ALBERT

Description: We simulate an equal-mass merger of two Milky Way-size galaxy discs with moderate gas fractions at parsec-scale resolution including a new model for radiative cooling and heating in a multiphase medium, as well as star formation and feedback from supernovae. The two discs initially have a 2.6 10 6 M supermassive black hole (SMBH) embedded in their centres. As the merger completes and the two galactic cores merge, the SMBHs form a pair with a separation of a few hundred pc that gradually decays. Due to the stochastic nature of the system immediately following the merger, the orbital plane of the binary is significantly perturbed. Furthermore, owing to the strong starburst the gas from the central region is completely evacuated, requiring ~10 Myr for a nuclear disc to rebuild. Most importantly, the clumpy nature of the interstellar medium has a major impact on the dynamical evolution of the SMBH pair, which undergo gravitational encounters with massive gas clouds and stochastic torquing by both clouds and spiral modes in the disc. These effects combine to greatly delay the decay of the two SMBHs to separations of a few parsecs by nearly two orders of magnitude, ~10 8  yr, compared to previous work. In mergers of more gas-rich, clumpier galaxies at high redshift stochastic torques will be even more pronounced and potentially lead to stronger modulation of the orbital decay. This suggests that SMBH pairs at separations of several tens of parsecs should be relatively common at any redshift.

Description: We present a new zoom-in hydrodynamical simulation, ‘Eris BH ’, which features the same initial conditions, resolution, and sub-grid physics as the close Milky Way-analogue ‘Eris’ (Guedes et al. 2011 ), but it also includes prescriptions for the formation, growth and feedback of supermassive black holes. This enables a detailed study of black hole evolution and the impact of active galactic nuclei (AGN) feedback in a late-type galaxy. At z = 0, the main galaxy of Eris BH hosts a central black hole of 2.6 x 10 6 M , which correlates to the bulge mass and the galaxy's central velocity dispersion similarly to what is observed in the Milky Way and in pseudobulges. During its evolution, the black hole grows mostly through mergers with black holes brought in by accreted satellite galaxies and very little by gas accretion (due to the modest amount of gas that reaches the central regions). AGN feedback is weak and it affects only the central $1\text{--}2 \,\rm {kpc}$ . Yet, it limits the growth of the bulge, which results in a rotation curve that, in the inner ~ 10 kpc, is flatter than that of Eris. We find that Eris BH is more prone to instabilities than Eris, due to its smaller bulge and larger disc. At z ~ 0.3, an initially small bar grows to be of a few disc scalelengths in size. The formation of the bar causes a small burst of star formation in the inner few hundred pc, provides new gas to the central black hole and causes the bulge to have a boxy/peanut morphology by z = 0.

Description: Using collisionless N -body simulations of dwarf galaxies orbiting the Milky Way, we construct realistic models of dwarf spheroidal (dSph) galaxies of the Local Group. The dwarfs are initially composed of stellar discs embedded in dark matter haloes with different inner density slopes and are placed on an eccentric orbit typical for Milky Way subhaloes. After a few Gyr of evolution, the stellar component is triaxial as a result of bar instability induced by tidal forces. Observing the simulated dwarfs along the three principal axes of the stellar component, we create mock data sets and determine the corresponding half-light radii and line-of-sight velocity dispersions. Using the estimator proposed by Wolf et al., we calculate the masses within half-light radii. The masses obtained in this way are over(under)estimated by up to a factor of 2 when the line of sight is along the longest (shortest) axis of the stellar component. We then divide the initial stellar distribution into an inner and outer population and trace their evolution in time. The two populations, although strongly affected by tidal forces, retain different density profiles even after a few Gyr of evolution. We measure the half-light radii and velocity dispersions of the stars in the two populations along different lines of sight and use them to estimate the slope of the mass distribution in the dwarf galaxies following the method recently proposed by Walker & Peñarrubia. The inferred slopes are systematically over- or underestimated, depending on the line of sight. In particular, when the dwarf is seen along the longest axis of the stellar component, a significantly shallower density profile is inferred than the real one measured from the simulations. Given that most dSph galaxies in the Local Group are non-spherical in appearance and their orientation with respect to our line of sight is unknown, but most probably random, the method can be reliably applied only to a large sample of dwarfs when these systematic errors are expected to be diminished.

Description: We present a phenomenological description of the properties of tidal tails forming around dwarf galaxies orbiting the Milky Way. For this purpose we use collisionless N -body simulations of dwarfs initially composed of a disc embedded in an NFW dark matter halo. The dwarfs are placed on seven orbits around the Milky Way like host, differing in size and eccentricity, and their evolution is followed for 10 Gyr. In addition to the well-studied morphological and dynamical transformation of the dwarf's main body, the tidal stripping causes them to lose a substantial fraction of mass both in dark matter and in stars which form pronounced tidal tails. We focus on the properties of the stellar component of the tidal tails thus formed. We first discuss the break radii in the stellar density profile defining the transition to tidal tails as the radii where the profile becomes shallower and relate them to the classically defined tidal radii. We then calculate the relative density and velocity of the tails at a few break radii as a function of the orbital phase. Next, we measure the orientation of the tails with respect to an observer placed at the centre of the Milky Way. The tails are perpendicular to this line of sight only for a short period of time near the pericentre. For most of the time the angles between the tails and this line of sight are low, with orbit-averaged medians below 42° for all, even the almost circular orbit. The median angle is typically lower while the maximum relative density higher for more eccentric orbits. The combined effects of relative density and orientation of the tails suggest that they should be easiest to detect for dwarf galaxies soon after their pericentre passage.