Observations of double or multiple at the central part of massive elliptical galaxies share common characteristics: (a) the stellar density profile of their host environment is shallow (b) the distance between the host and satellite nucleus is less than the influence radius of the central black hole and larger than the hardening radius of the massive binary (c) when possible observations indicate these binaries are low mass-ratio binaries.
Lifetime of an SMBH binary (color bar) as a function of its total mass and mass ratio for gamma=0.6. The lifetime of SMBH binaries with a low mass ratio becomes significantly longer when we include
in the calculation the timescale for dynamical friction inside the influence radius of the central black hole , which is often neglected in the literature.
Expected average number of stalled satellites as a function of the host galaxy SMBH mass for gamma=0.6. Solid red and
black lines are calculated based on empirical relations while filled stars use measurements directly from observations for the galaxy properties. Black points are derived
using the merger rate results of the ILLUSTRIS simulation. The average number of stalled satellites increases similarly with the galaxy mass in all three
different treatments. For massive galaxies, we expect a few stalled satellites within their inner cores.The purple stars refer to the galaxies with observed double and multiple nuclei shown in the first figure above, namely A2261-BCG, NGC 4696, NGC 5419, and NGC 6876.
