Tidal ring galaxies are observed rarely in the local universe due to theirintrinsically transient nature. The tidal ring structures are the result ofstrong interactions between gas-rich stellar disks and smaller galactic systemsand do not last longer than ~500~Myr therefore, these are perfect scenarioswhere to find the debris of recently accreted dwarf galactic systems. Wepresent new deep images of the NGC 922 tidal ring galaxy and its surroundingsfrom the DESI Legacy survey data and from our observations with an amateurtelescope. These observations are compared with results from high-resolutionN-body simulations designed to reproduce an alternative formation scenario forthis peculiar galaxy. Our new observations unveil that the low surfacebrightness stellar tidal structures around NGC 922 are much more complex thanreported in previous works. In particular, the formerly detected tidalspike-like structure at the northeast of the central galaxy disk is notconnected with the dwarf companion galaxy PGC3080368, which has been suggestedas the intruder triggering the ring formation of NGC 922. The deep imagesreveal that this tidal structure is mainly composed by a fainter giantumbrella-like shape and thus it was formed from the tidal disruption of adifferent satellite. Using the broad-band g, r and z DESI LS images, wemeasured the photometric properties of this stellar stream, estimating a totalabsolute magnitude in r-band of Mr= -17.0 +/- 0.03 magn and a total stellarmass for the stream between 6.9-8.5X10^8 Mo. We perform a set of N-bodysimulations to reproduce the observed NGC 922-intruder interaction, suggestinga new scenario for the formation of its tidal ring from the in-fall of a gasrich satellite around 150 Myr ago.

The outskirts of galaxies have been studied from multiple perspectives forthe past few decades. However, it is still unknown if all galaxies haveclear-cut edges like everyday objects. We address this question by developingphysically motivated criteria to define the edges of galaxies. Based on the gasdensity threshold required for star formation, we define the edge of a galaxyas the outermost radial location associated with a significant drop in eitherpast or ongoing in-situ star formation. We explore $\sim$1000 low-inclinationgalaxies with a wide range in morphology (dwarfs to ellipticals) and stellarmass ($10^7 M_{\odot} < M_{\star} < 10^{12}M_{\odot}$). The location of theedges of these galaxies ($R_{\rm edge}$) are visually identified as theoutermost cut-off or truncation in their radial profiles using deep multi-bandoptical imaging from the IAC Stripe82 Legacy Project. We find thischaracteristic feature at the following mean stellar mass density which varieswith galaxy morphology: $2.9\pm0.10\,M_{\odot}$/pc$^2$ for ellipticals,$1.1\pm0.04\,M_{\odot}/$pc$^2$ for spirals and $0.6\pm0.03\,M_{\odot}/$pc$^2$for present-day star forming dwarfs. Additionally, we find that $R_{\rm edge}$depends on its age (colour) where bluer galaxies have larger $R_{\rm edge}$ ata fixed stellar mass. The resulting stellar mass--size plane using $R_{\rmedge}$ as a physically motivated galaxy size measure has a very narrowintrinsic scatter ($\lesssim 0.06$ dex). These results highlight the importanceof new deep imaging surveys to explore the growth of galaxies and trace thelimits of star formation in their outskirts.