Deep HI Mapping of Stephan’s Quintet and Its Neighborhood

We carried out deep mapping observations of the atomic hydrogen (HI) 21 cm line emission in a ﬁeld centered on the famous galaxy group Stephan’s Quintet (SQ), using the Five-hundred-meter Aperture Spherical Telescope (FAST) equipped with the 19-Beam Receiver. The ﬁnal data cube reaches an HI column density sensitivity of 5 σ = 2 . 1 × 10 17 cm − 2 per 20 km s − 1 channel with an angular resolution of 4 (cid:48) . 0. The discovery of a large diﬀuse feature of the HI emission in the outskirt of the intragroup medium of SQ was reported in a previous paper (Xu et al. 2022). Here we present a new study of the total HI emission of SQ and the detection of several neighboring galaxies, exploiting the high sensitivity and the large sky coverage of the FAST observations. A total HI mass of M HI = 3 . 48 ± 0 . 35 × 10 10 M (cid:12) is found for SQ, which is signiﬁcantly higher than previous measurements in the literature. This indicates that, contrary to earlier claims, SQ is not HI deﬁcient. The excessive HI gas is mainly found in the velocity ranges of 6200 - 6400 km s − 1 and 6800 - 7000 km s − 1 , which was undetected in previous observations that are less sensitive than ours. Our results suggest that the “missing HI” in compact groups may be hidden in the low-density diﬀuse neutral gas instead of in the ionized gas.


INTRODUCTION
Galaxies are formed in hierarchical structures in the Universe.Galaxy-galaxy interaction plays an important role in the formation and evolution of galaxies.Compact groups, characterized by aggregates of 4 -8 galaxies with space densities as high as those in cluster cores (Hickson 1982), represent a special class of interacting systems completely different from merging pairs that trigger the most extreme starbursts (star formation rate, SFR > 100 M yr −1 ) in the local Universe (Sanders & Mirabel 1996).Galaxies in compact groups show very diversified SFR, but none with detected SFR > 100 M yr −1 (Lenkić et al. 2016).Their SFR depends sensitively on the HI abundance which is generally deficient (Verdes-Montenegro et al. 2001;Johnson et al. 2007).Discovered as an aggregation of nebulae in 1877 (Stephan 1877), SQ is arguably the best-studied compact group.It has been observed in nearly all accessible wavebands from X-rays, UV, visible, to IR and radio (Allen & Hartsuiker 1972;van der Hulst & Rots 1981;Shostak et al. 1984;Yun et al. 1997;Moles et al. 1997;Xu et al. 1999;Gao & Xu 2000;Sulentic et al. 2001;Gallagher et al. 2001;Williams et al. 2002;Xu et al. 2003;Lisenfeld et al. 2004;Xu et al. 2005;Trinchieri et al. 2005;Appleton et al. 2006), and high-resolution observations of the SQ's infrared (IR) emission were included in the James-Webb Space Telescope's first public release (Pontoppidan et al. 2022).It has been found that the HI gas is all in the intragroup medium outside member galaxies in SQ (Shostak et al. 1984;Williams et al. 2002), and its spiral members show non-enhanced SFR (Xu et al. 2005).Most strikingly, Allen & Hartsuiker (1972) discovered in the intragroup medium of SQ a very large shock front of ∼ 40 kpc in size which, among all known intergalactic shock fronts, is only second to that of the radio relics caused by cluster mergers (Enßlin 2002).This shock was triggered by a high velocity (∼1000 km s −1 ) intruder (NGC 7318b) currently colliding into a debris field in the intragroup medium, the latter being a product of previous interactions among other member galaxies of the group (Allen & Hartsuiker 1972;van der Hulst & Rots 1981;Moles et al. 1997;Sulentic et al. 2001).SQ has provided an excellent laboratory for studies of various processes associated with high-speed galaxy collisions, such as the heating and cooling of warm H 2 gas powered by large-scale shocks (Appleton et al. 2017;Guillard et al. 2022), SFR quenching by turbulence in post-shock molecular gas (Guillard et al. 2012), and the formation of Green Valley galaxies in galaxy groups (Cluver et al. 2013;Alatalo et al. 2014;Lisenfeld et al. 2017).These studies provide important constraints to galaxy formation/evolution models, in particular for the high z Universe when galaxies have very low metallicity and may collide with each other more frequently.
A comprehensive understanding of the properties of SQ depends crucially on how accurately we know its interaction history, which is very complex due to the relatively large number of participants (N ≥ 5).Theoretical simulations have yielded inconclusive and controversial results.The fiducial model in Renaud et al. (2010) involves a number of events in the last half Gyr, including collisions between spiral galaxies NGC 7319 and NGC 7320c creating the so-called outer tail, between NGC 7319 and the early-type galaxy NGC 7318a creating the inner tail, and between the high-speed intruder NGC 7318b and the intragroup medium producing shocks in the gas stripped by the previous interactions.However, the hydrodynamical simulations in Hwang et al. (2012) favor a different scenario in which both the outer tail and inner tail were triggered simultaneously by a close encounter between NGC 7320c and NGC 7319 about 9 × 10 8 years ago.It provides a better fit to the current positions of galaxies and tidal tails, though cannot explain the observed age difference between star formation regions and star clusters in the outer and inner tails (Xu et al. 2005;Fedotov et al. 2011).It predicts that most gas is still inside galaxies which contradicts with observations (Shostak et al. 1984;Williams et al. 2002).In both models, the role played by NGC 7317, an early-type galaxy, is neglected because of a lack of observed connection to any other SQ members.Recently a reddish, low surface brightness stellar halo surrounding SQ was discovered in deep optical imaging observations (Duc et al. 2018), which corresponds spatially very well to the diffuse X-ray halo (Trinchieri et al. 2005), in particular around NGC 7317.Its size and morphology are consistent with the formation of the galaxy group more than a billion years ago.
In order to better constrain the early history of SQ and explore the diffuse HI gas in and around the group, we carried out deep mapping observations of the diffuse HI emission in a region of ∼ 30 × 30 centered on SQ using the Five-hundred-meter Aperture Spherical Telescope (FAST) equipped with the 19-Beam Receiver (Jiang et al. 2019(Jiang et al. , 2020;;Qian et al. 2020).The HI is the least bound component of galaxies, so will be the easiest (and hence first) to be stripped off and spread around during interactions.Thus, the distribution of the diffuse HI and its velocity field can put new and crucial constraints to the interaction history models.The discovery of a large diffuse feature of the HI emission in the outskirt of intragroup medium of SQ was reported in a previous paper (Xu et al. 2022).In this paper we present other results from the same observations, with the focus on the contribution of the diffuse gas to the total HI mass in the central region of the intragroup medium, and the detection of several neighboring galaxies.
The paper is organized as follows: after this introduction, Section 2 describes the observations and the data reduction procedure.Sections 3 and 4 present our results on the HI properties of SQ and of the neighboring galaxies, respectively.Section 5 is devoted to a discussion.Finally, a summary is given in Section 6.A comoving distance of 85 Mpc for SQ is adopted from NED 1 .For neighboring galaxies in the field of view of the FAST observations, a Hubble constant of H 0 = 70 km s −1 Mpc −1 is assumed.[4, 8, 16, 32, 64, ...]×3.5 × 10 17 cm −2 .The green boundary delineates the coverage of the FAST observations.The central red circle (D = 12 ) shows the aperture with which the total HI spectrum presented in Figure 2 is extracted.The orange line delineates the diffuse HI feature whose system velocity is written in brown labels (Xu et al. 2022).Provided in the figure is also information for the member galaxies and main tidal features in SQ (in red labels), for neighboring galaxies detected by FAST (in blue labels) whose positions are marked by blue circles, and for neighboring galaxy KAZ 295 (in blue labels).For each source, the name, Hubble type (when available), and velocity in km s −1 are listed.

OBSERVATIONS AND DATA REDUCTION
The FAST observations and the data reduction, including the sidelobe correction, are presented in detail in Xu et al. (2022).Here we give only a brief recount: Observations: The observations were carried out using the FAST 19-beam receiver in the standard ON-OFF mode with a total observation time of 22.4 hours (including overheads).They consist of 16 pointings in a rectangular 4 × 4 grid and cover uniformly a region of ∼ 30 in size with 304 sky pixels (beam positions in the sky).The angular resolution is 2.9 and the separation between the nearest pixels is 1.4 in the right ascension (RA) direction and 1.2 in the declination (Decl.)direction, therefore the mapping satisfies the Nyquist sampling criterion.The observations have a central frequency of 1391.64 MHz and a frequency coverage of 1050 -1450 MHz with a frequency resolution of 7.63 kHz (∆v = 1.65 km s −1 ).
Data reduction: The spectral data of individual beams were reduced following a similar procedure as presented in Cheng et al. (2020).This includes: averaging of individual samplings, calibration (converting the digital counts to flux density in mJy beam −1 ), ON-minus-OFF, standing-waves and baseline removal, and re-binning.In addition, the sidelobes are corrected utilizing the images of individual beams (Jiang et al. 2020) which provide information on the point spread functions (PSF's) of the beams.The end product is a data cube containing 304 spectra covering the velocity range of 4600 -7600 km s −1 in ∆v = 20 km s −1 channels with a 1σ noise of 0.16 mJy beam −1 .The corresponding HI column density sensitivity is 1σ = 1.2 × 10 17 cm −2 per channel.The velocity system is in the optical redshift convention and the local standard of rest (LSR) reference frame.The calibration uncertainty of FAST data is 10% (Jiang et al. 2020).
Smoothing: The data cube obtained above is highly redundant in the sense that a sky area of the size of a single beam (D=2.9 ) is covered by multiple beams (beam-separation: 1.4 × 1.2 ).In order to take the advantage of the redundancy and improve the sensitivity for the diffuse emission, a Gaussian kernel of Full-Width-at-Half-Maximum (FWHM) = 2 .8 is applied to the data cube.This improves the HI column density sensitivity to 1σ = 4.2 × 10 16 cm −2 per channel with a slightly degraded angular resolution of 4 .0.
The channel maps of the FAST observations are presented in Appendix A.

HI EMISSION OF SQ
Figure 1 shows the contour map of the integrated HI emission between 5600 and 7000 km s −1 overlaid on the CFHT MegaCam r-band image.The large red circle in the center, which has a diameter of 12 (300 kpc in linear scale), represents the aperture within which we measure the SQ's HI spectrum.In Figure 2, the spectrum is compared with those obtained previously by mapping observations using the 305-meter Arecibo telescope (Peterson & Shostak 1980) and the Green Bank Telescope (GBT; Borthakur et al. 2015).Thanks to its high sensitivity, the FAST spectrum is much less noisy than the previous two results.From the FAST spectrum, the integrated HI flux of SQ is 20.5 ± 2.1 Jy km s −1 .This is 1.2 times higher than result of Arecibo observations (16.7 ± 1.6 Jy km s −1 ) and 1.7 times higher than that of the GBT (11.8 Jy km s −1 ) .The HI mass can be estimated using the formula M HI = 2.356 × 10 5 D 2 S∆v M , where D is the comoving distance in Mpc and S∆v the integrated HI flux in Jy km s −1 ).Accordingly, the FAST result corresponds to a HI mass of M HI = 3.48 ± 0.35 × 10 10 M for SQ.
With reference to previous high angular resolution synthetic observations by Shostak et al. (1984) and Williams et al. (2002), which resolved the HI emission in SQ into several spatially and kinematically separated features, we  of 5597-5789 km s −1 , 5959-6068 km s −1 , and 6475-6755 km s −1 , respectively. The contours levels are 5.8, 12, 18, 23,  29, 44, 58, and 73 ×10 19 cm −1 for a synthesized beam of 19.4 × 18.6 .divide the SQ spectrum into five velocity components: 5700 component (5600 -5800 km s −1 ), 6000 component (5800 -6200 km s −1 ), 6300 component (6200 -6400 km s −1 ), 6600 component (6400 -6800 km s −1 ), and 6900 component (6800 -7000 km s −1 .Note that these components do not include the emission outside the velocity range of 5600 -7000 km s −1 whose contribution to the total HI flux is negligible (see Figure 2).The 5700 component corresponds to the "West A" feature in Westerbork Synthesis Radio Telescope HI map of Shostak et al. (1984) and the "SW" feature in VLA map of Williams et al. (2002), the 6000 component to the "West B" feature in Shostak et al. (1984) and the "NW-LV" feature in Williams et al. (2002), and the 6600 component to the "Main" feature in Shostak et al. (1984) and the combination of the "NW-HV", "Arc-S", and "Arc-N" features in Williams et al. (2002).The 6300 and 6900 components are undetected in the two synthetic maps.The contour maps of the integrated emission (i.e.moment-0 maps) in the velocity ranges of the five components are presented in the five panels of Figure 3. Identified in these maps are also several neighboring galaxies of SQ which will be discussed in detail in the next section.For comparison, contours of the high-resolution VLA maps adopted from Williams et al. (2002) are plotted in the figures for the 5700, 6000, and 6600 components.
As shown in Figures 3a and 3b, the 5700 and 6000 components are only marginally resolved by FAST.The 2-D Gaussian fittings of the unsmoothed FAST maps find sizes of 3 .2and 3 .1 for the two components, respectively.These correspond to intrinsic sizes of 1 .5 and 1 .3after the PSF deconvolution, consistent with the VLA measurements (Williams et al. 2002).According to Williams et al. (2002), the 5700 component is associated with the "new intruder" NGC 7318b whose radial velocity is 5774 km s −1 .Its HI mass measured by FAST is M HI = 2.43 ± 0.24 × 10 9 M .This is consistent with the result of Westerbork Synthesis Radio Telescope (M HI = 2.4 ± 0.6 × 10 9 M ; Shostak et al. 1984) and higher than the VLA value (1.5 ± 0.2 × 10 9 M ; Williams et al. 2002).Peterson & Shostak (1980) found a higher HI mass for this component in their Arecibo mapping observations (M HI = 5.1 ± 1.8 × 10 9 M ), although the difference from our result is only at 1.5σ level because of the large error.
The origin of the 6000 component is still controversial.Based on the observations of Shostak et al. (1984), Moles et al. (1997) suggested that it is also associated with NGC 7318b and the redshift difference from the 5700 component is due to the rotation of an HI disk.This was disputed by Williams et al. (2002) who argued that the VLA observations, which have better angular resolution and flux sensitivity than those of the Westerbork Synthesis Radio Telescope, demonstrate that the 5700 and the 6000 components are distinctly separated both in the space and in the velocity domain, so are very unlikely to be associated with the same HI disk.We find an HI mass of (M HI = 8.34 ± 0.83 × 10 9 M ) for the 6000 component.This is higher than the results in the literature, which are 5.3 ± 1.5 × 10 9 M , 3.1 ± 0.4 × 10 9 M and 2.2 ± 0.3 × 10 9 M by Peterson & Shostak (1980), Shostak et al. (1984), and Williams et al. (2002), respectively.
Previously, the 6300 component of the HI 21 cm line emission was only marginally detected by GBT observations (Borthakur et al. 2010(Borthakur et al. , 2015)).On the other hand, emissions in the velocity range of 6200 -6400 km s −1 have been detected in SQ in many other bands, including the millimeter CO rotation line emission of molecular gas (Guillard et al. 2012;Yttergren et al. 2021), the FIR [CII] and [OI] fine structure line emissions (Appleton et al. 2013(Appleton et al. , 2017)), emissions of the Balmer recombination lines and other emission lines of ionized gas in the optical (Xu et al. 2003;Iglesias-Páramo et al. 2012;Duarte Puertas et al. 2019;Yttergren et al. 2021;Guillard et al. 2022), and the Lyα line emission in UV (Guillard et al. 2022).All of these detections of different emission lines, with angular resolutions ranging from sub-arcsec to 50 arcsec, are consistent with the shock excitation.The radiations are mainly from two locations: (1) the large shock front, (2) the so called "bridge" that links the Seyfert 2 nucleus of NGC 7319 and the shock front (Cluver et al. 2010).Some sub-regions in the large shock front show emissions in the velocity range of 6200 -6400 km s −1 in the wings of relatively broad lines (∆v > 300 km s −1 ) peaked near 6000 or 6600 km s −1 (Guillard et al. 2012;Iglesias-Páramo et al. 2012;Appleton et al. 2013Appleton et al. , 2017;;Guillard et al. 2022).Guillard et al. (2012) attributed these emissions to the post-shock gas condensed out of the shock-heated hot X-ray gas after the cooling-down.There is a major difference between the large shock front and the bridge: in the radio continuum the former is very bright but the latter is undetected (Xu et al. 2003), indicating that the shocks in the two regions are different.The fact that the bridge is spatially and kinematically linked to the 4-kpc outflow (Aoki et al. 1996) strongly suggests that the shocks in the bridge are powered by the outflow, and the radio quietness may be explained by the low efficiency of these shocks in accelerating cosmic-ray electrons.On the other hand, new JWST and ALMA observations with sub-arcsec resolutions detected in the bridge several filaments whose connection to the outflow is unclear (Appleton et al. 2023).The 2-D Gaussian fitting of the 6300 component finds a size of 4.0 × 3.4 , corresponding roughly to an intrinsic size of 2.9 × 1.9 after the beam-deconvolution under the assumption that the emission can be approximated by a Gaussian source.Its HI mass is M HI = 2.94 ± 0.30 × 10 9 M .The 6600 component, containing the majority of the HI emission associated with SQ (Figure 2), shows a strong central source plus low-level emissions in the outskirt.In particular, an arm/tail-like diffuse feature on the south of SQ has been discovered within this component (Xu et al. 2022).The HI gas in the diffuse feature, which is outside the aperture within which the SQ spectrum is extracted (Figure 1), is confined in the velocity range of 6550 -6750 km s −1 .The HI mass of the 6600 component of SQ is M HI = 1.91 ± 0.19 × 10 10 M , which is 56% of the total HI mass of the SQ system.
The 6900 component was not detected in previous HI observations, but appeared in the observations of the CO emission of the molecular gas (Guillard et al. 2012(Guillard et al. , 2022) ) and in the surveys of HII regions (Sulentic et al. 2001;Iglesias-Páramo et al. 2012;Duarte Puertas et al. 2019).The FAST map of the component (Figure 3e) centers very close to the intragroup starburst SQ-A (Xu et al. 1999), in agreement with the locations of the CO and Hα emissions.The FAST observations do not resolve the component.The Hα datacube of Duarte Puertas et al. ( 2019), which has an average angular resolution of 0.8 , shows that this component is extended from SQ-A toward the shock front, presumably associated with pre-shock gas in the debris field.The HI mass of the 6900 component is M HI = 1.38 ± 0.14 × 10 9 M .
The integrated fluxes and the corresponding HI masses of the five components are listed in Table 1.The errors include both the r.m.s and the calibration uncertainties (10%; Jiang et al. 2020).The latter is the dominant contributor of the error in all cases.
4. NEIGHBORING GALAXIES IN SQ FIELD HI emission of six neighboring galaxies are detected in the SQ field.They are identified in Figure 1, Figure 3 and listed in Table 2.These are low mass late-type galaxies with radial velocities in the range of 5700 -6800 km s −1 .Among them, Anon 8 and Anon 9, named following the convention in the literature (Shostak et al. 1984;Williams et al. 2002), are new detections of this work.Anon 2 and Anon 4 were detected in previous observations (Shostak et al. 1980;Williams et al. 2002), and the FAST detections of NGC 7320a and Anon 7 were reported by Xu et al. (2022).For Anon 2 and Anon 4, the HI velocities measured by FAST are consistent with those by VLA within 1-σ, and the values of M HI by FAST are 0.1 dex and 0.2 dex above the VLA measurements, respectively, after the adjustments for the different H 0 adopted (Williams et al. (2002) assumed H 0 = 75 km s −1 Mpc −1 whereas we assume H 0 = 70 km s −1 Mpc −1 ).
Listed in Table 2 are also the optical counterparts of these neighboring galaxies, which are identified on the SDSS images with two criteria: (1) offset < 1.5 and (2) r < 20 mag (corresponding roughly to M * > 10 7 M ).In order to confirm their correspondence to the HI detections, we carried out long slit spectroscopic observations using the Lijiang 2.4 meter telescope for these optical sources (Appendix B).Five sources, including NGC 7320a reported by Xu et al. (2022), are detected with their optical redshifts consistent with the HI velocities.Only the counterpart of Anon 7 is undetected spectroscopically.Anon 7 is very close to the bright background radio source B2 2233+33 (R.A. = 22h35m17.7s,Decl.=34d07m49s), with an offset of only 0.4'.Given the very high continuum flux density of B2 2233+33A (f 1.4GHz = 159.4± 4.8 mJy; Condon et al. 1998) which is about 100 times of the peak HI flux density of Anon 7, we cannot rule out the possibility that Anon 7 is a pseudo detection due to artifacts associated with B2 2233+33A.The optical redshifts and the stellar masses of the confirmed optical counterparts are presented in Table 2.The stellar masses are derived using the SDSS photometric data, the optical redshifts, and the code fastpp 1 (Kriek et al. 2009).The Chabrier IMF (Chabrier 2003) and the stellar population model of Bruzual & Charlot (2003) are adopted.

HI deficiency of compact groups
It has been well established that spiral galaxies in compact groups of galaxies are deficient in HI gas content (see the reviews in Verdes-Montenegro et al. 2001;Jones et al. 2023).The VLA study of Hickson compact groups (HCGs) of Verdes-Montenegro et al. (2001) show that many of them have a substantial amount of HI gas in the intragroup medium outside the member galaxies, and it is most likely that the intragroup HI is the stripped gas from the spiral galaxies in the groups.However, Verdes-Montenegro et al. (2001) found that the HCGs are still HI deficient even after including the intragroup HI, with a mean HI deficiency Def HI = 0.40 ± 0.07.The HI deficiency is defined by the equation Def HI = log(M HI,predicted ) − log(M HI,observed ), where M HI,predicted is the sum of the predicted HI mass of individual spiral galaxies in a group and the prediction is based on the optical luminosity and the morphology.Verdes-Montenegro et al. (2001) proposed that the HI deficiency of HCGs is mainly due to the phase transition of the atomic gas in the intragroup medium during the evolution of compact groups.The GBT survey of HCGs by Borthakur et al. (2010) found a lower HI deficiency (with a mean Def HI = 0.20), due both to the higher surface brightness sensitivity of GBT and the recovery of the missing fluxes in the VLA observations.SQ (also known by the name of HCG 92), as a prototype of HCGs, has all of its spiral members free of HI gas in their disks and its HI exclusively detected in the intragroup medium (Shostak et al. 1980;Williams et al. 2002).Verdes-Montenegro et al. ( 2001) found a Def HI = 0.49 for SQ, which was reduced to a Def HI = 0.28 by Borthakur et al. (2010).In this work, we found 1.7 times more HI than the GBT observations of Borthakur et al. (2015) and 2.4 times of that of Borthakur et al. (2010), thus SQ is not HI deficient anymore.The difference between our results and those of Borthakur et al. (2015) cannot be explained by the calibration uncertainty which is at 10% level for both FAST and GBT observations.A careful inspection of Figure 2, where we compare the FAST spectrum with that of Borthakur et al. (2015), reveals that our FAST observations detected significantly more HI in the 6300 and 6900 component, namely in the wings on the two sides of the 6600 km s −1 peak.Because of the lower sensitivity of GBT observations, these contributions (mainly due to low column The P-V plot.The contours start from 1 mJy beam −1 (beam-width = 2.9') with an increment of a factor of 2. density diffuse HI) were missed by Borthakur et al. (2015).In line with the results of Borthakur et al. (2010), our FAST results further demonstrate that most of (or even all of) the missing HI in compact groups is in the low-density diffuse gas.Namely, instead of transforming to ionized gas, the low-density diffuse HI is persistent as neutral gas in the intragroup medium during the group evolution.It is worth noting that the diffuse HI gas discussed here is different from the low-density HI gas in the diffuse feature discussed by Xu et al. (2022): the former is measured within the central aperture in Figure 1 and the latter is outside the aperture.Xu et al. (2022) found that the mass of the HI gas in the diffuse feature is only ∼ 3% of the HI mass of SQ and is therefore too low to affect the HI deficiency index of SQ significantly.

Anon 4 and 6000 component
One of the possible scenarios proposed by Xu et al. (2022) for the formation of the large diffuse feature in the 6600 component (Figure 3) is that, like the large Leo Ring (Michel-Dansac et al. 2010), the diffuse feature could be the product of a high-speed head-on collision between an unknown old intruder and one of the core members of SQ (i.e. NGC 7319,NGC 7318a,and NGC 7317).In this scenario, the collision triggers an expanding density wave that pushes gas in an extended HI disk of the target galaxy outwards to form a very large ring, of which the diffuse feature is the high-density part.A candidate for such an intruder could be Anon 4 (v = 6057 km s −1 ), which has a relative radial velocity of about 600 km s −1 and a projected distance of approximately 0.2 Mpc from the SQ centre.If the relative transverse velocity is about 200 km s −1 , it would have taken approximately 1 Gyr for Anon 4 to move to the current position after the collision.If this scenario is true then it is conceivable that the 6000 component of the HI emission in the debris field of SQ is due to stripped gas from Anon 4 when it passed through the group center.In order to check this hypothesis we make a P-V plot along the line connecting Avon 4 and the center of the 6000 component (Figure 5).The plot shows that the 6000 component (the emission around the position of x = 0 and velocity = 6000 km s −1 ) has about the same velocity as Anon 4 (the emission around x = -2 arcmin), but there is no connection between the two emissions.This does not confirm any relationship between the 6000 component and Anon 4. Meanwhile, in the PV plot, there is a spur of the 6600 component toward Anon 4. The lower density gas between 6200 and 6400 km s −1 (i.e. the 6300 component) has a similar (albeit weaker) trend.Compared to its counterparts in the 6000 and 6600 components, the central source of the 6300 component (Figure 3c) is slightly stretched toward the east, making its contours slightly elongated in the east-west direction (Figure 3).These trends might be suggestive of a tidal influence of Anon 4 on the HI gas in the debris field.et al. (1997) proposed a comprehensive picture for the interaction history of SQ based on a 'two-intruders' scenario (also see Sulentic et al. 2001): an old intruder (NGC 7320c) stripped most of the gas from group members, and a new intruder (NGC 7318b) is currently colliding with this gas and triggering the large scale shock.The 'old intruder' passed the core of SQ twice, one in ∼ 5 -7 × 10 8 yr ago and pulled out the outer tail (also known as 'old tail'), another about 2 × 10 8 yr ago and triggered the inner tail ('young tail') .Xu et al. (2005) , based on newer observations, argued that it is more likely that the inner tail is triggered by NGC7318a which has a closer projected distance to NGC7319 than NGC7320c.However, the simulations of Renaud et al. (2010) and Hwang et al. (2012) found that, if indeed the inner tail is generated later in a different encounter, it is very difficult for the outer tail to survive the strong tidal influence during the second passage.Instead, Hwang et al. (2012) concluded that it is most likely that the two tails are triggered by the same passage of NGC 7320c from a very close vicinity of NGC 7319 about 9 × 10 8 years ago.The fiducial model of Hwang et al. (2012) can reasonably reproduce the the positions of the galaxies involved in the encounter between NGC 7319 and NGC 7320c and that between NGC 7318a and NGC 7318b, and the locations and the orientations of the inner and older tails.However, it cannot reproduce the overall HI distribution in SQ, in particular it predicts that NGC 7319 and NGC 7318b still retain most of their gas, in contradiction with the observations.For NGC 7318b, the fiducial model of Hwang et al. (2012) assumes that it has a relative radial velocity with NGC 7318a of ∆v ∼ 300 km s −1 , which is significantly lower than the observed value (∆v ∼ 900 km s −1 ).This may cause an under-estimate of the strength of density waves triggered by the head-collision and result in less gas removal in NGC 7318b.For NGC 7319, the assumption of the initial gas distribution as in a normal disk is unrealistic.As argued by Xu et al. (2022), the diffuse HI feature attached to the south edge of the debris field was most likely produced by an interaction between a core galaxy of SQ and an earlier intruder (e.g.NGC 7320a or Anon 4) prior to the invasion of NGC 7320c.If the involved core galaxy were NGC 7319, then its gas disk had been already distorted when NGC 7320c arrived.Alternatively, NGC 7319 could have had been involved in a collision with the elliptical galaxy NGC7317 which seems to be linked with NGC 7319 with a diffuse optical halo (Moles et al. 1997;Duc et al. 2018).The recently published JWST MIRI images of SQ in the JWST Early Release Observations (Pontoppidan et al. 2022) show some peculiar features in NGC 7319 in the 7.7 µm band in incredible details, which is presumably dominated by the emission of PAH molecules.These features don't look like any tidal arms/tails at all, but are very similar to the spokes in a ring galaxy which are triggered by a head-on collision with an intruder galaxy (e.g.those in the Cartwheel Galaxy; Renaud et al. 2018).The rather high molecular gas content and very low SFR in the disk of NGC 7319 (Gao & Xu 2000;Yttergren et al. 2021;Xu et al. 2005) correspond to a very low star formation efficiency, similar to the situation in Arp 142, a ring galaxy involved in a head-on collisions previously (Xu et al. 2021).On the other hand, it is not clear whether the spoke-like features seen in JWST images were indeed produced by a head-on collision ∼ 1 Gyr ago, because according to the current theory the effects of such collisions shall fade away in a few 10 8 yrs (Renaud et al. 2018).And even if they were, could they survive the tidal force during the subsequent passage of NGC 7320c?These question can only be answered in future simulations.

Origin of the diffuse HI feature
Given its spatial and kinematical connections to the debris field produced by interactions among core members of SQ and by collisions with numerous intruders, Xu et al. (2022) argued that the diffuse feature was most likely due to gas ejected from one of the core galaxies during an interaction with an early intruder (plausibly NGC 7320a or Anon 4) more than 1 Gyr ago.Xu et al. (2022) ruled out the hypothesis that the diffuse feature is merely a collection of unresolved 'dark' or 'almost dark' galaxies (Leisman et al. 2021) on the ground that it is very extended (∼ 0.5 Mpc) and has a narrow velocity range (∆v 20 = 160 km s −1 ).However, there are other possibilities.
First of all, it is possible that the diffuse feature is a tidal tail pull out from the neighboring spiral galaxy NGC 7320a instead of from a core member of SQ.The diffuse HI feature is right next to NGC 7320a, and its radial velocity (v = 6633 km s −1 ) is very close to that of NGC 7320a (v = 6702 km s −1 ).The simulations of Duc & Bournaud (2008) suggested that high-speed encounters (relative velocity of ∼ 1000 km s −1 ) between two galaxies of nearly equal mass can produce long HI streams similar to the diffuse feature.Indeed there is an early type galaxy, KAZ 295, at the other end of the diffuse feature opposite to NGC 7320a.Our new optical spectroscopy finds a redshift velocity of 6555 km s −1 for the galaxy (see Appendix B), confirming it to be at about the same distance of NGC 7320a.Its stellar mass estimated from the SDSS photometry is M * = 10 9.71 M which is slightly lower than that of NGC 7320a (M * = 10 10.08 M ).In analog to the simulation of Duc & Bournaud (2008), KAZ 295 may have a high transverse velocity relative to NGC 7320a, and the encounter between the two may have pulled a long HI stream from NGC 7320a that is identified by us as the diffuse feature.However there are some inconsistencies which cast doubts on the analogy.The case simulated by Duc & Bournaud (2008) are for 2 massive galaxies of M * > 10 11 M , and NGC 7320a and KAZ 295 are low mass galaxies of M * ∼ 10 10 M .Also the simulated HI stream is only ∼ 200 kpc whereas the length of the diffuse feature is ∼ 500 kpc.Whether the putative interaction between the two low-mass galaxies can trigger the formation of an HI feature as long as 500 kpc needs to be investigated by future studies.
Another possibility is that the diffuse feature might be due to cold gas accretion by the SQ group from the intergalactic medium (IGM) in the large-scale filaments.The diffuse feature locates at the bottom of the FAST map and may well reach beyond the map.In the large-scale environment, SQ is in a rather high density region embedded within one of the main filaments in Perseus supercluster (Gregory et al. 2000).Tammann (1970) suggested that SQ is perhaps part of the Zwicky Cluster 22h31m.2+ 37 • 32 (Zwicky & Kowal 1968).That explains why there are many neighboring galaxies in the SQ field in a relatively large velocity range (5600 -7000 km s −1 ), and why SQ has been intruded many times in its past history.
Smooth accretion of intergalactic gas from the surrounding filaments is a major growth mode for galaxy clusters and groups (Walker et al. 2019).The cold intergalactic gas, which is mostly ionized and is ∼ 15% of the intergalactic medium (IGM) in the filaments (Cantalupo et al. 2012), has been detected by observations of the Lyman-α emission and absorption (Cantalupo et al. 2014;Finley et al. 2014;Umehata et al. 2019), and by studies on the cross-correlations between neutral hydrogen (HI) 21 cm intensity maps and galaxy surveys (Chang et al. 2010;Masui et al. 2013).On the other hand, there is no confirmed detection of the HI 21 cm line emission of the filaments in the literature.Although the "HI bridge" detected in the intergalactic space between M31 and M33, which has a characteristic column density of N HI ∼ 10 18 cm −2 , was first interpreted as the product of condensation within an intergalactic filament (Braun 2004;Lockman et al. 2012;Wolfe et al. 2016), later simulations by Tepper-García et al. (2022) suggests that it is more likely a tidal feature triggered by a close encounter between the two galaxies ∼ 6.5 Gyr ago.Indeed, theoretical models predicted very low neutral HI column density of the filaments, with a mean of N HI ∼ 10 14 cm −2 for the total and a mean of N HI ∼ 10 16 cm −2 for the brightest 10% of the filaments (Kooistra et al. 2019).This is significantly lower than the characteristic HI column density of the diffuse feature (N HI ∼ 7 × 10 17 cm −2 ; Xu et al. 2022).Nevertheless, this does not completely reject the accretion hypothesis for the origin of the diffuse feature.Future HI observations exploring the regions beyond the south boundary of our FAST map, preferably with even higher sensitivity, will provide crucial constraints to this hypothesis.
6. SUMMARY HI mapping observations were carried out using the FAST 19-beam receiver.The observations cover a sky region of size of ∼ 30 × 30 around SQ with an angular resolution of 2.9 and a sampling that satisfies the Nyquist criterion.The smoothed sensitivity of the data (resolution = 4 ) reaches 1σ = 4.3 × 10 16 cm −2 for 20 km s −1 channels.The discovery of a large HI structure of radial velocity of 6633 km s −1 was presented in Xu et al. (2022).In this paper.we report the following new results: • The HI spectrum of SQ covers a wide velocity range between 5600 and 7000 km s −1 , with an integrated flux of 20.5 ± 2.1 Jy km s −1 .The corresponding HI mass is M HI = 3.48 ± 0.35 × 10 10 M (for a comoving distance of 85 Mpc).This is significantly higher than the previous measurements by the VLA and GBT observations (Verdes-Montenegro et al. 2001;Williams et al. 2002;Borthakur et al. 2010Borthakur et al. , 2015) ) and indicates that, when including the HI gas in the intragroup medium, SQ is not HI deficient.
• The HI emission of six neighboring galaxies are detected in the SQ field.These include NGC 7320a, Anon 2, Anon 4, Anon 7, Anon 8 and Anon 9.These are low mass late-type galaxies with radial velocities in the range of 5700 -6800 km s −1 .Among them, Anon 2 and Anon 4 were detected in previous observations (Shostak et al. 1980;Williams et al. 2002), and the FAST detections of NGC 7320a and Anon 7 were reported by Xu et al. (2022).We obtain the optical redshifts for optical counterparts of Anon 2, Anon 4, Anon 8 and Anon 9 which are consistent with the HI radial velocities.
• The FAST observations provide new evidence for a complex and long history of SQ, particularly before the formation of the inner and outer tails.The newly discovered diffuse feature is most likely due to a gas ejection from one of the core galaxies during an interaction with an early intruder (NGC 7320a or Anon 4) more than 1 Gyr ago.However the possibilities that the diffuse feature is an HI stream pull out from the neighboring lowmass spiral NGC 7320a by a high-speed encounter with another low-mass galaxy (KAZ 295), and that the diffuse feature might be due to cold gas accretion by the SQ group from the intergalactic medium in the large-scale filaments, warrant further investigations.

APPENDIX
A. HI CHANNEL MAPS The Figure A1 presents the channel maps of the FAST observation, covering the velocity range between 5500 and 7080 km s −1 with 80 channels of width ∆v = 20km s −1 .In each channel map, contours of the HI emission (with smoothed angular resolution of 4') are overlaid on the inverted grey-scale map of the deep CFHT MegaCam r-band image (Duc et al. 2018).The contours start from N HI = 1.3 × 10 17 cm −2 (at 3σ level) with the increment of a factor of 2. The central source associated with the SQ proper is detected in most channels.The diffuse feature reported in Xu et al. (2022) is visible close to the bottom of the field in channels centered at v = 6620, 6640, and 6660 km s −1 .Several neighboring galaxies are also detected in different channels across the velocity range, which are discussed in details in section 4.

B. OPTICAL SPECTROSCOPIC OBSERVATIONS OF NEIGHBORING GALAXIES
We performed long slit spectroscopic observations for the optical counterparts of ANON 2, ANON 4, ANON 7, ANON 8 and ANON 9, and for the optical galaxy KAZ 295, using Lijiang 2.4m telescope on Oct. 30-31 and Dec. 14, 2022.A slit of 1.8 wide was chosen which matches the seeing.We used the G14 grating with a wavelength coverage from 3500 -7000 with a resolution about 1000, which can help us to cover the bright emission lines such as Hα, [OIII], as well as the [OII], Ca II H, K lines at bluer wavelengths.The exposure time is about 2 hours for each target.We reduced the long slit spectra by the standard IRAF script.The HeNe lamp was used for wavelength calibration.However, bright lines of the lamp spectra were saturated, which affected our wavelength accuracy.Thus, conservatively, we included a wavelength calibration error of 1σ = 3 Å (137 km/s).Since the spectra are for redshift measurements only, no flux calibration was performed.The spectra of the optical counterparts of Anon 2, Anon 4, and Anon 8, and of the optical galaxy KAZ 295, are presented in Figure B1.The spectrum of the optical counterpart of Anon 9, which is a weak detection, is presented in Figure B2.The units of the flux density (F λ ) in the figures are arbitrary.

Figure 1 .
Figure1.Contour map (in blue contours) of the integrated HI emission between 5600 -7000 km s −1 over-plotted on the r band image.The contour levels are[4, 8, 16, 32, 64, ...]×3.5 × 10 17 cm −2 .The green boundary delineates the coverage of the FAST observations.The central red circle (D = 12 ) shows the aperture with which the total HI spectrum presented in Figure2is extracted.The orange line delineates the diffuse HI feature whose system velocity is written in brown labels(Xu et al. 2022).Provided in the figure is also information for the member galaxies and main tidal features in SQ (in red labels), for neighboring galaxies detected by FAST (in blue labels) whose positions are marked by blue circles, and for neighboring galaxy KAZ 295 (in blue labels).For each source, the name, Hubble type (when available), and velocity in km s −1 are listed.

Figure 2 .
Figure 2. HI spectra of SQ obtained by FAST (red), Arecibo (green, Peterson & Shostak 1980) and GBT (blue, Borthakur et al. 2015).The red vertical dashed lines mark the boundaries of the five kinematical components.We have smoothed the Arecibo and GBT spectra to a resolution of 20 km s −1 for comparison, from their original resolutions of 8.8 km s −1 and 10 km s −1 , respectively.

Figure 4 .
Figure 4. HI spectra of four neighboring galaxies.The dash gray lines show the position of zero flux.HI spectra of NGC 7320a and Anon 7 are presented in Xu et al. (2022).

Figure 5 .
Figure 5. Left: The contour and false color map of the 6000 component.The dashed white lines delineate the stripe within which the data of the P-V plot (in the right panel) are taken.The yellow cross marks the sky position of x = 0 in the P-V plot, whose coordinates are 22h36m00.8s+33d57m47s.The width of the stripe is 5' and the position angle is P.A. = 99 deg.Right:The P-V plot.The contours start from 1 mJy beam −1 (beam-width = 2.9') with an increment of a factor of 2.

Figure A1 .
Figure A1.FAST HI channel maps (contours) over-plotted on the CFHT r-band image(Duc et al. 2018).The channel maps cover the velocity range of 5480 -7040 km s −1 with a channel width of 20 km s −1 .The contour levels are[3, 6, 12, 24, 48,  96...]×4.2 × 10 16 cm −2 .The blue boundaries delineate the FAST coverage.The small red circle at lower left corner of the first map in the last row shows the angular resolution of the HI channel maps (FWHM = 4 ).

Figure
Figure A1.(Continued) Contours of HI channel maps over-plotted on the r-band image.

Figure
Figure A1.(Continued) Contours of HI channel maps over-plotted on the r-band image.

Table 1 .
HI kinematical components of SQ.

Table 2 .
Neighboring galaxies detected by FAST observations Stellar mass derived from SDSS photometry and optical redshift.