NGC 4993 is the shell galaxy host of the GRB170817A short gamma-ray burst and the GW170817 gravitational-wave event produced during a binary-neutron-star coalescence. The galaxy shows signs, ...including the stellar shells, that it has recently accreted a smaller, late-type galaxy. The accreted galaxy might be the original host of the binary neutron star. We measured the positions of the stellar shells of NGC 4993 in an HST/ACS archival image and use the shell positions to constrain the time of the galactic merger. According to the analytical model of the evolution of the shell structure in the expected gravitational potential of NGC 4993, the galactic merger happened at least 200 Myr ago, with a probable time roughly around 400 Myr and the estimates higher than 600 Myr being improbable. This constitutes the lower limit on the age of the binary neutron star, because the host galaxy was probably quenched even before the galactic merger, and the merger has likely shut down the star formation in the accreted galaxy. We roughly estimate the probability that the binary neutron star originates in the accreted galaxy to be around 30%.
(Abridged) Dynamical friction can be used to distinguish Newtonian gravity and modified Newtonian dynamics (MOND) because it works differently in these frameworks. This concept, however, has yet to ...be explored very much with MOND. Previous simulations showed weaker dynamical friction during major mergers for MOND than for Newtonian gravity with dark matter. Analytic arguments suggest the opposite for minor mergers. In this work, we verify the analytic predictions for MOND by high-resolution \(N\)-body simulations of globular clusters (GCs) moving in isolated ultra-diffuse galaxies (UDGs). We test the MOND analog of the Chandrasekhar formula for the dynamical friction proposed by Sánchez-Salcedo on a single GC. We also explore whether MOND allows GC systems of isolated UDGs to survive without sinking into nuclear star clusters. The simulations are run using the adaptive-mesh-refinement code Phantom of Ramses. The mass resolution is \(20\,M_\odot\) and the spatial resolution \(50\,\)pc. The GCs are modeled as point masses. Simulations including a single GC reveal that, as long as the apocenter of the GC is over about 0.5 effective radii, the Sánchez-Salcedo formula works excellently, with an effective Coulomb logarithm increasing with orbital circularity. Once the GC reaches the central kiloparsec, its sinking virtually stops, likely because of the core stalling mechanism. In simulations with multiple GCs, many of them sink toward the center, but the core stalling effect seems to prevent them from forming a nuclear star cluster. The GC system ends up with a lower velocity dispersion than the stars of the galaxy. By scaling the simulations, we extend these results to most UDG parameters, as long as these UDGs are not external-field dominated.
The missing mass problem has not been solved decisively yet. Observations show that if gravity is to be modified, then the MOND theory is its excellent approximation on galactic scales. MOND suggests ...an adjustments of the laws of physics in the limit of low accelerations. Comparative simulations of interacting galaxies in MOND and Newtonian gravity with dark matter revealed two principal differences: 1) galaxies can have close flybys without ending in mergers in MOND because of weaker dynamical friction, and 2) tidal dwarf galaxies form very easily in MOND. When this is combined with the fact that many interacting galaxies are observed at high redshift, we obtain a new perspective on tidal features: they are often formed by non-merging encounters and tidal disruptions of tidal dwarf galaxies. Here we present the results from our self-consistent MOND \(N\)-body simulation of a close flyby of two galaxies similar to the Milky Way. It turns out that most types of the structures that are traditionally assigned to galaxy mergers can be formed by non-merging encounters, including tidal arms, bridges, streams, shells, disk warps, thick disks, and most probably also disks of satellites. The success of MOND in explaining the dynamics of galaxies hints us that this way of formation of tidal structures should be considered seriously.
A small fraction of early-type galaxies (ETGs) show prolate rotation, i.e. they rotate around their long photometric axis. In simulations, certain configurations of galaxy mergers are known to ...produce this type of rotation. We investigate the association of prolate rotation and signs of galaxy interactions among the observed galaxies. We collected a sample of 19 nearby ETGs with distinct prolate rotation from the literature and inspected their ground-based deep optical images for interaction signs - 18 in archival images and one in a new image obtained with the Milanković telescope. Tidal tails, shells, asymmetric/disturbed stellar halos, or on-going interactions are present in all the 19 prolate rotators. Comparing this with the frequency of tidal disturbance among the general sample of ETGs of a roughly similar mass range and surface-brightness limit, we estimate that the chance probability of such an observation is only 0.00087. We also found a significant overabundance of prolate rotators that are hosting multiple stellar shells. The visible tidal features imply a relatively recent galaxy interaction. That agrees with the Illustris large-scale cosmological hydrodynamical simulation, where prolate rotators are predominantly formed in major mergers during the last 6 Gyr. In the appendix, we present the properties of an additional galaxy, NGC 7052, a prolate rotator for which no deep images are available, but for which an HST image revealed the presence of a prominent shell, which had not been reported before.
The lenticular galaxy NGC474 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 observed in integrated starlight. The authors studied the stellar populations of the shell, of the center of the galaxy and of its globular clusters. 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. Shell radii are analyzed with the shell identification method, and we run self-consistent simulations of the formation of the tidal features. We consider Newtonian as well as MOND gravity. Observations suggest that the tidal features originate from the accretion of a spiral galaxy. The shell identification method yields that the merging galaxies collided first 1.3Gyr ago and then again 0.9Gyr ago, thereby forming the shells in two generations. This would also explain the young ages of stellar populations in the center of the galaxy and the young age of the globular clusters. The analytic models of shell propagation, that underlie the shell identification method, are verified by a simulation. The simulations reproduce well the observed morphology of the tidal features. The accreted spiral likely reached NGC474 nearly radially, in the plane of the sky, from the south, its rotation axis pointing toward us. It should have had a stellar mass of around 1/6 of NGC474, i.e. \(10^{9.8}\,M_\odot\). It seems that all tidal features in the galaxy originate from one merger.
To understand the origin of extended disks of low-surface brightness (LSB) galaxies, we studied in detail 4 such systems with large disks seen edge-on. Two of them are edge-on giant LSB galaxies ...(gLSBGs) recently identified by our team. The edge-on orientation of these systems boosts their surface brightnesses that provided an opportunity to characterize stellar populations spectroscopically and yielded the first such measurements for edge-on gLSBGs. We collected deep images of one galaxy using the 1.4-m Milanković Telescope which we combined with the archival Subaru Hyper Suprime-Cam and DESI Legacy Surveys data available for the three other systems, and measured the structural parameters of the disks. We acquired deep long-slit spectra with the Russian 6-meter telescope and the 10-m Keck II telescope and estimated stellar population properties in the high- and low-surface brightness regions as well as the gas-phase metallicity distribution. The gas metallicity gradients are shallow to flat in the range between 0 and -0.03 dex per exponential disk scale length, which is consistent with the extrapolation of the gradient -- scale length relation for smaller disk galaxies. Our estimates of stellar velocity dispersion in the LSB disks as well as the relative thickness of the disks indicate the dynamical overheating. Our observations favor mergers as the essential stage in the formation scenario for massive LSB galaxies.
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. Each LSB structure was delineated and labeled according to its likely nature: stellar shells, streams associated to a disrupted satellite, tails formed in major mergers, ghost reflections or cirrus. From our database containing 8441 annotations, the area, size, median surface brightness and distance to the host of 228 structures were computed. The results confirm the fact that tidal structures defined as streams are thinner than tails, as expected by numerical simulations. In addition, tidal tails appear to exhibit a higher surface brightness than streams (by about 1 mag), which may be related to different survival times for the two types of collisional debris. We did not detect any tidal feature fainter than 27.5 mag.arcsec\(^{-2}\), while the nominal surface brightness limits of our surveys range between 28.3 and 29 mag.arcsec\(^{-2}\), a difference that needs to be taken into account when estimating the sensitivity of future surveys to identify LSB structures. Our annotation database of observed LSB structures may be used for quantitative analysis and as a training set for machine learning algorithms (abbreviated).
Kinematical and morphological features observed in early-type galaxies provide valuable insights into the evolution of their hosts. We studied the origin of prolate rotation (i.e., rotation around ...the long axis) in Illustris large-scale cosmological hydrodynamical simulations. We found that basically all the simulated massive prolate rotators were created in relatively recent major mergers of galaxies. Such mergers are expected to produce tidal features such as tails, shells, asymmetric stellar halos. We investigated deep optical images of prolate rotators, including newly obtained Milanković data, revealing signs of galaxy interaction in all of them. This correlation proves to be statistically very significant when compared with a general sample of early-type galaxies from the MATLAS deep imaging survey. In an ongoing project, we use Milanković to assemble deep images of the complete sample of all known nearby massive prolate rotators. Additionally, we searched these data for asteroids to improve the accuracy of trajectories and even discover one previously unknown main-belt asteroid. The most frequent tidal features among the prolate rotators happen to be shells. We developed methods to calculate the probable time of the merger from optical images. This will allow us to compare the merger history of the sample with predictions from Illustris. Our plan is to expand these methods to even larger samples of shell galaxies supplied by upcoming large surveys like LSST at Rubin Observatory. This will provide an unprecedented amount of statistically significant data on the recent merger history of our Universe and allow extensive investigation of the impact of mergers to a wide range of other astrophysical phenomena.
(abridged) The Milky Way (MW) and Andromeda (M31) galaxies possess rotating planes of satellites. Their formation has not been explained satisfactorily yet. It was suggested that the MW and M31 ...satellites are ancient tidal dwarf galaxies, which could explain their configuration. This suggestion gained support by an analytic backward calculation of the relative MW-M31 orbit in the MOND modified dynamics paradigm by Zhao et al. (2013) implying their close flyby 7-11 Gyr ago. Here we explore the Local Group history in MOND in more detail using a simplified first-ever self-consistent simulation. We note the features induced by the encounter in the simulation and identify their possible real counterparts. The simulation was set to approximately reproduce the observed MW and M31 masses, effective radii, separation, relative velocity and disk inclinations. We used the publicly available adaptive-mesh-refinement code Phantom of RAMSES. In the simulation, matter was transferred from the MW to M31 along a tidal tail. The encounter induced formation of several structures resembling the peculiarities of the Local Group. Most notably: 1) A rotating planar structure formed around M31 from the transferred material. It had a size similar to the observed satellite plane and was oriented edge-on to the simulated MW, just as the real one. 2) The same structure also resembled the tidal features observed around M31 by its size and morphology. 3) A warp in the MW developed with an amplitude and orientation similar to that observed. 4) A cloud of particles formed around the simulated MW, with the extent of the actual MW satellite system. The encounter did not end by merging in a Hubble time. The simulation thus demonstrated that MOND can possibly explain many peculiarities of the Local Group and, moreover, that non-merging galaxy encounters in MOND can produce tidal features in galaxies.
Our work has two recherchive parts. The first is devoted to the shell galaxies and we describe the observational facts here, different models of origin and the possibility of determining the ...gravitational potential with the use of shells. The second part is on the modified Newtonian dynamics (MOND). We explain what it consists in, its implications and its experimental tests. The third part is devoted to our numerical simulations of a shell system evolution both in the classical and modified dynamics. Our mission is to explain the differences in the two simulation theoretically and to verify, whether the result of the modified simulation oppose the observed shells around the galaxy NGC 3923. The conclusion is it doesn't, but our test is not very strong.
