Discovery of a Large-scale H i Plume in the NGC 7194 Group

We present the discovery of a new H i structure in the NGC 7194 group from the observations using the Karl G. Jansky Very Large Array. NGC 7194 group is a nearby (z ∼ 0.027) small galaxy group with five quiescent members. The observations reveal a 200 kpc long H i plume that spans the entire group with a total mass of M H I = 3.4 × 1010 M ⊙. The line-of-sight velocity of the H i gas gradually increases from south (7200 km s−1) to north (8200 km s−1), and the local velocity dispersion is up to 70 km s−1. The structure is not spatially coincident with any member galaxies but it shows close associations with a number of blue star-forming knots. Intragroup H i gas is not rare, but this particular structure is still one of the unusual cases in the sense that it does not show any clear connection with sizable galaxies in the group. We discuss the potential origins of this large-scale H i gas in the NGC 7194 group and its relation with the intergalactic star-forming knots. We propose that this H i feature could have originated from tidal interactions among group members or the infall of a late-type galaxy into the group. Alternatively, it might be leftover gas from flyby intruders.


INTRODUCTION
Cosmological simulations predict that galaxies and groups are assembled into massive clusters hierarchically.Thus it is likely that a considerable fraction of present-day cluster galaxies have experienced morphological transformation in galaxy groups, which allow more violent mergers between galaxies (pre-processing, Zabludoff & Mulchaey 1998;De Lucia et al. 2012).Recent observational and theoretical studies emphasize the importance of galaxy groups as the environments for pre-processing (Cortese et al. 2006;Just et al. 2019;Kleiner et al. 2021;Scott et al. 2022).
Neutral Hydrogen (H I), a low column density diffuse atomic gas, is a great tool to probe gravitational and/or hydrodynamic interactions in galaxy groups (Yun et al. 1994;Kilborn et al. 2006;Serra et al. 2013;Oosterloo et al. 2018;Saponara et al. 2018;Kleiner et al. 2019;Koribalski et al. 2020).The H I that resides in galaxies is typically a factor of 2-3 larger than the stellar component (Pisano et al. 2000;Chung et al. 2009;Koribalski et al. 2018), therefore it is more susceptible to external processes such as tidal and hydro-dynamical processes in galaxies and the intracluster medium (Gunn & Gott 1972;Cowie & Songaila 1977;Larson et al. 1980;Nulsen 1982;Moore et al. 1996;Chung et al. 2007Chung et al. , 2009;;Rasmussen et al. 2008).During interactions, H I gas is scattered to circumgalactic regions such as isolated dark cloud (Cannon et al. 2015), and then forms new stars and dwarf galaxies (Thilker et al. 2009;Corbelli et al. 2021).Therefore, studying the properties of H I in galaxy groups is important to understand the preprocessing.
The H I observations in many galaxy groups have been widely used to study galaxy evolution and therefore the structures of H I clouds, streams, plumes and tidal tails are commonly found in nearby groups.(e.g., Koribalski et al. 2003;Kormendy & Kennicutt 2004;English et al. 2010;Lee-Waddell et al. 2019;Serra et al. 2019;Kleiner et al. 2021;Namumba et al. 2021).The triplet of M81 group is a well-known example of tidal interactions among the galaxies revealed by tidal H I bridges and disturbed H I distributions (Gottesman & Weliachew 1975;Yun et al. 1994).Serra et al. (2013) reported a long H I tail in HCG 44 group, which may be the result of an interaction between the group and a spiral galaxy.The H I ring of the NGC 5291 galaxy is known to have formed through a violent collision with a neighbor (Bournaud et al. 2007).The active star formation has been triggered, particularly in the form of giant HII regions, which also take place as the formation of dwarf galaxies (Malphrus et al. 1997;Duc & Mirabel 1998;Boquien et al. 2007;Fensch et al. 2016).One of the most well-known examples is the giant H I ring (∼200 kpc in diameter) in the Leo I group (Schneider et al. 1983) but its origin still shrouded in mystery.The Leo ring may be a collisional ring (Michel-Dansac et al. 2010) or the stripped remnants of tidal interactions between galaxies (Bekki et al. 2005;Corbelli et al. 2021).The Leo ring is isolated, not associated with any luminous galaxy in the group, and is much more quiescent than many other collisional ring galaxies (Higdon 1995;Corbelli et al. 2021;Fensch et al. 2016).Meanwhile, ever since the discovery of H I in even early-type galaxies (ETGs, Gouguenheim 1969), the idea that ETGs can harbor H I gas has become well-known now, supported by numerous observational studies of nearby ETGs (Oosterloo et al. 2010;Serra et al. 2012;Bait et al. 2020.)In this Letter, we report the discovery of an extremely extended H I structure in the NGC 7194 group from our the Karl G. Jansky Very Large Array (VLA) observations.The NGC 7194 group is a nearby (z ∼ 0.027) galaxy group with velocity dispersion of σ ∼ 201 km s −1 , mass of 2 × 10 13 M ⊙ and virial radius of 460 kpc (Tully 2015).An interesting point is that four of the five members are obviously quiescent and one shows only a very weak sign of star formation, which are not expected to hold a large amount of H I gas.Intriguingly, there are two distinct galaxies in transition: a passive spiral (PSp) galaxy and a shell galaxy, which may have recently experienced the transformation of morphologies by group environment effects.Thus, the NGC 7194 group is an excellent target to investigate group environment processes acting on galaxies with the combination of optical and H I observations.The H I data from the Arecibo Legacy Fast ALFA Extragalactic H I Source Catalog (ALFALFA; Haynes et al. 2018) give strong evidence of an amount of H I in the NGC 7194 group.Did the massive H I gas originate from the passive members?Otherwise, where did the gas come from?Finding an answer to the origin of the H I and those intergalactic features must be the key to understanding the current dynamical status of this environment and the evolution of galaxies in the group.Throughout the paper, we adopt a standard ΛCDM cosmology with Ω m = 0.3, Ω Λ = 0.7, and H 0 = 67.8km s −1 Mpc −1 .This gives a scale of 0.557 kpc/ ′′ .

THE KARL G. JANSKY VERY LARGE ARRAY OBSERVATIONS AND ANALYSIS
We observed the NGC 7194 group with the VLA Dconfiguration in L-band (project id: 22A-161).The pointing center is the position of the NGC 7194 at R.A.: 22 h 03 m 30.s 936, DEC.: +12 • 38 ′ 12 .′′ 415 (J2000).Two observation runs performed on 2022 Aug. 29 and Sep.04 were used.The total on-source time is 100 min.A single band of 16 MHz width was centered at 1.385 GHz with 31.25 kHz channel separation, corresponding to ∼6.8 km s −1 at z = 0.027.3C 48 was observed as flux and bandpass calibrator and J2148+0657 was observed as complex gain calibrator The data were processed using the standard CASA (5.6.2-3)pipeline.We first manually flagged bad data in the time and frequency domain using tfcrop.Then we ran the pipeline for flux, bandpass, and complex gain calibration.After the pipeline calibration of each dataset, we merge two epoch data for imaging using tclean.In order to find H I detect the H I line, we subtracted the continuum in the image plane using line free channels by imcontsub.In this Letter, Briggs weighting of a robust 1 in the CASA scale was adopted, yielding a synthesized beam of 58 .′′ 2 × 51 .′′ 7 (32.4 × 28.8 kpc).The velocity of the spectral cube was smoothed to ∼20 km s −1 (3 channels) to maximise the signal-to-noise ratio (S/N) while keeping the spectral resolution good enough to study the kinematics of the H I gas.The smoothed 20 km/s channel rms is 0.6 mJy/beam.H I moment maps were extracted by applying a 5-channel Hanning smoothing filter in the spectral axis and a 4-pixel Gaussian smoothing filter in spatial axes, with a cutoff level of 1.5 times channel map rms.This allows us to probe the faint H I structure in the outskirt.

EXTENDED H I STRUCTURE IN THE NGC 7194 GROUP
We present the H I structure from the VLA observations overlaid on the optical image from DECam Legacy Survey (DECaLS) in Figure 1.The NGC 7194 group consists of five confirmed member galaxies (Tully 2015) and none of them form stars actively with no strong nebular emissions in their optical spectroscopy.Aside from the passive member galaxies, Figure 1a reveals several blue star-forming knots and a faint, long, linear stellar stream.These features may indicate evidence of gravitational interactions.B1, close to the brightest group galaxy (NGC 7194), is the biggest star-forming knots in this group.A long arrow from B1 to B2 (and B3) marks the faint stellar stream with a projected distance of 160 kpc, which may be a connecting bridge between B1

and B2 (and B3
).There are blue and diffuse branches in the south-western outskirts of the NGC 7194 and the largest one extends to a projected distance of 45 kpc (see the inset of Figure 1).The H I distribution in Figure 1b reveals that the structure extends across the group.It is highly interesting that the largest blue knot B1 and the peak surface density of H I gas almost overlap in position.It is also noticeable that all the blue knots are found inside the H I gas.In addition, the diffuse branches in the south-western outskirts of the NGC 7194 also seems to be associated with the extended H I structure.
The densest H I clump embracing B1 at ∼ 7480 km s −1 has H I flux = 2.51 Jy km s −1 , M HI = 8.7 ×10 9 M ⊙ .The upper area including B2 and B3 is more massive with H I flux = 4.34 Jy km s −1 , M HI = 1.5 × 10 10 M ⊙ .The H I is extended further to the north-west without obvious optical counterparts.

DISCUSSION AND CONCLUSION
It is common to detect H I gas in nearby galaxy groups, but the H I structure in the NGC 7194 group is different from those previously known one, because those groups mostly consist of late-type and/or dusty galaxies (English et al. 2010;Lee-Waddell et al. 2019;Namumba et al. 2021).Moreover, H I gas is distributed on a scale of galaxies and the donor of H I gas is rather obvious (Malphrus et al. 1997;Oosterloo et al. 2018;Bait et al. 2020).Based on the analysis currently available in this letter, we discuss several possible origins of the huge H I structure in the NGC 7194 group.
One plausible scenario is that the H I gas originates from the member galaxies.For several decades, it has been reported that a significant number of early-type galaxies do indeed harbor H I gas (van Gorkom & Schiminovich 1997; Moran et al. 2006;Geréb et al. 2016;Oosterloo et al. 2010;Serra et al. 2012).We confirmed that there is no strong sign of star formation for members by inspecting spectroscopy data using the Calar Alto Legacy Integral Field Area survey (NGC 7194), the Sloan Digital Sky Survey (NGC 7195, PGC 67935 and NSA 24868), and Mapping Nearby Galaxies at APO (NSA 24489) and photometry data using Galaxy Evolution Explorer (GALEX) and Wide-field Infrared Survey Explorer (WISE).The IDs of NSA 24868 and NSA 24889 are taken from the NASA-Sloan Atlas catalog (NSA, Blanton et al. 2011).PGC 67935 is optically red but has a weak Hα emission line at the center in the SDSS spectrum, which is a weak star-forming galaxy from the BPT diagram (Baldwin et al. 1981).Its WISE color of W2 -W3 is 2.19, which lies in the boundary between non star forming and weak star-forming galaxies.There is no detection in FUV.While all five members appear quiescent (see bottom panels for Figure 1), several pieces of evidence indicate they may have recently experienced morphological transformation.
Even though the existence of H I gas in a PSp galaxy has not been reported to date, NGC 7195 might be the most prime candidate.NGC 7195 is a typical PSp galaxy having no sign of significant star formation and distinct spiral arms.From simulations and observations, spiral arm structures are found to fade over several Gyrs, after gas is stripped (Bekki et al. 2002;Pak et al. 2019;Pak et al. 2021).Since the stellar stream is located at the north-eastern vicinity of the NGC 7195, H I gas may be stripped from the NGC 7195 by interaction with the NGC 7194 galaxy and/or other members.For instance, PGC 67935 also has a clear shell structure and a diffuse stellar stream in the east, which is an evidence of merging or interactions.Unfortunately, the observed GALEX field is too shallow (exposure time of ∼ 192s) to detect faint features between members.The expected H I mass range of NGC 7195 is from 4.3 × 10 8 M ⊙ to 2.1 × 10 10 M ⊙ at the stellar mass of log(M ⋆ /M ⊙ ) = 10.53 (Catinella et al. 2018).However, even if we consider the upper limit of the expected H I mass, this amount is smaller than the total H I mass that we found in this group.
Secondly, we suspect that a gas-rich late-type progenitor may have fallen into the group from north to south of the group and it ends up B1 now.During infall, loosely bound H I gas of the progenitor is stripped by the gravity of the group and/or the group members, and left behind along the trajectory of the progenitor, extending up to 200 kpc.In the tails, blue knots are formed where the H I gas is condensed enough to form new stars.Since there was no available spectroscopic data for these blue knots to check their group membership, we confirmed it using long-slit spectroscopy for B1, B2 and B3 with the Doyak 1.8−m telescope of Bohyunsan Optical Astronomy Observatory in Korea.From these observations, we detected strong Hα emission centered at 7496, 7872 and 7766 km s −1 for B1, B2 and B3, respectively, which are in good agreement with the H I velocity distribution (Figure 3).
In order to test the feasibility of this scenario, we estimate the crossing time between the progenitor, B1, and the nearest Hα knot, B2.Assuming a typical galaxy speed in groups (∼300 km s −1 ; Smith et al. 2007;Piffl et al. 2014), the crossing time is at least ∼520 Myr to move the projected distance of 160 kpc from B2 to B1.This timescale is much longer than the star formation timescale traced by Hα emission which is ∼10 Myr (Leroy et al. 2012).Therefore, it is less likely that a gasrich late-type infaller is a donor of HI gas in the group if the star formation in B2 occurs instantly from stripped gas.
In addition to the timescales, this scenario does not explain detailed features of the H I structure.If the progenitor has fallen simply from north to south, it is hard to explain how the blue streams exist at the southwestern outskirts of the NGC 7194 galaxy and why the H I mass is greater in the northern region around B2 and B3.In addition, B1 is too small and amorphous to be a progenitor.The stellar mass of the B1 approximated from its SDSS luminosity and color is [M ⋆ /M ⊙ ] ∼ 10 7.5 , which is within the stellar mass range of that expected of a dwarf galaxy.Though low mass late-type galaxies can hold an amount of H I mass up to [M HI /M ⊙ ] ∼ 10 9 (Hunter et al. 2019), this is thirty times smaller than the H I mass found in the group.
It is also possible that a gas-rich progenitor has flown by and interacted with the group.One example is a long H I tail of ∼ 300 kpc in HCG 44 group (Serra et al. 2013).Serra et al. (2013) suggested that the tail may be the result of an interaction between the group and a spiral galaxy, NGC 3162, separated by 650 kpc.According to their hypotheses, the NGC 3162 passed through the group at high velocity and its gas was stripped in its trajectory.The lopsided morphology in the optical image, indicating recently perturbed via interactions, may support their hypotheses.
There is a candidate, PGC 67927, which is located in a projected distance of ∼ 600 kpc apart from the NGC 7194 to the north-west.PGC 67927 is a latetype galaxy with stellar mass log(M ⋆ /M ⊙ ) ∼ 10.5 from the NSA catalog.The morphology of PGC 67927 in optical image shows asymmetry in its spiral arm and quenching in the bulge.PGC 67927 contains H I flux of S HI = 1.36 Jy km s −1 (Springob et al. 2005).Assuming a distance of 118 Mpc from the NASA/IPAC Extragalactic Database, this corresponds to the H I mass of 4.5 × 10 9 M ⊙ using the equation (2) in Chung et al. (2009), which means PGC 67927 is already a gas-rich system at its stellar mass based on H I scaling relation from Catinella et al. (2018).Therefore, it is unlikely that PGC 67927 left a significant amount of H I over the NGC 7194 group.
In conclusion, we have discovered a huge H I structure in the small NGC 7194 galaxy group.This isolated H I cloud is extremely rare especially in a group environment.Our new discovery in this letter highlights that the origin of the structure is still shrouded in mystery, but evokes that the pre-processing in group environments can be very violent.In a forthcoming paper (Baek et al. in preparation), deeper insights on the detailed H I properties of structures will come from the higher-resolution VLA observations in B-and Cconfigurations.We also will investigate the ionized gas properties of star-forming knots in optical data, which will allow us to probe their formation histories.We gratefully thank the anonymous referee for constructive comments that have significantly improved this manuscript.We are grateful to Lachlan Marnoch for helpful discussions.The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.We thank the developers of the Bohyunsan Observatory and all staff of the Bohyunsan Optical Astronomy Observatory (BOAO).This re-

Figure 1 .
Figure 1.(a) The DECaLS image of the NGC 7194 group.North is at the top and east is to the left.White dotted circles are the five previously-known members of the group.Cyan circles are blue knots.The long arrow is drawn parallel with the long diffuse and blue stream, and the small arrows mark diffuse blue branches in the south-western outskirts of the N7194.(b) H I distributions from VLA D-configuration is overlaid on the DECaLS image.Circle in the right-bottom corner denotes the beam size of D-configuration.The contour levels of H I column density go from 1.5 to 47.0 × 10 19 cm −2 in the interval with 10 steps in linear scale.The DECaLS images (70 ′′ × 70 ′′ ) of five quiescent galaxies are shown in the bottom panel.The IDs of NSA 24868 and NSA 24889 are taken from the NASA-Sloan Atlas catalog (NSA, Blanton et al. 2011).
Figure  2 reveals a gradient with velocities increasing from 7200 km s −1 in the south to 8200 km s −1 in the north.In the right panel of Figure2, the H I velocity dispersion arise up to (∼ 70 km s −1 ).While the intensity weighted velocities (left panel of Figure2) increase gradually from south to west, there are locally peaked high velocity dispersion regions (∼70 km s −1 ) between bright H I clumps, suggesting the presence of small kinematic discontinuities between bright H I clumps.The extent of the total H I is 200 kpc with H I flux = 9.85 Jy km s −1 and M HI = 3.4 × 10 10 M ⊙ .NGC 7194 group was covered by the Arecibo Legacy Fast Arecibo L-band Feed Array (AL-FALFA) survey(Haynes et al. 2018).From the AL-FALFA source catalogue fromHaynes et al. (2018),

Figure 2 .
Figure 2. The H I intensity weighted velocity (left panel) and velocity dispersion (right panel) from VLA D-configuration.H I column density contours are overlaid with solid lines on both maps.The H I velocities increase smoothly from 7200 km s −1 in the south to 8200 km s −1 in the north.

Figure 3 .
Figure3.The spectra of three blue knots (B1 -B3) observed from the Doyak 1.8-m telescope of Bohyunsan Optical Astronomy Observatory long-slit spectrograph.We detected Hα emission centered at 7496, 7872, and 7766 km s −1 , respectively.Instrumental flux is shown since calibration is not applied since signal to noise of the continuum is not sufficient enough to analyze flux calibration.
search was partially supported by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013.J.B. and A.C. acknowledge support by the National Research Foundation of Korea (NRF), grant No. 2022R1A2C100298212, and No. 2022R1A6A1A03053472.This work was also supported by National R&D Program through the NRF funded by the Korea government (Ministry of Science and ICT) (RS-2022-00197685).J.H.L., H.J. and Y.K.S. acknowledge support from the NRF grants funded by the Korea government (MSIT) (No. 2022R1A2C1004025, No. NRF-2019R1F1A1041086 and No. 2019R1C1C1010279, respectively).This research was supported by the Korea Astronomy and Space Science Institute under the R&D program (Projects No. 2023-1-830-01) supervised by the Ministry of Science and ICT.