Abstract
We investigated whether the two recently discovered nuclei in NGC 7727 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 NGC 7727. We detect a SMBH in the photometric center of the galaxy in Nucleus 1, with a mass of MSMBH = 1.54−0.15+0.18 × 108 M⊙. In the second nucleus, which is 500 pc offset from the main nucleus, we also find a clear signal for a SMBH with a mass of MBH = 6.33−1.40+3.32 × 106 M⊙. Both SMBHs are detected at high significance. The off-axis nature of Nucleus 2 makes modeling the system challenging; however, a number of robustness tests suggest that a black hole is required to explain the observed 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 MBH − MBulge scaling relation. This confirms it as the surviving nuclear star cluster of a galaxy that has merged with NGC 7727. 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 27.4 Mpc. 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. 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.
Highlights
When two galaxies with super-massive black hole (SMBH) in their centers merge, they are expected to form a dual SMBH with a separation of 1−100 kpc
In the second nucleus, which is 500 pc offset from the main nucleus, we find a clear signal for a SMBH with a mass of MBH = 6.33+−31..3420 × 106 M
We present adaptive-optics narrow-field MUSE IFU kinematic data on the two nuclei of NGC 7727
Summary
When two galaxies with SMBHs in their centers merge, they are expected to form a dual SMBH with a separation of 1−100 kpc (e.g., see De Rosa et al 2019 for a review). They discovered that it might be a triple AGN system with three accreting black holes within a separation of ∼1 kpc This galaxy is too distant for a dynamical BH mass measurement, and they can only infer the SMBH masses from the M-σ relation. BH masses in dual SMBHs have been estimated using AGN indicators (Greene & Ho 2007b; Schulze et al 2015) or using the M-σ relation (Greene & Ho 2007a; Xiao et al 2011) These estimates are uncertain, due to the nonequilibrium conditions stripped galaxies show during merging (e.g., Forbes et al 2014). Stripped nuclei with SMBHs in their centers have been confirmed multiple times in massive star clusters in the halo of larger galaxies (Seth et al 2014; Ahn et al 2017, 2018; Afanasiev et al 2018) Their SMBHs were all confirmed through direct stellar dynamical measure-
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