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Stephan’s Quintet (SQ, distance=85 Mpc) is unique among compact groups of galaxies. Observations have previously shown that interactions between multiple members, including a high-speed intruder galaxy currently colliding into the intragroup medium, have likely generated tidal debris in the form of multiple gaseous and stellar filaments, the formation of tidal dwarfs and intragroup-medium starbursts, as well as widespread intergalactic shocked gas. The details and timing of the interactions/collisions remain poorly understood because of the multiple nature. Here we report atomic hydrogen (HI) observations in the vicinity of SQ with a smoothed sensitivity of 1σσ=4.2 ×1016cm−2×1016cm−2 per channel (ΔΔv=20 km s−1−1; angular-resolution=4′), which are about two orders of magnitude deeper than previous observations. The data reveal a large HI structure (linear scale ~0.6 Mpc) encompassing an extended source of size ~0.4 Mpc associated with the debris field and a curved diffuse feature of length ~0.5 Mpc attached to the south edge of the extended source. The diffuse feature was likely produced by tidal interactions in early stages of SQ (>1 Gyr ago), though it is not clear how the low density HI gas (NHI≤1018cm−2HI≤1018cm−2) can survive the ionization by the inter-galactic UV background on such a long time scale. Our observations require a rethinking of gas in outer parts of galaxy groups and demand complex modeling of different phases of the intragroup medium in simulations of group formation.

Xu et al., 2022, Nature 610, 461

In this paper, we address the challenge of segmenting global contaminants in large images. The precise delineation of such structures requires ample global context alongside un- derstanding of textural patterns. CNNs specialise in the latter, though their ability to generate global features is limited. At- tention has been used to measure long range dependencies in images, capturing global context, however this incurs a large computational cost.

Context:  Early-type galaxies (ETGs) are divided into slow and fast rotators (FRs and SRs) according to the degree of ordered rotation of their stellar populations. Cosmological hydrodynamical simulations indicate that galaxies form as FRs before their rotational support decreases, usually because of mergers.
Aims: We aimed to investigate this process observationally for galaxies outside of clusters..
Methods:  We made use of the fact that different merger types leave different traces that have different lifetimes. We statistically analyzed multiple characteristics of galaxies that are expected to be influenced by mergers, such as tidal features, kinematically distinct cores, and stellar ages. They were taken from the MATLAS and ATLAS^3D databases. Through multilinear regression we identified the quantities that, at a fixed mass and environmental density of the galaxy, significantly correlate with a measure of the ordered rotation of the galaxy, λ_Re^N.

The characterization of Low Surface Brightness (LSB) stellar structures around galaxies such as tidal debris of on-going or past collisions is essential to constrain models of galactic evolution. Our goal is to obtain quantitative measurements of LSB structures identified in deep images of samples consisting of hundreds of galaxies. We developed an online annotation tool that enables contributors to delineate the shapes of diffuse extended stellar structures, as well as artefacts or foreground structures. All parameters are automatically stored in a database which may be queried to retrieve quantitative measurements. We annotated LSB structures around 352 nearby massive galaxies with deep images obtained with the CFHT as part of two large programs: MATLAS and UNIONS/CFIS.

Context. The lenticular galaxy NGC 474 hosts a rich system of tidal shells and streams, some of which are exceptionally bright. Two teams recently presented spectroscopic observations of the brightest shells. These were the first shell spectra ever published. They studied the stellar populations of the shell, of the center of the galaxy and of its globular clusters.
Aims. The precise formation scenario for the tidal features of this prominent galaxy however still remained unclear. Here, we add further clues on their formation from the radii of the shells, and we present a scenario for the formation of the tidal features that seems to be unique and explaining all available data.
Methods. Shell radii are analyzed with the shell identification method, and we run self-consistent simulations of the formation of the tidal features.

Context. The HI content of dwarf galaxies provides valuable insights into the effects of the internal processes and the environment on galaxy morphology and evolution.
Aims. We investigate the HI properties of the 2210 dwarf and ultra-diffuse galaxies (UDGs) in the low to moderate density environ- ments of the Mass Assembly of early-Type GaLAxies with their fine Structures (MATLAS) survey. We examine how these properties correlate with the optical properties, morphology and local environment of the dwarfs.
Methods. We make use of the HI data obtained with the Westerbork Synthesis Radio Telescope in the context of the ATLAS3D HI survey. These HI observations are combined with the extragalactic HI sources from the ALFALFA survey study the HI content in the MATLAS dwarfs located in the targeted regions of the sky.