The Doubling of Stellar Black Hole Nuclei
  It is strongly believed that Andromeda's double nucleus signals a disk of
stars revolving around its central super-massive black hole on eccentric
Keplerian orbits with nearly aligned apsides. A self-consistent stellar
dynamical origin for such apparently long-lived alignment has so far been
lacking, with indications that cluster self-gravity is capable of sustaining
such lopsided configurations if and when stimulated by external perturbations.
Here, we present results of N-body simulations which show unstable
counter-rotating stellar clusters around super-massive black holes saturating
into uniformly precessing lopsided nuclei. The double nucleus in our featured
experiment decomposes naturally into a thick eccentric disk of apo-apse aligned
stars which is embedded in a lighter triaxial cluster. The eccentric disk
reproduces key features of Keplerian disk models of Andromeda's double nucleus;
the triaxial cluster has a distinctive kinematic signature which is evident in
HST observations of Andromeda's double nucleus, and has been difficult to
reproduce with Keplerian disks alone. Our simulations demonstrate how the
combination of eccentric disk and triaxial cluster arises naturally when a star
cluster accreted over a pre-existing and counter-rotating disk of stars, drives
disk and cluster into a mutually destabilizing dance. Such accretion events are
inherent to standard galaxy formation scenarios. They are here shown to double
stellar black hole nuclei as they feed them.
