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We investigated whether the two recently discovered nuclei in NGC7727 bothhost a super-massive black hole (SMBH). We used the high spatial resolutionmode of the integral-field spectrograph MUSE on the VLT in adaptive optics modeto resolve the stellar kinematics within the sphere of influence of bothputative black holes. We combined the kinematic data with an HST-based massmodel and used Jeans models to measure their SMBH mass. We report the discoveryof a dual SMBH system in NGC7727. We detect a SMBH in the photometric center ofthe 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 themain nucleus, we also find a clear signal for a SMBH with a mass of $M_{\rmBH}=6.33^{+3.32}_{-1.40}\times10^{6}M_{\odot}$. Both SMBHs are detected at highsignificance. The off-axis nature of Nucleus 2 makes modeling the systemchallenging; but a number of robustness tests suggest that a black hole isrequired to explain the kinematics. The SMBH in the offset Nucleus 2 makes up3.0% 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 survivingnuclear star cluster of a galaxy that has merged with NGC7727. This discoveryis the first dynamically confirmed dual SMBH system with a projected separationof less than a kiloparsec and the nearest dynamically confirmed dual SMBH at adistance 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. Opticalemission lines suggest Nucleus 2 is a Seyfert galaxy, making it alow-luminosity Active Galactic Nuclei (AGN). There are likely many morequiescent SMBHs as well as dual SMBH pairs in the local Universe that have beenmissed by surveys that focus on bright accretion signatures.

First direct dynamical detection of a dual supermassive black hole system at sub-kiloparsec separation