Abstract
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 the 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 (TDEs). Order of magnitude calculations predict extremely high rates in recently formed eccentric nuclear disks (∼0.1–1 ). 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 TDEs in recently merged and post-merger (E+A/K+A) galaxies.
Highlights
The tidal gravity of supermassive black holes (SMBHs) gives rise to some of the most energetic phenomena in the universe: tidal disruption events (TDEs), hyper-velocity stars, and gravitational wave inspirals of compact stellar remnants
As the mean eccentricity of an eccentric nuclear disk decreases with time, the stellar orbits move to the left of this plot until they drop below the threshold, de (1 - e) = 1, and the disk no longer produces TDEs
This paper focuses on the dynamics of eccentric nuclear disks, in which the stars orbiting the SMBH in the center of a galaxy all have eccentric, apsidally aligned orbits
Summary
The tidal gravity of supermassive black holes (SMBHs) gives rise to some of the most energetic phenomena in the universe: tidal disruption events (TDEs), hyper-velocity stars, and gravitational wave inspirals of compact stellar remnants. Tremaine (1995) showed that the double nucleus is best modeled as an eccentric stellar disk orbiting the SMBH. In the Lauer et al (2005) sample of 65 nondust-obscured early-type galaxies, ∼20% show features consistent with eccentric nuclear stellar disks seen from different angles (offset nuclei, nuclei with central minima, and double nuclei). This is despite the observational challenges of detecting such signatures; stable eccentric nuclear disks live within the radius of influence of the SMBH, a scale difficult to resolve except for the closest and/or most massive galaxies (see Section 5).
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