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Using parsec scale resolution hydrodynamical adaptive mesh refinement simulations, we have studied the mass transport process throughout a Galactic merger. The aim of such study is to connect both the peaks of mass accretion rate on to the BHs and star formation bursts with both gravitational and hydrodynamic torques acting on the galactic gaseous component. Our merger initial conditions were chosen to mimic a realistic system. The simulations include gas cooling, star formation, supernovae feedback, and AGN feedback. Gravitational and hydrodynamic torques near pericentre passes trigger gas funneling to the nuclei that is associated with bursts of star formation and black hole growth. Such episodes are intimately related with both kinds of torques acting on the galactic gas. Pericentres trigger both star formation and mass accretion rates of ∼few |$(1\!-\!10)\, \mathrm{ M}_\odot$|  yr−1. Such episodes last |$\sim \, (50\!-\!75)$| Myr. Close passes also can produce black hole accretion that approaches and reaches the Eddington rate, lasting ∼few Myrs. Our simulation shows that both gravitational and hydrodynamic torques are enhanced at pericentre passes with gravitational torques tending to have higher values than the hydrodynamic torques throughout the merger. We also find that in the closest encounters, hydrodynamic and gravitational torques can be comparable in their effect on the gas, the two helping in the redistribution of both angular momentum and mass in the galactic disc. Such phenomena allow inward mass transport on to the BH influence radius, fuelling the compact object and lighting up the galactic nuclei. [ABSTRACT FROM AUTHOR]

Discoveries of low-mass galaxy pairs and groups are increasing. Studies indicate that dwarf galaxy pairs are gas rich in the field and exhibit elevated star formation rates, suggestive of interactions. Lacking are dynamical models of observed dwarf galaxy pairs to disentangle the physical processes regulating their baryon cycles. We present new optical data and the first detailed theoretical model of an observed tidal encounter between two isolated low-mass galaxies (NGC 4490 and NGC 4485). This system is an isolated analogue of the Magellanic Clouds and is surrounded by a ∼50 kpc extended H  i envelope. We use hybrid N -body and test-particle simulations along with a visualization interface (Identikit) to simultaneously reproduce the observed present-day morphology and kinematics. Our results demonstrate how repeated encounters between two dwarf galaxies can 'park' baryons at very large distances, without the aid of environmental effects. Our best match to the data is an 8:1 mass ratio encounter where a one-armed spiral is induced in the NGC 4490-analogue, which we postulate explains the nature of diffuse starlight presented in the new optical data. We predict that the pair will fully merge in ∼370 Myr, but that the extended tidal features will continue to evolve and return to the merged remnant over ∼5 Gyr. This pre-processing of baryons will affect the efficiency of gas stripping if such dwarf pairs are accreted by a massive host. In contrast, in isolated environments this study demonstrates how dwarf–dwarf interactions can create a long-lived supply of gas to the merger remnant. [ABSTRACT FROM AUTHOR]

We quantify the frequency of companions of low-redshift (0.013 < $$z$$ < 0.0252) dwarf galaxies (2 × 108 M⊙ < Mstar < 5 × 109 M⊙) that are isolated from more massive galaxies in SDSS and compare against cosmological expectations using mock observations of the Illustris simulation. Dwarf multiples are defined as two or more dwarfs that have angular separations >55 arcsec, projected separations rp < 150 kpc, and relative line-of-sight velocities Δ V LOS < 150 km s−1. While the mock catalogues predict a factor of two more isolated dwarfs than observed in SDSS, the mean number of observed companions per dwarf is Nc  ∼ 0.04, in good agreement with Illustris when accounting for SDSS sensitivity limits. Removing these limits in the mock catalogues predicts Nc  ∼ 0.06 for future surveys (LSST, DESI), which will be complete to Mstar = 2 × 108 M⊙. The 3D separations of mock dwarf multiples reveal a contamination fraction of ∼40 per cent in observations from projection effects. Most isolated multiples are pairs; triples are rare and it is cosmologically improbable that bound groups of dwarfs with more than three members exist within the parameter range probed in this study. We find that <1 per cent of LMC-analogues in the field have an SMC-analogue companion. The fraction of dwarf "Major Pairs" (stellar mass ratio >1:4) steadily increases with decreasing Primary stellar mass, whereas the cosmological "Major Merger rate" (per Gyr) has the opposite behaviour. We conclude that cosmological simulations can be reliably used to constrain the fraction of dwarf mergers across cosmic time. [ABSTRACT FROM AUTHOR]

We explore the effect of using different kinematic tracers (HI and Hα) on reconstructing the encounter parameters of the Mice major galaxy merger (NGC 4676A/B). We observed the Mice using the SparsePak Integral Field Unit (IFU) on the WIYN telescope, and compared the Hα velocity mapwith VLA HI observations. The relatively high spectral resolution of our data (R ≈ 5000) allows us to resolve more than one kinematic component in the emission lines of some fibres.We separate the Hα-[NII] emission of the star-forming regions from shocks using their [NII]/H α line ratio and velocity dispersion. We show that the velocity of star-forming regions agree with that of the cold gas (H I), particularly, in the tidal tails of the system. We reconstruct the morphology and kinematics of these tidal tails utilizing an automated modelling method based on the IDENTIKIT software package. We quantify the goodness of fit and the uncertainties of the derived encounter parameters. Most of the initial conditions reconstructed using Hα and HI are consistent with each other, and qualitatively agree with the results of previous works. For example, we find 210±5040 Myr, and 180±5040 Myr for the time since pericentre, when modelling Hα and HI kinematics, respectively. This confirms that in some cases, Hα kinematics can be used instead of HI kinematics for reconstructing the initial conditions of galaxy mergers, and our automated modelling method is applicable to some merging systems. [ABSTRACT FROM AUTHOR]