We perform high-resolution N-body+SPH (smoothed particle hydrodynamics) simulations of gas-rich dwarf galaxy satellites orbiting within a Milky Way-sized halo and study for the first time the ...combined effects of tides and ram pressure. The structure of the galaxy models and the orbital configurations are chosen in accordance with those expected in a Lambda cold dark matter (ΛCDM) universe. While tidal stirring of disky dwarfs produces objects whose stellar structure and kinematics resembles that of dwarf spheroidals after a few orbits, ram pressure stripping is needed to entirely remove their gas component. Gravitational tides can aid ram pressure stripping by diminishing the overall potential of the dwarf, but tides also induce bar formation which funnels gas inwards making subsequent stripping more difficult. This inflow is particularly effective when the gas can cool radiatively. Assuming a low density of the hot Galactic corona consistent with observational constraints, dwarfs with Vpeak < 30 km s−1 can be completely stripped of their gas content on orbits with pericenters of 50 kpc or less. Instead, dwarfs with more massive dark haloes and Vpeak > 30 km s−1 lose most or all of their gas content only if a heating source keeps the gas extended, partially counteracting the bar-driven inflow. We show that the ionizing radiation from the cosmic ultraviolet (UV) background at z > 2 can provide the required heating. In these objects, most of the gas is removed or becomes ionized at the first pericenter passage, explaining the early truncation of the star formation observed in Draco and Ursa Minor. Galaxies on orbits with larger pericenters and/or falling into the Milky Way halo at lower redshift can retain significant amounts of the centrally concentrated gas. These dwarfs would continue to form stars over a longer period of time, especially close to pericenter passages, as observed in Fornax and other dwarf spheroidal galaxies (dSphs) of the Local Group. The stripped gas breaks up into individual clouds pressure confined by the outer gaseous medium that have masses, sizes and densities comparable to the H-i clouds recently discovered around M31.
Giant impacts (GIs) are common in the late stage of planet formation. The Smoothed Particle Hydrodynamics (SPH) method is widely used for simulating the outcome of such violent collisions, one ...prominent example being the formation of the Moon. However, a decade of numerical studies in various areas of computational astrophysics has shown that the standard formulation of SPH suffers from several shortcomings such as artificial surface tension and its tendency to promptly damp turbulent motions on scales much larger than the physical dissipation scale, both resulting in the suppression of mixing. In order to estimate how severe these limitations are when modeling GIs we carried out a comparison of simulations with identical initial conditions performed with the standard SPH as well as with the novel Lagrangian Meshless Finite Mass (MFM) method using the multimethod code, GIZMO. We confirm the lack of mixing between the impactor and target when SPH is employed, while MFM is capable of driving vigorous subsonic turbulence and leads to significant mixing between the two bodies. Modern SPH variants with artificial conductivity, a different formulation of the hydro force or reduced artificial viscosity, do not improve mixing as significantly. Angular momentum is conserved similarly well in both methods, but MFM does not suffer from spurious transport induced by artificial viscosity, resulting in a slightly higher angular momentum of the protolunar disk. Furthermore, SPH initial conditions unphysically smooth the core-mantle boundary, which is easily avoided in MFM.
ABSTRACT
The origin of Uranus and Neptune remains a challenge for planet formation models. A potential explanation is that the planets formed from a population of a few planetary embryos with masses ...of a few Earth masses which formed beyond Saturn’s orbit and migrated inwards. These embryos can collide and merge to form Uranus and Neptune. In this work, we revisit this formation scenario and study the outcomes of such collisions using 3D hydrodynamical simulations. We investigate under what conditions the perfect-merging assumption is appropriate, and infer the planets’ final masses, obliquities, and rotation periods, as well as the presence of proto-satellite discs. We find that the total bound mass and obliquities of the planets formed in our simulations generally agree with N-body simulations therefore validating the perfect-merging assumption. The inferred obliquities, however, are typically different from those of Uranus and Neptune, and can be roughly matched only in a few cases. In addition, we find that in most cases, the planets formed in this scenario rotate faster than Uranus and Neptune, close to break-up speed, and have massive discs. We therefore conclude that forming Uranus and Neptune in this scenario is challenging, and further research is required. We suggest that future planet formation models should aim to explain the various physical properties of the planets such as their masses, compositions, obliquities, rotation rates, and satellite systems.
We perform a detailed investigation into the disruption of central cusps via the transfer of energy from sinking massive objects. Constant density inner regions form at the radius where the enclosed ...mass approximately matches the mass of the infalling body. We explore parameter space using numerical simulations and give an empirical relation for the size of the resulting core within structures that have different initial cusp slopes. We find that infalling bodies always stall at the edge of these newly formed cores, experiencing no dynamical friction over many dynamical times. As applications, we consider the resulting decrease in the dark matter annihilation flux due to centrally destroyed cusps, and we present a new theory for the formation of close binary nuclei--the 'stalled binary' model. We focus on one particularly interesting binary nucleus system, the dwarf spheroidal galaxy VCC 128 which is dark matter dominated at all radii. We show that its nuclei would rapidly coalesce within a few million years if it has a central dark matter cusp slope steeper than r --1. However, if its initial dark matter cusp is slightly shallower than a logslope of --0.75 at ~0.1% of the virial radius, then the sinking nuclei naturally create a core equal to their observed separation and stall. This is close to the logslope measured in a recent billion particle cold dark matter halo simulation.
The formation of Uranus' regular moons has been suggested to be linked to the origin of its enormous spin axial tilt (~98o). A giant impact between proto-Uranus and a 2–3 MEarth impactor could lead ...to a large tilt and to the formation of an impact generated disc, where prograde and circular satellites are accreted. The most intriguing features of the current regular Uranian satellite system is that it possesses a positive trend in the mass-distance distribution and likely also in the bulk density, implying that viscous spreading of the disc after the giant impact plays a crucial role in shaping the architecture of the final system. In this paper, we investigate the formation of Uranus' satellites by combining results of SPH simulations for the giant impact, a 1D semi-analytic disc model for viscous spreading of the post-impact disc, and N-body simulations for the assembly of satellites from a disc of moonlets. Assuming the condensed rock (i.e., silicate) remains small and available to stick onto the relatively rapid growing condensed water-ice, we find that the best case in reproducing the observed mass and bulk composition of Uranus' satellite system is a pure-rocky impactor with 3 MEarth colliding with the young Uranus with an impact parameter b = 0.75. Such an oblique collision could also naturally explain Uranus' large tilt and possibly, its low internal heat flux. The giant impact scenario can naturally explain the key features of Uranus and its regular moons. We therefore suggest that the Uranian satellite system formed as a result of an impact rather than from a circumplanetary disc. Our results also suggest that objects beyond the water snow-line could be dominated by rocky objects similar to Pluto and Triton. Future missions to Uranus and its satellite system would further constrain the properties of Uranus and its moons and provide further insight on their formation processes.
•Giant impact could explain both Uranus' properties and its satellite system.•Viscous spreading of the impact generated disc is required to explain the formation of the current Uranian system.•A pure-rocky impactor with 3 MEarth is required to form the current mass and composition of the satellites.•Massive objects beyond the ice line could be rock dominated.
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