASHI J030428.71-115315.1 or (ID 20240000480): This absorber seems to have been first discovered by FASHI. The optical redshift <cit.> is $\sim2266\,\kms$ larger than the measured redshift ($z_{\rm HI}=0.02322$).

FASHI J050451.78-101501.7 or (ID 20240000482): This absorber seems to have been first detected by FASHI. The radio source is the Seyfert 2 galaxy, IRAS 05025-1018 (Simbad).

FASHI J141310.09-031140.8 or (ID 20240000405): This absorber was reported e.g. by Dickey, 1986. There are two absorption lines, located at $V_{\rm heli} \approx 1800\,\kms$ and $V_{\rm heli} \approx 2000\,\kms$. The measured spectral profile is in good agreement with that in Gallimore et al., 1999. The radio source belongs to the Seyfert galaxy [Gallimore et al., 1999, Nagar et al., 2002].

FASHI J105833.00-023559.1 or (ID 20240000476): This absorber seems to have been first detected by FASHI. The source is a radio galaxy.

FASHI J031558.84-022532.1 or (ID 20240000399): This absorber was reported e.g. by Alatalo et al., 2011. The measured flux density agrees well with that of Alatalo et al., 2011. The radio source is a lenticular galaxy NGC 1266 in the constellation Eridanus. The galaxy hosts an obscured active galactic nucleus [Alatalo et al., 2011].

FASHI J152121.51+042039.1 or (ID 20240000093): This absorber seems to have been first detected by FASHI. The radio source belongs to the Seyfert 2 galaxy.

FASHI J160329.87+171133.9 or (ID 20240000483): This absorber was first detected in absorption by VLA observations [Dickey, 1997]. The radio source is hosted by an S0 optical galaxy in cluster A2151 of the Hercules Supercluster [Geréb et al., 2015].

FASHI J081937.14+210636.1 or (ID 20240000003): The observations were presented by Bicay & Giovanelli, 1986, Springob et al., 2005. The radio source belongs to the Seyfert 2 galaxy.

FASHI J153455.59+233013.2 or (ID 20240000408): This absorber has been widely detected, e.g. by Dickey, 1986. The radio source is a well-known starburst galaxy, Arp 220 or IC 4553 [Baan et al., 1982].

FASHI J131501.12+243651.4 or (ID 20240000409): The observations were presented by Mirabel & Sanders, 1988, Courtois & Tully, 2015. The radio source belongs to a LINER-type Active Galaxy Nucleus.

FASHI J005922.61+270337.1 or (ID 20240000116): This absorber was first reported with FAST by Zhang et al., 2021, but also detected by Hu et al., 2023. Our measured flux density is in good agreement with that of Hu et al., 2023.

FASHI J012406.73+295238.5 or (ID 20240000122): This absorber appears to have been first discovered by FASHI. There is no optical spectroscopic redshift in the previous literature.

FASHI J005747.27+302108.9 or (ID 20240060788): The observations were presented by Haynes et al., 2011. The radio source belongs to a LINER-type active galactic nucleus.

FASHI J135215.88+312631.6 or (ID 20240000140): This absorber was reported with FAST by Hu et al., 2023. This object is the famous disk galaxy 3C 0293. The outflow driven by the radio jet was reported by Mahony et al., 2013. The extremely broad and multi-component absorption feature may be indicative of a rotating disk <cit.>.

FASHI J074735.82+313625.2 or (ID 20240000155): This absorber seems to have been first detected by FASHI. The radio source is weak with $S_{\rm 1.4GHz}=7.3\pm0.5{\rm\,mJy}$, while the linewidth is broad with $W_{50}=381.4\pm10.9\,\kms$. There are three optical counterparts near the radio source.

FASHI J053117.26+315418.4 or (ID 20240000014): This absorber was first detected with FAST by Hu et al., 2023, whose measured flux density is in good agreement with our detection in this paper. Hu et al., 2023 suggested unsettled gas structures or gas accretion onto the supermassive black hole (SMBH).

FASHI J011932.90+321101.9 or (ID 20240000159): This absorber was reported by van Gorkom et al., 1989, Conway, 1996, Gupta et al., 2006, Struve & Conway, 2012, Morganti & Oosterloo, 2018. It has two absorption velocity components. The low velocity is slightly blueshifted ($\sim$40 $\kms$), while the high velocity is redshifted ($\sim$200 $\kms$), relative to the systemic velocity of the galaxy. The low-velocity component may trace the rotating disk, and the high-velocity component may be due to small clouds evaporating from the inner edge of the disk [Gupta et al., 2006].

FASHI J230222.13+321111.5 or (ID 20240000027): The line was presented by Courtois & Tully, 2015, Yu et al., 2022, but its absorption feature was first discovered by FASHI. The radio source belongs to the Low Surface Brightness Galaxy.

FASHI J225442.56+321324.2 or (ID 20240000026): This absorber seems to have been first detected by FASHI. The galaxy is in a group of galaxies. The background is the Milky Way.

FASHI J122512.74+321353.4 or (ID 20240000129): This absorber seems to have been first detected by FASHI. The radio source belongs to the Seyfert 2 galaxy.

FASHI J090733.90+325729.4 or (ID 20240000141): This absorber seems to have been first detected by FASHI. There is no further information about this object.

FASHI J135036.30+334216.5 or (ID 20240000427): This absorber was reported by Mirabel, 1983. The radio source is a LINER-type active galactic nucleus. That the observed absorption occurs in clouds close to the nucleus [Mirabel, 1983].

FASHI J120547.43+335016.9 or (ID 20240000196): This absorber seems to have been first discovered by FASHI. There is also a emission line that agrees well with the redshift of the optical counterpart.

FASHI J150803.69+342339.2 or (ID 20240000174): This absorber was reported by Baan et al., 1978. The optical counterpart is a binary galaxy system that is connected to each other by a visible bridge (see Figure <ref>).

FASHI J103327.36+360550.9 or (ID 20240000243): This absorber appears to have been first discovered by FASHI. There is no further information on this object.

FASHI J143235.35+361806.7 or (ID 20240000024): The radio source is a known Seyfert 2 galaxy, NGC 5675. This source was observed by Dickey, 1986, but they did not detect a absorption feature. With FAST we detected a faint and narrow absorption line at the center of the emission line.

FASHI J150033.79+364829.3 or (ID 20240000037): This absorber was reported by Geréb et al., 2015. The line is close to the systemic velocity of the host galaxy and may trace a regularly rotating disk [Geréb et al., 2015].

FASHI J160542.81+371049.2 or (ID 20240000039): This absorber seems to have been first detected by FASHI. The radio source is a LINER-type active galactic nucleus.

FASHI J051419.90+371721.2 or (ID 20240000036): This absorber seems to have been first detected by FASHI. There is no further information about this object. The background is the Milky Way.

FASHI J095055.65+375800.8 or (ID 20240000500): This absorber seems to have been first detected by FASHI. The radio source belongs to the Seyfert galaxy [Toba et al., 2014].

FASHI J132849.73+383419.9 or (ID 20240000022): This absorber seems to have been first discovered by FASHI. The line shows a narrow absorption feature at the center of the broad emission line.

FASHI J031646.12+395955.4 or (ID 20240000443): This absorber seems to have been first detected by FASHI. The line shows a prominent absorption feature that nearly coincides with the mean velocity of the emission signal. The line profile of this source is almost identical to that of NGC 984 reported by Mirabel, 1983. NGC 984 has a compact bright core with an extended halo. It is a low surface brightness galaxy.

FASHI J123943.52+401459.7 or (ID 20240000264): This absorber seems to have been first detected by FASHI. There is also a emission line that agrees well with the redshift of the optical counterpart.

FASHI J071908.19+403355.0 or (ID 20240000051): This absorber seems to have been first detected by FASHI. There is also a emission line at a low velocity position. The background is the Milky Way. The radio source belongs to the Seyfert galaxy [Wang et al., 2018].

FASHI J032421.15+404714.8 or (ID 20240000445): This absorber seems to have been first discovered by FASHI. The background is the Milky Way.

FASHI J031946.81+413034.4 or (ID 20240000501): This absorber has been widely reported e.g. by De Young et al., 1973, Ferruit et al., 1997. The radio source is the well-known Seyfert 2 galaxy, 3C 84. The absorbing systems have two very different velocities. The high velocity system is a relatively narrow absorption line redshifted $\sim$3000 $\kms$ from the systemic velocity of 3C 84 [De Young et al., 1973].

FASHI J075134.30+425242.1 or (ID 20240000013): This absorber seems to have been first discovered by FASHI. The absorption line is located at the blueshift wing of the double peak emission profile. The radio source belongs to a LINER-type active galactic nucleus.

FASHI J044226.82+440615.0 or (ID 20240000454): This absorber seems to have been first detected by FASHI. There is no further information about this object. The background is the Milky Way.

FASHI J033337.92+445011.9 or (ID 20240000059): This absorber seems to have been first discovered by FASHI. There is no optical spectroscopic redshift in the previous literature. The background is the Milky Way.

FASHI J060801.90+450814.2 or (ID 20240000507): This absorber seems to have been first detected by FASHI. There is no further information about this object. The background is the Milky Way.

FASHI J032614.32+492357.3 or (ID 20240000078): : This absorber appears to have been first detected by FASHI. There is no optical spectroscopic redshift in the previous literature. The background is the Milky Way.

FASHI J024600.14+511458.1 or (ID 20240000512): This absorber seems to have been first discovered by FASHI. There is no further information about this object. The background is the Milky Way.

FASHI J044433.30+525213.5 or (ID 20240000007): This absorber seems to have been first detected by FASHI. There is no further information about this object. The background is the Milky Way.

FASHI J030630.87+541208.7 or (ID 20240000083): This absorber seems to have been first discovered by FASHI. There is no optical spectroscopic redshift in the previous literature. The background is the Milky Way.

FASHI J043636.25+542201.5 or (ID 20240000005): This absorber seems to have been first detected by FASHI. The line shows a narrow absorption feature at the center of the broad emission line. There is no optical spectroscopic redshift in the previous literature.

FASHI J134440.84+555306.2 or (ID 20240000023): This absorber has been reported in detail by Geréb et al., 2015. In Figure <ref> we can see that the line width is extremely broad with $W_{50}=521.8\pm11.9\,\kms$ and the optical morphology shows a long tidal tail extending 40 kpc to the south [Iwasawa et al., 2011]. This object is the host of an ongoing merger.

FASHI J125614.58+565222.9 or (ID 20240000513): absorption was reported by Heckman et al., 1978, Gallimore et al., 1999. The radio source belongs to the Seyfert 2 galaxy.

FASHI J085519.79+575155.5 or (ID 20240000515): This absorber has been reported in detail by Zwaan et al., 2015, Curran et al., 2016. The absorption line is very narrow and strong.

FASHI J162636.67+580849.8 or (ID 20240000472): This absorber seems to have been first discovered by FASHI. The line shows a narrow but strong absorption feature at the broad emission line center. There is no optical spectroscopic redshift in the previous literature.

FASHI J112832.57+583323.0 or (ID 20240000016): This absorber was reported by Dickey, 1986, Baan & Haschick, 1990. The interaction system IC 694/NGC 3690 (Arp 299-Mrk 171) has been studied in absorption and OH emission with the VLA in the A configuration [Baan & Haschick, 1990]. The line shows not only an absorption feature but also an emission profile (see Figure <ref>). The emission component may come from the nearby interacting galaxy.

FASHI J114852.43+592501.9 or (ID 20240000471): This absorber was reported by van Gorkom et al., 1989, Dickey, 1986, Peck & Taylor, 2000. This galaxy is classified as elliptical by de Vaucouleurs et al., 1976 and as SO by Nilson, 1973. The galaxy has a close binary SO companion, NGC 3895 [van Gorkom et al., 1989]. The emission component at $\sim3500\,\kms$ may originate from the nearby galaxy (see Figure <ref>).

§ FIGURES ON INDIVIDUAL SOURCES

21 cm H I absorption galaxy FASHI 030428.71-115315.1 or ID 20240000480.

21 cm H I absorption galaxy FASHI 050451.78-101501.7 or ID 20240000482.

21 cm H I absorption galaxy FASHI 141310.09-031140.8 or ID 20240000405.

21 cm H I absorption galaxy FASHI 105833.00-023559.1 or ID 20240000476.

21 cm H I absorption galaxy FASHI 031558.84-022532.1 or ID 20240000399.

21 cm H I absorption galaxy FASHI 152121.51+042039.1 or ID 20240000093.

21 cm H I absorption galaxy FASHI 160329.87+171133.9 or ID 20240000483.

21 cm H I absorption galaxy FASHI 081937.14+210636.1 or ID 20240000003.

21 cm H I absorption galaxy FASHI 153455.59+233013.2 or ID 20240000408.

21 cm H I absorption galaxy FASHI 131501.12+243651.4 or ID 20240000409.

21 cm H I absorption galaxy FASHI 005922.61+270337.1 or ID 20240000116.

21 cm H I absorption galaxy FASHI 012406.73+295238.5 or ID 20240000122.

21 cm H I absorption galaxy FASHI 005747.27+302108.9 or ID 20240060788.

21 cm H I absorption galaxy FASHI 135215.88+312631.6 or ID 20240000140.

21 cm H I absorption galaxy FASHI 074735.82+313625.2 or ID 20240000155.

21 cm H I absorption galaxy FASHI 053117.26+315418.4 or ID 20240000014.

21 cm H I absorption galaxy FASHI 011932.90+321101.9 or ID 20240000159.

21 cm H I absorption galaxy FASHI 230222.13+321111.5 or ID 20240000027.

21 cm H I absorption galaxy FASHI 225442.56+321324.2 or ID 20240000026.

21 cm H I absorption galaxy FASHI 122512.74+321353.4 or ID 20240000129.

21 cm H I absorption galaxy FASHI 090733.90+325729.4 or ID 20240000141.

21 cm H I absorption galaxy FASHI 135036.30+334216.5 or ID 20240000427.

21 cm H I absorption galaxy FASHI 120547.43+335016.9 or ID 20240000196.

21 cm H I absorption galaxy FASHI 150803.69+342339.2 or ID 20240000174.

21 cm H I absorption galaxy FASHI 103327.36+360550.9 or ID 20240000243.

21 cm H I absorption galaxy FASHI 143235.35+361806.7 or ID 20240000024.

21 cm H I absorption galaxy FASHI 150033.79+364829.3 or ID 20240000037.

21 cm H I absorption galaxy FASHI 160542.81+371049.2 or ID 20240000039.

21 cm H I absorption galaxy FASHI 051419.90+371721.2 or ID 20240000036.

21 cm H I absorption galaxy FASHI 095055.65+375800.8 or ID 20240000500.

21 cm H I absorption galaxy FASHI 132849.73+383419.9 or ID 20240000022.

21 cm H I absorption galaxy FASHI 031646.12+395955.4 or ID 20240000443.

21 cm H I absorption galaxy FASHI 123943.52+401459.7 or ID 20240000264.

21 cm H I absorption galaxy FASHI 071908.19+403355.0 or ID 20240000051.

21 cm H I absorption galaxy FASHI 032421.15+404714.8 or ID 20240000445.

21 cm H I absorption galaxy FASHI 031946.81+413034.4 or ID 20240000501.

21 cm H I absorption galaxy FASHI 075134.30+425242.1 or ID 20240000013.

21 cm H I absorption galaxy FASHI 044226.82+440615.0 or ID 20240000454.

21 cm H I absorption galaxy FASHI 033337.92+445011.9 or ID 20240000059.

21 cm H I absorption galaxy FASHI 060801.90+450814.2 or ID 20240000507.

21 cm H I absorption galaxy FASHI 032614.32+492357.3 or ID 20240000078.

21 cm H I absorption galaxy FASHI 024600.14+511458.1 or ID 20240000512.

21 cm H I absorption galaxy FASHI 044433.30+525213.5 or ID 20240000007.

21 cm H I absorption galaxy FASHI 030630.87+541208.7 or ID 20240000083.

21 cm H I absorption galaxy FASHI 043636.25+542201.5 or ID 20240000005.

21 cm H I absorption galaxy FASHI 134440.84+555306.2 or ID 20240000023.

21 cm H I absorption galaxy FASHI 125614.58+565222.9 or ID 20240000513.

21 cm H I absorption galaxy FASHI 085519.79+575155.5 or ID 20240000515.

21 cm H I absorption galaxy FASHI 162636.67+580849.8 or ID 20240000472.

21 cm H I absorption galaxy FASHI 112832.57+583323.0 or ID 20240000016.

21 cm H I absorption galaxy FASHI 114852.43+592501.9 or ID 20240000471.