Princeton University Gravity Initiative Lunch

Abstract: The structure and kinematics of galactic nuclei are fossil relics of the merger histories of galaxies and of the interaction between stars and SMBH binaries. Stars ejection during the SMBH binary evolution should reduce a galaxy’s central density and expand its core. This has been suggested as a plausible explanation for why large elliptical galaxies have lower central densities and weaker density cusps than small ellipticals. An important element that determines the effect of a massive binary on a galactic core is the time it takes for the binary separation to decay to the regime where GW emission will take over. But whether or not a massive binary manages to coalesce, it will leave behind an imprint on both the mass density and kinematics of the galaxy: (i) a mass deficit - a lowered density of stars near the centre of the galaxy; (ii) a tangentially anisotropic velocity distribution - an excess of stars in the core moving along tangential directions. I use N-body simulations to simulate the merger of two galaxies and study the destruction of density cusps around SMBHs in multiple mergers of elliptical galaxies. I follow the details of the merger from its earliest stages, when the two galaxies are distinct, to its late stages, when the SMBHs have formed a hard binary and the binary has decayed via energy exchange with stars to the GW dominated regime. I present the morphological and kinematical structure of the merged galaxy as a function of the merger mass-ratio, the galaxy density profile slope and the number of mergers and discuss observational signatures associated with the formation picture.