Effective power-law dependence of Lyapunov exponents on the central mass in galaxies

TL;DR

This paper demonstrates a power-law relation between Lyapunov exponents and black hole mass in galaxies, supported by numerical and analytical methods, with implications for galaxy evolution.

Contribution

It introduces a theoretical and numerical analysis of the power-law dependence of Lyapunov exponents on black hole mass in galaxies, extending to different galaxy models.

Findings

Lyapunov exponent scales as a power law with black hole mass, with exponent approximately 0.3 to 0.5.

Theoretical prediction of the power-law exponent as 2/3 minus a small correction.

The power-law relation holds in elliptical, cusp, and disc galaxy models, affecting orbital chaos understanding.

Abstract

Using both numerical and analytical approaches, we demonstrate the existence of an effective power-law relation $L\propto m^p$ between the mean Lyapunov exponent $L$ of stellar orbits chaotically scattered by a supermassive black hole in the center of a galaxy and the mass parameter $m$, i.e. ratio of the mass of the black hole over the mass of the galaxy. The exponent $p$ is found numerically to obtain values in the range $p \approx 0.3$--$0.5$. We propose a theoretical interpretation of these exponents, based on estimates of local `stretching numbers', i.e. local Lyapunov exponents at successive transits of the orbits through the black hole's sphere of influence. We thus predict $p=2/3-q$ with $q\approx 0.1$--$0.2$. Our basic model refers to elliptical galaxy models with a central core. However, we find numerically that an effective power law scaling of $L$ with $m$ holds also in models with central cusp, beyond a mass scale up to which chaos is dominated by the influence of the cusp itself. We finally show numerically that an analogous law exists also in disc galaxies with rotating bars. In the latter case, chaotic scattering by the black hole affects mainly populations of thick tube-like orbits surrounding some low-order branches of the $x_1$ family of periodic orbits, as well as its bifurcations at low-order resonances, mainly the Inner Lindbland resonance and the 4/1 resonance. Implications of the correlations between $L$ and $m$ to determining the rate of secular evolution of galaxies are discussed.