General concept of the project

One of the most common types of observed galaxies in the universe are elliptical galaxies. They are found predominantly in galaxy clusters and in compact galaxy groups. Their masses and luminosities can vary greatly: from dwarf galaxies containing from 105 solar masses to massive elliptical galaxies of the order of 1013 solar masses with a supermassive black hole (SMBH) in the center. Elliptical galaxies are collisionless systems, meaning that one can ignore the exchange of energy and angular momentum between stars, with the exception of the central regions, where the stellar density can be up to a hundred millions higher. This high value implies the possibility of the existence of anisotropy of the velocity dispersion, which is the main mechanism causing the nonsphericity of this type of galaxy. Such galaxies, with their characteristic flattening, can have a shape close to an oblate, an elongated or triaxial ellipsoid, not exceeding certain critical flattening. Their shape and dynamical stability is an interesting and important problem in astrophysics. The dynamical equations of an arbitrary collisionless ellipsoidal system are complex and analytically impossible to solve, therefore today most of the research in this area uses computer simulations. In our project, the main effort will focus on studying the dynamic stability of various triaxial equilibrium configurations of the N-body system with respect to external perturbations. And here one of the main goals of the perturbed analysis consists in modelling and understanding of the actually observed features in the structure and shape of elliptical galaxies. However, we also want to investigate in a parallel way two other important phenomena. Namely, (1) the process of stellar accretion onto a central massive black hole, because it plays an important role in the distribution of the phase density of stars and orbital structure in the central part of an elliptical galaxy. Also, (2) the dwarf type may be the breeding ground of sources of gravitational waves. It has been put forward that these galaxies, in particular ultra-compact ones, are the outcome of an agglomeration of smaller building blocks. If black holes in the range of intermediate masses such as the one detected by LIGO/Virgo are present in these smaller building blocks, then it is expected that they will merge via gravitational radiation. These gravitational waves are measurable with current detectors and provide us with supplementary information inaccessible via light due to obscuration (gas and dust). In this sense, we may envisage these relativistic mergers as a tomography of the host galaxy.

Goal of the project

Understanding the effect of a SMBH and external perturbations on the stability of orbits in elliptical galaxies; studying the destruction of periodic orbits under the influence of perturbations, their stochastization and role in maintaining and changing an ellipsoidal shape; estimation of the event-rate of gravitational wave sources involving intermediate-mass black holes.

Expected results

The numerical study of the dynamic evolution of the elliptical galaxy model will allow us to better understand the shape and stability of elliptical galaxies. It will provide us with the initial density- and mass distributions corresponding to equilibrium states. The numerical analysis will deliver a detailed analysis of stability vs. instability of the initial system as a function of external perturbations. Additionally, the numerical simulations combined with the quantitative analysis mentioned in “main approaches” will shed light on the mechanism of transition from oblate to elongated configurations. By processing the data, we will be in the position of performing a nonlinear analysis of stellar orbits, which is of utmost importance to comprehend the processes of parametric resonance and destruction of periodic orbits as well as their stochasticity. Since we will also address stellar accretion in the simulations, we will derive the process of mass growth of the SMBH. We will derive the event rate of relativistic mergers of binaries of intermediate-mass black holes and also of stellar-mass compact objects with intermediate-mass black holes, of big interest in view of the recent detections of the ground-based detectors LIGO/Virgo.