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The interactions and mergers of gas rich galaxies are known to produce star formation which often leads to nuclear activity as well. The star formation is ideally mapped using FUV and NUV emission, since UV traces star formation for longer timescales compared to Hα emission. It is also emitted over a broader range of stellar masses in galaxies. In this study we present FUV and NUV observations of merging and interacting galaxies in our nearby universe conducted using the UVIT. We present the example of a merging system MRK212 that has dual AGN and the triple AGN system NGC7733-7734. The UV emission is associated with the tidal arms, individual nuclei, resonance rings, nuclear spirals as well as AGN/stellar feedback. We also find that radio emission is often closely associated with the UV emission, arising from both star formation as well as AGN activity, and perhaps kpc-scale AGN feedback. We find that a comparison of optical IFU imaging with FUV in NGC7733-7734 reveals unique properties associated with the interaction including the third AGN buried in a tidal arm.

The discovery of a distant, low-mass satellite galaxy helps to constrain the mass function for substructure beyond the local Universe to a form that agrees at the 95 per cent confidence level with predictions based on cold dark matter. A distant dark galaxy unveiled Simulations of galaxy formation are based on the assumption that much of the Universe consists of unobservable cold dark matter. Such models predict the presence of many more dwarf galaxies than we see in the Local Group, which may, however, be anomalous. Dwarf galaxies cannot normally be seen at large distances because their stellar light is too faint, but Vegetti et al . now report the presence of a dwarf galaxy companion to a galaxy at the cosmological distance of redshift 0.881, using gravitational lensing. The dark satellite galaxy is similar to the Sagittarius dwarf around our own Galaxy, and allows an estimate of a mass function consistent with the predictions from cold dark matter simulations at the 95% confidence level. The mass function of dwarf satellite galaxies that are observed around Local Group galaxies differs substantially from simulations 1 , 2 , 3 , 4 , 5 based on cold dark matter: the simulations predict many more dwarf galaxies than are seen. The Local Group, however, may be anomalous in this regard 6 , 7 . A massive dark satellite in an early-type lens galaxy at a redshift of 0.222 was recently found 8 using a method based on gravitational lensing 9 , 10 , suggesting that the mass fraction contained in substructure could be higher than is predicted from simulations. The lack of very low-mass detections, however, prohibited any constraint on their mass function. Here we report the presence of a (1.9 ± 0.1) × 10 8 dark satellite galaxy in the Einstein ring system JVAS B1938+666 (ref. 11 ) at a redshift of 0.881, where denotes the solar mass. This satellite galaxy has a mass similar to that of the Sagittarius 12 galaxy, which is a satellite of the Milky Way. We determine the logarithmic slope of the mass function for substructure beyond the local Universe to be , with an average mass fraction of per cent, by combining data on both of these recently discovered galaxies. Our results are consistent with the predictions from cold dark matter simulations 13 , 14 , 15 at the 95 per cent confidence level, and therefore agree with the view that galaxies formed hierarchically in a Universe composed of cold dark matter.

Reshaping the Galaxy Since its discovery more than a decade ago, the Sagittarius dwarf galaxy (Sgr), a satellite galaxy of our own Milky Way, has been recognized as a local analogue to the numerous mergers thought to be common in galaxies throughout the Universe. Traditionally, Sgr has been treated as a negligible perturber to the Galactic disk. New simulations of the response of the Milky Way to the infall of the Sgr reveal that, on the contrary, Sgr has played an important part in shaping the disk morphology. Past impacts have triggered the formation of spiral structure and influenced bar evolution. Like many galaxies of its size, the Milky Way is a disk with prominent spiral arms rooted in a central bar 1 , although our knowledge of its structure and origin is incomplete. Traditional attempts to understand our Galaxy’s morphology assume that it has been unperturbed by major external forces. Here we report simulations of the response of the Milky Way to the infall of the Sagittarius 2 dwarf galaxy (Sgr), which results in the formation of spiral arms, influences the central bar and produces a flared outer disk. Two ring-like wrappings emerge towards the Galactic anti-Centre in our model that are reminiscent of the low-latitude arcs observed in the same area of the Milky Way. Previous models have focused on Sgr itself 3 , 4 to reproduce the dwarf’s orbital history and place associated constraints on the shape of the Milky Way gravitational potential, treating the Sgr impact event as a trivial influence on the Galactic disk. Our results show that the Milky Way’s morphology is not purely secular in origin and that low-mass minor mergers predicted to be common throughout the Universe 5 probably have a similarly important role in shaping galactic structure.

Milky way satellites: galaxies with a dark side Sophisticated optical astronomy projects such as the Sloan Digital Sky Survey are reaching a new threshold in detecting the least luminous galaxies in the Universe, and now at least twenty-three faint satellite galaxies are known in the region of the Milky Way. They range in luminosity from about a thousand to more than 100 million times that of the Sun. The velocities of the stars in these galaxies reveal that despite this variation in luminosity, each of the galaxies is similar in mass, at about 10 million times the mass of the Sun within their central 300 parsecs. The faintest of the Milky Way satellites are accordingly the most dark-matter-dominated galaxies known in the Universe. The Milky Way has at least twenty-three known satellite galaxies that shine with luminosities ranging from about a thousand to a billion times that of the Sun. Half of these galaxies were discovered 1 , 2 in the past few years in the Sloan Digital Sky Survey, and they are among the least luminous galaxies in the known Universe. A determination of the mass of these galaxies provides a test of galaxy formation at the smallest scales 3 , 4 and probes the nature of the dark matter that dominates the mass density of the Universe 5 . Here we use new measurements of the velocities of the stars in these galaxies 6 , 7 to show that they are consistent with them having a common mass of about 10 7 within their central 300 parsecs. This result demonstrates that the faintest of the Milky Way satellites are the most dark-matter-dominated galaxies known, and could be a hint of a new scale in galaxy formation or a characteristic scale for the clustering of dark matter.

We present 3D observations of three interacting galaxies in order derive their extended kinematics and to trace events such as violent star-formation (SF), mass-transfer, structure perturbation and the presence of energetic sources induced by the interacting processes.

It was found that satellites of nearby galaxies can form flattened co-rotating structures called disks of satellites or planes of satellites. Their existence is not expected by the current galaxy formation simulations in the standard dark matter-based cosmology. On the contrary, modified gravity offers a promising alternative: the objects in the disks of satellites are tidal dwarf galaxies, that is, small galaxies that form from tidal tails of interacting galaxies. After introducing the topic, we review here our work on simulating the formation of the disks of satellites of the Milky Way and Andromeda galaxies. The initial conditions of the simulation were tuned to reproduce the observed positions, velocities and disk orientations of the galaxies. The simulation showed that the galaxies had a close flyby 6.8 Gyr ago. One of the tidal tails produced by the Milky Way was captured by Andromeda. It formed a cloud of particles resembling the disk of satellites at Andromeda by its size, orientation, rotation and mass. A hint of a disk of satellites was formed at the Milky Way too. In addition, the encounter induced a warp in the disk of the simulated Milky Way that resembles the real warp by its magnitude and orientation. We present here, for the first time, the proper motions of the members of the disk of satellites of Andromeda predicted by our simulation. Finally, we point out some of the remaining open questions which this hypothesis, for the formation of disks of satellites, brings up.

We present an analysis of bar structures in a sample of 140 strongly interacting galaxies with bridge features and 57 M51-type galaxies using high-resolution images from the Hubble Space Telescope. This study explores the influence of gravitational interactions on bar formation and stability. Our results show that the bar fraction ( f bar ) is significantly low in both samples, with only 16% of galaxies exhibiting bar structures. By examining the relationship between bar fraction and key parameters such as angular separation, redshift, and surface brightness, we find that bars are more likely to form in larger, brighter galaxies with weaker interactions. Galaxies with close companions, high redshift, or fainter surface brightness show a reduced likelihood of developing or maintaining bar structures. These findings suggest that strong gravitational interactions inhibit or disrupt the formation of bars in galaxies, contributing to our understanding of galactic evolution and morphology. Our results provide valuable insights into how external forces shape galactic structures and offer important implications for studying galaxy formation and dynamics.

The RAD@home RGB-maker Tool is a python-based web application that enables citizen science research through collaboration using open-source technology. The tool fetches FITS image data from NASA SkyView and generates false colour images in Red-Green-Blue channels with contour. This helps in the basic multi-wavelength understanding and characterization of extragalactic objects, and further analysis along with reporting of potential new discoveries in a uniform format. Students from Universities or science-graduate citizens gain skills in RGB-C image analysis and identify unique features in objects via either one-day online-weekend events or week-long in-person training. Trained citizen scientists in turn are part of rare discoveries such as the jet-galaxy interaction system RAD12. The tool has been successfully used by thousands of citizen scientists in India since its launch on 26th January 2021 and has demonstrated the potential during IAU symposium 375 to be part of citizen science efforts with international participants.

Collisional debris around interacting and post-interacting galaxies often display condensations of gas and young stars that can potentially form gravitationally bound objects: Tidal Dwarf Galaxies (TDGs). We summarise recent results on TDGs, which are originally published in Lelli et al. (2015, A&A).We study a sample of six TDGs around three different interacting systems, using high-resolution HI observations from the Very Large Array. We find that the HI emission associated to TDGs can be described by rotating disc models. These discs, however, would have undergone less than one orbit since the time of the TDG formation, raising the question of whether they are in dynamical equilibrium. Assuming that TDGs are in dynamical equilibrium, we find that the ratio of dynamical mass to baryonic mass is consistent with one, implying that TDGs are devoid of dark matter. This is in line with the results of numerical simulations where tidal forces effectively segregate dark matter in the halo from baryonic matter in the disc, which ends up forming tidal tails and TDGs.

To clarify the sources of outer gas accretion onto disk galaxies, we study the vicinity of four interacting galaxy systems in the Hα emission line by using the scanning Fabry–Perot interferometer of the 6m telescope of the Special Astrophysical Observatory RAS. We find perspective accretion flows seen as ionized-gas emission filaments between the galaxies. We discuss the whole kinematics and origin of these flows.