The formation of extremely diffuse galaxy cores by merging supermassive
  black holes
  Given its velocity dispersion, the early-type galaxy NGC 1600 has an
unusually massive ($M_\bullet = 1.7 \times 10^{10} M_\odot$) central
supermassive black hole (SMBH), surrounded by a large core ($r_\mathrm{b} =
0.7$ kpc) with a tangentially biased stellar distribution. We present
high-resolution equal-mass merger simulations including SMBHs to study the
formation of such systems. The structural parameters of the progenitor
ellipticals were chosen to produce merger remnants resembling NGC 1600. We test
initial stellar density slopes of $\rho \propto r^{-1}$ and $\rho \propto
r^{-3/2}$ and vary the initial SMBH masses from $8.5 \times 10^8$ to $8.5
\times 10^9$ $M_\odot$. With increasing SMBH mass the merger remnants show a
systematic decrease in central surface brightness, an increasing core size, and
an increasingly tangentially biased central velocity anisotropy.
Two-dimensional kinematic maps reveal decoupled, rotating core regions for the
most massive SMBHs. The stellar cores form rapidly as the SMBHs become bound,
while the velocity anisotropy develops more slowly after the SMBH binaries
become hard. The simulated merger remnants follow distinct relations between
the core radius and the sphere-of-influence, and the SMBH mass, similar to
observed systems. We find a systematic change in the relations as a function of
the progenitor density slope, and present a simple scouring model reproducing
this behavior. Finally, we find the best agreement with NGC 1600 using SMBH
masses totaling the observed value of $M_\bullet = 1.7 \times 10^{10} M_\odot$.
In general, density slopes of $\rho \propto r^{-3/2}$ for the progenitor
galaxies are strongly favored for the equal-mass merger scenario.
