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 _• = 1.7 × 10¹⁰ M _⊙) central supermassive black hole (SMBH) surrounded by a large core (r _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 ρ ∝ r ^-1 and ρ ∝ r ^-3/2 and vary the initial SMBH masses from 8.5 × 10⁸ to 8.5 × 10⁹ M _⊙. 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 _• = 1.7 × 10¹⁰ M _⊙. In general, density slopes of ρ ∝ r ^-3/2 for the progenitor galaxies are strongly favored for the equal-mass merger scenario.