Dynamical Properties of Eccentric Nuclear Disks: Stability, Longevity, and Implications for Tidal Disruption Rates in Post-merger Galaxies

Abstract

In some galaxies, the stars orbiting the supermassive black hole take the form of an eccentric nuclear disk, in which every star is on a coherent, apsidally-aligned orbit. The most famous example of an eccentric nuclear disk is the double nucleus of Andromeda, and there is strong evidence for many more in the local universe. Despite their apparent ubiquity however, a dynamical explanation for their longevity has remained a mystery: differential precession should wipe out large-scale apsidal-alignment on a short timescale.Here we identify a new dynamical mechanism which stabilizes eccentric nuclear disks, and explain for first time the negative eccentricity gradient seen in the Andromeda nucleus. The stabilizing mechanism drives oscillations of the eccentricity vectors of individual orbits, both in direction (about the mean body of the disk) and in magnitude. Combined with the negative eccentricity gradient, the eccentricity oscillations push some stars near the inner edge of the disk extremely close to the black hole, potentially leading to tidal disruption events.Order of magnitude calculations predict extremely high rates in recently-formed eccentric nuclear disks (∼ 0.1 − 1 yr −1 gal −1 ). Unless the stellar disks are replenished, these rates should decrease with time as the disk depletes in mass. If eccentric nuclear disks form during gas-rich major mergers, this may explain the preferential occurrence of tidal disruption events in recently-merged and post-merger (E+A/K+A) galaxies.

Black hole masses of tidal disruption event host galaxies II

“…The post-merger evolution likely includes an AGN phase that expels the remaining gas and thereby quenches the star burst (Hopkins et al 2006), after which unusual stellar dynamical processes can enhance the TDE rate. It is not yet clear which of the many proposed dynamical processes (Arcavi et al 2014;Stone et al 2018;Madigan et al 2018) is predominantly responsible for elevating post-starburst TDE rates, but many of them are expected to arise prior to the cessation of star formation. Post-merger galaxies may therefore display similarly elevated TDE rates at all phases, including in the immediate progenitors of E+A galaxies, which include star-forming (e.g.…”

Mass Segregation in Eccentric Nuclear Disks: Enhanced Tidal Disruption Event Rates for High-mass Stars

“…Out-of-plane inclinations allows us to simply consider the angle an orbit makes with the midplane of the disk, and not which direction the star itself is moving along that orbit, as a significant fraction of the orbits in a stable END can be expected to be retrograde at any given time (Madigan et al 2018, Wernke & Madigan 2019.…”

“…To quantify the the TDE rate across different populations we use the disrupted fraction: the fraction of stars that are disrupted over the course of a simulation. Stars at the inner edge of the disk are preferentially disrupted, as they have lower angular momentum due to the negative eccentricity gradient in ENDs and lower angular momentum stars are more easily torqued to a high eccentricity (Madigan et al 2018). Therefore, we should expect to see a larger fraction of heavy stars disrupting than light stars, as they are more central concentrated due to radial mass segregation.…”

“…ENDs are kept stable via secular, or orbit-averaged torques (Madigan et al 2018). These secular torques drive orbits to oscillate about their equilibrium eccentricity, with orbits commonly reaching e > 0.99 at the peak of an oscillation, leading to a high rate of close encounters between disk stars and the central SMBH.…”

Secular Eccentricity Oscillations in Axisymmetric Disks of Eccentric Orbits

“…Some of these bodies in our simulations also evolve onto retrograde orbits. This occurs when they are torqued to low enough orbital angular momenta that they undergo "inclination flips" (Li et al 2014;Madigan et al 2018a). This is interesting given the existence of retrograde orbiters in the outer Solar System such as Drac (Gladman et al 2009) and Niku (Chen et al 2016).…”