Theoretical astrophysics and plasma physics at RPC

Abstract:

Fitting equilibrium dynamical models to observational data is an essential step in understanding the structure of the gaseous hot haloes that surround our own and other galaxies. However, the two main categories of models that are used in the literature are poorly suited for this task: (i) simple barotropic models are analytic and can therefore be adjusted to match the observations, but are clearly unrealistic because the rotational velocity vϕ(R, z⁠) does not depend on the distance z from the galactic plane, while (ii) models obtained as a result of cosmological galaxy formation simulations are more realistic, but are impractical to fit to observations due to high computational cost. Here we bridge this gap by presenting a general method to construct axisymmetric baroclinic equilibrium models of rotating galactic coronae in arbitrary external potentials. We consider in particular a family of models whose equipressure surfaces in the (R, z⁠) plane are ellipses of varying axis ratio. These models are defined by two one-dimensional functions, the axial ratio of pressure qaxis(⁠z⁠) and the value of the pressure Paxis(⁠z⁠) along the galaxy’s symmetry axis. These models can have a rotation speed vϕ(R, z⁠) that realistically decreases as one moves away from the galactic plane, and can reproduce the angular momentum distribution found in cosmological simulations. The models are computationally cheap to construct and can thus be used in fitting algorithms. We provide a python code that given qaxis(⁠z⁠), Paxis(⁠z⁠), and Φ(R, z⁠) returns ρ(R, z⁠), T(R, z⁠), P(R, z⁠), vϕ(R, z⁠). We show a few examples of these models using the Milky Way as a case study.

Abstract:

An encounter between a passing star and a massive black hole at the centre of a galaxy, a so-called tidal disruption event or TDE, may leave a debris disc that subsequently accretes onto the hole. We solve for the time evolution of such a TDE disc, making use of an evolutionary equation valid for both the Newtonian and Kerr regimes. The late time luminosity emergent from such a disc is of interest as a model diagnostic, as it tends to follow a power law decline. The original simple ballistic fallback model, with equal mass in equal energy intervals, produces a −5/3 power law, while standard viscous disc descriptions yield a somewhat more shallow decline, with an index closer to −1.2. Of four recent, well-observed tidal disruption event candidates however, all had fall-off power law indices smaller than 1 in magnitude. In this work, we revisit the problem of thin disc evolution, solving this reduced problem in full general relativity. Our solutions produce power law indices that are in much better accord with observations. The late time observational data from many TDEs are generally supportive, not only of disc accretion models, but of finite stress persisting down to the innermost stable circular orbit.

Abstract:

Gravitational torques among objects orbiting a supermassive black hole drive the rapid reorientation of orbital planes in nuclear star clusters (NSCs), a process known as vector resonant relaxation. In this Letter, we determine the statistical equilibrium of systems with a distribution of masses, semimajor axes, and eccentricities. We average the interaction over the apsidal precession time and construct a Monte Carlo Markov chain method to sample the microcanonical ensemble of the NSC. We examine the case of NSCs formed by 16 episodes of star formation or globular cluster infall. We find that the massive stars and stellar mass black holes form a warped disk, while low mass stars resemble a spherical distribution with a possible net rotation. This explains the origin of the clockwise disk in the Galactic center and predicts a population of black holes (BHs) embedded within this structure. The rate of mergers among massive stars, tidal disruption events of massive stars by BHs, and BH-BH mergers are highly increased in such disks. The first two may explain the origin of the observed G1 and G2 clouds, the latter may be important for gravitational wave detections with LIGO and VIRGO. More generally, black holes are expected to settle in disks in all dense spherical stellar systems assembled by mergers of smaller systems including globular clusters.