First direct dynamical detection of a dual supermassive black hole
  system at sub-kiloparsec separation
  We investigated whether the two recently discovered nuclei in NGC7727 both
host a super-massive black hole (SMBH). We used the high spatial resolution
mode of the integral-field spectrograph MUSE on the VLT in adaptive optics mode
to resolve the stellar kinematics within the sphere of influence of both
putative black holes. We combined the kinematic data with an HST-based mass
model and used Jeans models to measure their SMBH mass. We report the discovery
of a dual SMBH system in NGC7727. We detect a SMBH in the photometric center of
the galaxy in Nucleus 1, with a mass of $M_{\rm SMBH}=1.54^{+0.18}_{-0.15}
\times10^{8}M_{\odot}$. In the second nucleus, which is 500pc offset from the
main nucleus, we also find a clear signal for a SMBH with a mass of $M_{\rm
BH}=6.33^{+3.32}_{-1.40}\times10^{6}M_{\odot}$. Both SMBHs are detected at high
significance. The off-axis nature of Nucleus 2 makes modeling the system
challenging; but a number of robustness tests suggest that a black hole is
required to explain the kinematics. The SMBH in the offset Nucleus 2 makes up
3.0% of its total mass, which means its SMBH is over-massive compared to the
$M_{\rm BH}-M_{\rm Bulge}$ scaling relation. This confirms it as the surviving
nuclear star cluster of a galaxy that has merged with NGC7727. This discovery
is the first dynamically confirmed dual SMBH system with a projected separation
of less than a kiloparsec and the nearest dynamically confirmed dual SMBH at a
distance of 27.4Mpc. The second Nucleus is in an advanced state of inspiral,
and it will eventually result in a 1:24 mass ratio SMBH merger. Optical
emission lines suggest Nucleus 2 is a Seyfert galaxy, making it a
low-luminosity Active Galactic Nuclei (AGN). There are likely many more
quiescent SMBHs as well as dual SMBH pairs in the local Universe that have been
missed by surveys that focus on bright accretion signatures.
