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A giant umbrella-like stellar stream around the tidal ring galaxy NGC 922

Tidal ring galaxies are observed rarely in the local universe due to their intrinsically transient nature. The tidal ring structures are the result of strong interactions between gas-rich stellar disks and smaller galactic systems and do not last longer than ~500~Myr therefore, these are perfect scenarios where to find the debris of recently accreted dwarf galactic systems. We present new deep images of the NGC 922 tidal ring galaxy and its surroundings from the DESI Legacy survey data and from our observations with an amateur telescope. These observations are compared with results from high-resolution N-body simulations designed to reproduce an alternative formation scenario for this peculiar galaxy. Our new observations unveil that the low surface brightness stellar tidal structures around NGC 922 are much more complex than reported in previous works. In particular, the formerly detected tidal spike-like structure at the northeast of the central galaxy disk is not connected with the dwarf companion galaxy PGC3080368, which has been suggested as the intruder triggering the ring formation of NGC 922. The deep images reveal that this tidal structure is mainly composed by a fainter giant umbrella-like shape and thus it was formed from the tidal disruption of a different satellite. Using the broad-band g, r and z DESI LS images, we measured the photometric properties of this stellar stream, estimating a total absolute magnitude in r-band of Mr= -17.0 +/- 0.03 magn and a total stellar mass for the stream between 6.9-8.5X10^8 Mo. We perform a set of N-body simulations to reproduce the observed NGC 922-intruder interaction, suggesting a new scenario for the formation of its tidal ring from the in-fall of a gas rich satellite around 150 Myr ago.


Introduction
The widely accepted theory for galaxy formation and evolution proposes that galaxies grow through two main channels, the gas accretion through filaments, and mergers with other galactic systems (e.g., L'Huillier et al. 2012). These processes conduct to the formation of a zoo of galaxy morphologies that was described by the classical Hubble sequence (Willett et al. 2013). Furthermore, observations show that a non-negligible fraction of the Local Universe galaxies have peculiar morphologies not initially included in this classification (Nair & Abraham 2010;Willett et al. 2013). Among them, ring galaxies are particularly abundant (Willett et al. 2013) as they are present in more than one fifth of spiral galaxies (Buta & Combes 1996). Their origin Talentia Senior Fellow, dmartinez@iaa.es has been studied from the early development of numerical astronomy using N-body simulations (e.g., Lynds & Toomre 1976;Gerber et al. 1992;Hernquist & Weil 1993). Also experts on galactic dynamics used analytical approaches to unveil the origin of such structures (Struck-Marcell 1990;Kormendy & Kennicutt 2004;Romero-Gómez et al. 2007). At the light of results from these many works the researchers agree that ring-like galactic systems can form through two mechanisms (Buta & Combes 1996), a secular process triggered by the presence of a galactic bar (Romero-Gómez et al. 2007), and strong interactions with other galactic systems (Wu & Jiang 2012). Although both mechanisms produce similar morphologies, the properties of the resulting rings are well differentiated. Rings from secular origin are Θ-shaped (O-Type), do not show expansion velocity, and are not or marginally star forming. Rings resulting from interactions Article number, page 1 of 8 arXiv:2209.01583v1 [astro-ph.GA] 4 Sep 2022 (P-Type) show strong star formation within the ring and the central region of the galaxy, in most cases show a clear expansion velocity outwards, and present a variety of shapes.
Although collisions during minor mergers are common in the CDM paradigm, most collisional rings at the low-z universe are of secular origin. In fact, the statistics of observed and simulated ring galaxies show that the presence of ring-like galaxies by collisions (Tidal Ring Galaxies, TRG hereafter) is specially relevant at high redshift when mergers dominate the galaxy growth (Elagali et al. 2018a), while the secular processes are prevalent at lower-z and in low-density environments were galaxies tend to be less perturbed. This agrees well with the most recent observations of collisional rings in galaxies of the local universe (e.g., CSRG and CZ2- CNRG Buta 1995, 2017 that according to Madore et al. (2009a) show a lower limit of only the 0.001%. In Madore et al. (2009a) the authors also state that if assuming a constant minor mergers rate it is expected that in the local volume the majority of galaxies should have suffered a collisional ring formation over a Hubble time (see also Theys & Spiegel 1977). Therefore, the low number of detections is due to the short dynamical time of its formation, evolution, and decay.
After several years of research many studies showed that the TRGs are produced by an impulsive interaction between a small galaxy (intruder) in an almost radial orbit towards the gaseous disk of a larger galaxy (e.g., Madore et al. 2009a). The short but strong interaction of the intruder with the disk of the larger galaxy make it to contract first and expand later resulting in an outwardly propagating wave (Lynds & Toomre 1976;Hernquist & Weil 1993). This compression wave triggers star formation in an expanding ring that can reach velocities up to ∼113 km s −1 just after the collision ( ∼50 Myr, e.g., Arp 147 Fogarty et al. 2011), slowing down to ∼50 km s −1 for older rings ( ∼200 Myr, e.g., Cartwheel galaxy Higdon 1996). Properties of the ring and the survival of the bar/bulge in the center of the disk are determined by the mass of the intruder and its impact parameter with respect to the center of the disk (Gerber et al. 1996;Madore et al. 2009b;Fiacconi et al. 2012;Elagali et al. 2018b). All models and simulations showed that these star forming rings only lasts for 0.2−0.5 Gyr (Wong et al. 2006;Pellerin et al. 2010;Renaud et al. 2018;Elagali et al. 2018a) and became only marginally observable up to 0.7 Gyr after the collision (Wu & Jiang 2015), which makes them transient features. The identification of the intruder can also be challenging. Interacting dwarf galaxies can be quickly disrupted and mixed with the central regions of the TRG (Madore et al. 2009a), either in their first or second pericenter (Wu & Jiang 2015). In this regards Elmegreen & Elmegreen (2006) showed that some observed TRGs have no obvious companions and that this can be due that observations are not deep enough or because they have already merged with the main galaxy. Some works also pointed out the possibility that multiple perturbers, or multiple interactions by a single intruder can generate prominent rings.
From an observational point of view the most prominent example of a TRG is the Cartwheel galaxy. This system has been studied and simulated by many research groups (e.g., Charmandaris et al. 1999;Mayya et al. 2005;Barway et al. 2020), which helped to settle down the picture of the formation and evolution of such systems. Another example of a Cartwheel-like system is the less studied NGC 922. Wong et al. (2006) proposed that the nearby compact dwarf PGC3080368 (named S2 in their work) could be the intruder that generated the observed TRG morphology on the NGC 922. In their work, the authors present an N-body simulation showing that an off-center impact of a point mass on the NGC 922-like disk galaxy can generate a TRG. However, their models can not fully explain the formation and morphology of the detected stellar plume around the NGC 922 (Pellerin et al. 2010). More recently, HI observations of the NGC 922 and its outskirts have found an HI tail that is not aligned with the S2 orbit neither with the stellar plume (Elagali et al. 2018b). This result suggest that other interaction than the one proposed by Wong et al. (2006) occurred to the NGC 922. In Elagali et al. (2018b) the authors have not found the HI bridge predicted by all hydrodynamical models of similar interacting systems that should connect the S2 and the NGC 922. In addition, although showing a Cartwheel-like morphology in observations in the optical, this system does not show the single drop-out TRG typical morphology not in HI neither in the stellar component as predicted by simulations (Elagali et al. 2018a;Renaud et al. 2018). All these inconsistencies found when comparing data with results from numerical experiments points towards a more complex formation scenario than the one proposed by Wong et al. (2006).
In this paper, we present new deep images of the NGC 922 tidal ring galaxy and its surroundings from the DESI Legacy survey data and from our observations with an amateur telescope. These observations are compared with results from highresolution N-body simulations designed to reproduce an alternative formation scenario for this peculiar galaxy. This paper is organized as follows. In Sec. 2 we describe the observations we use in this work to support our new hypothesis on the formation of the TRG NGC 922. In Sec. 3 we present our main results obtained after analysing the observations. The N-body models used in this work are described in Sec. 4. Finally in Sec. 5 and 6 we discuss our results and present our conclusions.

Stan Watson Observatory South RC-60cm telescope
A wide-field deep image of NGC 922 and its tidal stream was obtained at the Stan Watson Observatory South (Dark Sky New Mexico, USA) with an 60 cm aperture f /6.7 Planewave CDK Ritchey-Chrétien telescope. We used a SBIG STX16803 CCD camera was that provided a pixel scale of 0.46 pixels −1 over a 32.0 × 32.0 field of view. We obtained a set of 40 individual 900 second images with an Astrodon Gen2 Tru-Balance E-series luminance filter 1 over several nights between 2021 November 25-29 by remote observations. Each individual exposure was reduced following standard image processing procedures for dark subtraction, bias correction, and flat fielding (Martínez-Delgado et al. 2010). The images were combined to create a final co-added luminance-filter image with a total exposure time of 36000 s (see Fig. 1 top panel).

DESI Legacy surveys imaging data
NGC 922 is one of the target galaxies of the Stellar Stream Legacy Survey (Martinez-Delgado et al. 2021), which is carrying on a systematic search for stellar tidal streams around massive galaxies of the local Universe. The imaging source for this project is the DESI Legacy Imaging Surveys (DESI LS), that compile optical data in three optical bands (g, r, and z) obtained by three different imaging projects: The DECam Legacy Survey  Wong et al. (2006) intruder. This deep image reveals an outer shell in its Southwest side, also detected in the amateur data shown in Fig. 1 (top panel). The blue smaller galaxy to the South of S2 is possibly a background distant galaxy. The field-of-view is about 4 × 4 arcmin. North is up, East is left. each image of this survey including NGC 922 was astrometrically calibrated to Gaia-DR2 and photometrically calibrated to the Pan-STARRS PS1 survey. Then they are subsequently resampled to a common pixel grid and summed with inverse-variance weighting.
The depth of the DESI LS images in each band has been determined by calculating the surface brightness limit following the standard method of Román et al. (2020), i.e. the surface brightness corresponding to 3σ of the signal in the non detection areas of the image for a 100 arcsec 2 aperture (Martinez-Delgado et al. 2021). This yield 29.15, 28.73 and 27.52 mag arcsec −2 for the g, the r and the z passbands, respectively.

Photometry of the NGC 922 stream
The photometry of the stellar stream around NGC 922 in the g, r, and z-bands has been derived with the GNU Astronomy Utilities (Gnuastro) 2 using resulting coadded image cutout of this galaxy obtained from the DESI LS data (see Sec.2.2). The measurements have been carried out with Gnuastro's MakeCatalog on the basis of the sky-subtracted image generated by Gnuastro's NoiseChisel (Akhlaghi & Ichikawa 2015;Akhlaghi 2019).
In addition to the photometry of the NGC 922 stream obtained from this custom Legacy Surveys cutout image, we also fitted a Sérsic model to the disk of NGC 922 in order to subtract spurious flux where the stream is located. Although this modeling does not contain structural information of NGC 922 given its irregular morphology, it is useful to reduce systematic errors and provide the cleanest possible photometry in external regions of the disk, where the stream stands out and is susceptible to analysis. Since the galaxy model used for the subtraction was 2 http://www.gnu.org/software/gnuastro not fully optimized for this type of irregular spiral galaxy, we only use it in our photometric analysis to quantify the possible contamination of the outer disk of the host galaxy on the surface brightness, colour and luminosity measurements on the stream, as it is discussed in Sec. 3.2

The umbrella-like stellar stream of NGC 922
The deep images of NGC 922 detect tidal structures in its outskirts at much lower surface brightness than previous studies and, thus, allow us to interpret their origin in more detail than in previous works. Fig. 1 show that the halo of NGC 922 contains a giant, complex low surface brightness umbrella-like substructure connected to the aforementioned bright, collimated tidal plume detected in many previous studies (see Sec. 1) and also visible in the HST data of this galaxy (Fig. 1, bottom-right panel). Interestingly, both amateur ( Fig. 1, top panel) and DESI LS (Fig. 3) images also reveal a very faint, tail-like feature in the southeast side of NGC 922 (marked with T in Fig. 1 top panel), which seems to be the extension of the plume in the other side of the disk, following the path of the radial orbit of the disrupted satellite (see Sec. 4). Thus, our observations suggests that all these tidal structures were formed by a different satellite than the nearby compact dwarf (named S2) proposed by Wong et al. (2006), which appears clearly isolated and with no signature of a tidal tail emanating in the direction of NGC 922. Because of their shallower data, they assumed that the brighter tidal plume, roughly in the direction of the S2 dwarf, was the only signature of a recent strong interaction of those galactic systems.
The overall tidal structure observed around NGC 922 is quite similar to that of the well-known umbrella-like galactic systems (e.g., NGC 4651, Foster et al. 2014) are the consequence of the recent tidal disruption of a satellite galaxy orbiting in a radial orbit in the main galaxy outskirts. Our simulations agree with these previous works (see Fig. 4). Therefore, we state that there is no causal connection between the S2 and the formation of this low surface brightness structure.
Our deep amateur image also includes the S2 dwarf galaxy, clearly visible at the right upper side of our amateur image (marked with S2 in Fig. 1 top panel). This kind of systems is expected to suffer a strong star formation burst after interacting with a central galaxy, followed by a fast depletion of gas as a consequence of the gas consumption in star formation and the clearance by supernovae feedback. Less than a Gyr after the interaction the satellite galaxy usually shows a low SFR and so an old stellar population (Wetzel et al. 2013). However, S2 looks very bright in the GALEX UV images, thus, it is actively forming stars. This result also disfavors the hypothesis that it interacted with NGC 922 to produce the observed tidal ring. In addition, the low surface brightness features detected in our image suggest that this dwarf galaxy holds its own tidal structure. Fig. 2 shows an image cutout from the DESI LS data centered in this galaxy, showing the presence of a strong shell around S2. This shell-like features look like very similar to those reported in deep images of the outskirts of the Large Magellanic Cloud (Besla et al. 2016), possibly originated by its interaction with the Small Magellanic Cloud. This S2 tidal features gives additional support that this satellite cannot be the intruder that generated the TRG, since a low-mass system like S2 should not hold such an unbound stellar structure after interacting with another galactic system. The detection of this structure could also be a direct proof that dwarf satellites suffer similar accretion events as their larger counterparts. Although more data are needed this may also be a sign that the S2 it just in its first infall towards NGC 922.

Stream surface brightness and colours
The photometry measurements for the stream were carried out in circular apertures placed on clearly detected parts of the stream. Regions where the stream surface brightness could be contaminated by the outer disk of NGC 922 were avoided. Figure 3 illustrates the photometry measurement approach used in our analysis. After subtracting the sky from the input images, the first step is to perform the detection of the signal. Then all foreground and background objects, identified as clumps in Gnuastro's Segment program (Akhlaghi & Ichikawa 2015; Akhlaghi 2019), were masked and apertures were placed on the resulting image to measure the photometry parameters (e.g., surface brightness and colours). The shell on the NE side of the disk of NGC 922 (labelled as S in Fig.1) and the tail on the SW side (labelled as T in Fig.1) have been measured separately. For comparison, the photometry parameters have also been measured in an aperture placed on the galaxy.
The results for the average surface brightness and colours along with the corresponding errors are given in Table 1, computed by Gnuastro's MakeCatalog 3 . The measurements have been averaged separately for the shell and the tail described above. The surface brightness of the tail is approximately 2 mag arcsec −2 fainter than in the shell. The average colours differ somewhat between the shell and the tail, but are close enough (the shell colours are at 0.75 σ for (g − z) and 1.6 σ for (g − r) within the tail colour error distribution) to reinforce the notion that both parts belong to the same stream. The results are given for the DESI LS custom image, as the host galaxy-subtracted image suffers from over-subtraction and yields surface brightnesses up to 0.1 mag arcsec −2 fainter than for the image without subtraction for the tail and up to 0.2 mag arcsec −2 fainter for the shell, as expected, while the relative comparison between the shell and the tail remains the same.

Luminosity and Stellar Mass
Unfortunately, it is not possible to derive the total luminosity of the NGC 922 stream, mainly because of some parts of the stream are hidden (or overlapping) the host disk or are too faint to be detected due to the surface brightness limit of our images. However, it is worth attempting to approximate it in order to be able to properly constrain the stellar mass of its progenitor and to better understand the impact of its interaction with NGC 922. With this purpose, the apparent magnitude and total luminosity of the stream of the stream was measured using larger apertures covering as much area as possible of their different detected pieces. Two separate polygonal apertures were placed overlapping the shell and the tail respectively, as depicted in Figure 3. The measured apparent magnitudes are given in Table 1.
From the apparent magnitudes given in Table 1 the NGC 922 distance (d=43.1 Mpc from Meurer et al. 2006), and the Galactic extinction obtained from NASA/IPAC Extragalactic Database 4 ), we derive an absolute magnitude of M g = -16.5 ± 0.04 magn, M r = -17.0 ± 0.03 magn and M z = -17.3 ± 0.03 magn for the whole stream (shell plus tail). We compute the luminosity of these two parts of the stream by using the solar absolute magnitudes for the g, the r and the z passbands from Willmer (2018). We obtain the values of 4.14×10 8 L for the g-passband, 4.42×10 8 L for the r-passband and 5.35×10 8 L for the zpassband. We calculate the mass-to-light ratio (M/L λ ) from the 3 colours measured for the stream (see Section 3.2), using the correlations between SDSS ugriz colors and SDSS/2MASS M/L ratios given in Bell et al. (2003) (i.e., the coefficients given in Table A7 of their paper). Following this method, we obtain an estimate for the stellar mass of the stream progenitor between 6.87 × 10 8 M and 8.51 × 10 8 M . The ratio between the stellar masses of the dwarf progenitor and the host spiral galaxy is key to determine the kind of tidal interaction between them and to characterize the resulting merger event. Considering the stellar mass of NGC922 is 5.47×9 8 M (Wong et al. 2006), this yield a (stellar) mass merger ratio between 0.13 and 0.16.

N-BODY SIMULATIONS
We performed a set of N-body simulations using the ART code (Kravtsov et al. 1997;Colín et al. 2010) starting with initial conditions designed to reproduce the NGC 922-intruder interaction that generated the observed stellar plume (Pellerin et al. 2010). We used a single dark matter particle species with a mass of 2.5×10 4 M , which is the same we set as the maximum star particle mass. The spatial resolution (one AMR cell side) is 40 pc. The simulated system includes a NGC 922-like and a compact dwarf galaxies, both inside a box of 1 Mpc/h side. The NGC 922like galaxy is simulated as a stellar exponential disk embedded in a NFW dark matter halo. In Tab. 2 we show the parameters we used to generate the stellar disk and the dark matter halo A&A proofs: manuscript no. aanda Table 1. Surface brightnesses, colours and apparent magnitude for the tidal features detected around NGC 922. For the shell (S) and for the tail (T), the measurements listed are the average of the measurements on the circular apertures placed along the stream showed in Fig.3. We also include surface brightness and color for NGC 922 measured using a single aperture in its nominal center. profiles. The initial conditions of the collisionless components have been obtained using the Jeans equation moments method as introduced by Hernquist (1993). We chose these parameters to be consistent with results from our own observations of the NGC 922 system and also the ones by Wong et al. (2006); Pellerin et al. (2010); Elagali et al. (2018b). In contrast with previous works, we simulated the compact dwarf system as an extended distribution of particles, which allowed us to reproduce the observed stellar stream. The initial condition of this system is a simple stellar structure that follows a compact NFW profile (see Tab. 2). We located the compact dwarf at (1.0,0.0,100.0) kpc, being the NGC 922-like system at (0.0,0.0,0.0) kpc, and with a relative velocity of (0.0,0.0,-500) km/s. The initial location and velocity of the intruder have been chosen to produce an off-center collision, this following the works by Wong et al. (2006) and Renaud et al. (2018). We evolved the system for 2 Gyr which allows us to study the intruder's orbit from its first infall to an almost complete disruption (see Fig. 4).

DISCUSSION
In this section we analyse two scenarios for the formation of the NGC 922's tidal ring structure. First, we study the currently accepted scenario where the S2 dwarf galaxy is the intruder. Later, we propose a new scenario where the intruder is a much closer satellite that is now almost completely disrupted, that also generated the umbrella-like stellar stream, and the detected HI tail.
In the scenario proposed by Wong et al. (2006), the S2 strongly interacted with NGC 922 and quickly moved to its current location. If we assume NGC 922 and S2 are roughly at the same distance from the Sun (i.e., 43.1 Mpc according to Meurer et al. 2006), this means that S2 moved at a minimum distance of ∼100 Kpc from the NGC 922's center. If S2 is not gravitationally bound to the central galaxy (parabolic orbit), the time interval between two positions d 1 and d 2 of the trajectory is roughly ∆t ∼ (d 3/2 2 − d 3/2 1 )/ √ 4.5GM. Therefore, assuming a close encounter between NGC 922 and S2 and no mass-loss, dynamical friction or other slow-down processes, the S2 dwarf would need, at least, 0.7 Gyr to reach 100 Kpc distance from the NGC 922 center. If S2 is gravitationally bound, this time interval would be even larger. All models and simulations showed that the star forming rings associated with the tidal ring structures only lasts for 0.2−0.5 Gyr (Wong et al. 2006;Pellerin et al. 2010;Renaud et al. 2018;Elagali et al. 2018a) and became only marginally observable up to 0.7 Gyr after the collision (Wu & Jiang 2015). The NGC 922's star forming ring is clearly visible (see Fig. 1), so it is in an early stage of its evolution. Both results are not compatible, therefore we state that this first scenario  We also warn the reader that although Wong et al. (2006) reproduces such scenario using pure N-body simulations, in their models they simulate the intruder as a point-mass, thus, they do not capture most of the tidal effects that affect its evolution. In this simplified model the effects over the central galaxy structure are also artificially magnified. In our models, the intruder has internal structure and it still induces a tidal ring on the central galaxy. The largest difference between our model and the model of Wong et al. (2006) is the fate of the intruder. In agreement with previous models (see e.g., Foster et al. 2014) the intruder is partially or totally disrupted and generates a variety of tidal structures around the central galaxy (see Fig. 4). We also want to notice that the TRG morphology obtained in our simulations lasts for less than 500 Myr after the intruder's first pericenter, as predicted by previous works. This result reinforces the hypothesis that a recent interaction is responsible for the tidal ring structure. Finally, it is also important to mention that in most of the studied TRG the authors detect HI tails in the direction of the intruder (see e.g., Higdon 1996). In Elagali et al. (2018b) the authors found an HI tail but it points in other direction extending towards north of the disk instead of east, and for ∼0.7 arcmin (8 Kpc). This result is also inconsistent with the simple collisional scenario proposed by Wong et al. (2006) for the NGC 922 formation.
The discovery of the new stream morphology (see Sec 3.1) and the results from our N-body models (see Fig. 4) brought us to a new interpretation for the origin of the NGC 922 morphology. From an observational point of view we detect that a satellite merged recently with the host producing two shells one evident in the north-west, another more diffuse in the south-east, with a total mass of ∼6.9-8.5×10 8 M (this work, in agreement with resuts by Pellerin et al. 2010). We expect that stars of this dwarf satellite are now partially mixed with stars of the central. If the mixing truly occurred this would be reflected by a bi-modal distribution in metallicity. In Kouroumpatzakis et al. (2021) the authors found evidences of two metallicity populations in NGC 922, result that is consistent with the new scenario presented here. Additionally, as the NGC 922 is a relatively low-mass galaxy, the infalling satellites do not suffer strong rampressure stripping and bring fresh HI gas that fuels star formation in the main system. The recent infall of a gas rich satellite also explains why NGC 922 has M HI /M * ∼0.32 that is much higher than the one observed in galaxies with similar masses (∼0.13)(see Elagali et al. 2018b). The presence of these tidal shells can also explain the strange kinematic signatures detected in the external regions of the host and that made some authors propose that the system was already perturbed before the formation of the tidal ring (e.g., Renaud et al. 2018). Our N-body models also support this scenario that contradicts the formerly accepted one, where S2 was the intruder (see Fig. 4).

CONCLUSIONS
We present a new merger history for the formation of the tidal ring galaxy NGC 922 after obtaining a set of deep images revealing its low-surface brightness outskirts. The main conclusions of this work are the following: -We report the presence of an umbrella-like tidal structure where previous observations proposed the presence of a connecting tidal tail with a nearby dwarf galaxy. -The absence of such a tidal tail connecting NGC 922 with the dwarf companion S2 (PGC 3080368), both in stars or in HI, may discard the hypothesis of the latter being the trigger of the tidal ring formation. Recent works also raised the question of whether a scenario as simple as a single off-shot could explain the many peculiar properties of this system. -We propose a new scenario for the formation of the tidal ring in NGC 922, that is a merger with an already totally or partially stripped dwarf galaxy. This new scenario explains many of the previous peculiar results like high velocities in the outskirts of the galaxy, the presence of two metallicity populations, and the relatively recent formation of the ring, if the expansion velocities and lifetimes predicted by numerical models are correct. Also a recent gas rich wet merger can explain the observed high HI mass fraction of the galactic system. -Finally we present the discovery of a tidal structure around the dwarf galaxy S2, which holds its own tidal shell structure. This, together with its high star formation rate shown by the GALEX-UV data, could be an evidence that the system is in its first in-fall towards NGC 922. This result can be confirmed by future estimations of the radial velocity and proper motions of this satellite.
Our work also showed as deep imaging of the TRG's properties, reaching a enough surface brightness regime for detecting the presence of giant tidal shells in their outskirts, can provide valuable information on the recent evolution of galaxies undergoing a minor merger. In the Stellar Tidal Stream Survey (Martínez-Delgado et al. 2022, in preparation), we have detected several nearby spiral galaxies with both tidal ring and tidal shell features in different evolutionary stages. Our observations include galaxies with recently formed tidal rings and single tenuous shells, others with well-developed tidal rings and a single shell, and also some with evidence of multiple tidal rings and shells. However, to make a systematic study of the relation between the formation of tidal rings and the almost radial minor mergers that generate tidal shells, it is fundamental to carry out a systematic survey of TRGs with a surface brightness limit fainter than 28 mag arcsec −2 . Once completed, this deep imaging survey combined with future space missions like EUCLID will provide us with enough data to fully characterize the TRG in the local volume and, thus, to better understand the processes that shape galaxies during and after a radial minor merger. This information will also open the door to study whether the Milky Way suffered such a process in its recent history.