The Simultaneous Formation of Cored, Tangentially Biased, and Kinematically Decoupled Centers in Massive Early-type Galaxies

We study the impact of merging supermassive black holes (SMBHs) on the central regions of massive early-type galaxies (ETGs) using a series of merger simulations with varying mass ratios. The ETG models include realistic stellar and dark matter components and are evolved with the GADGET-3 based regularized tree code KETJU. We show that observed key properties of the nuclear stellar populations of massive ETGs, namely flat stellar density distributions (cores), tangentially biased velocity distributions, and kinematically decoupled (counter-)rotation, can naturally result from a single process—the scouring by SMBHs. Major mergers with mass ratios of q > 1/3 produce flat, tangentially biased cores with kinematically distinct components. These kinematic features originate from reversals of the SMBH orbits caused by gravitational torques after pericenter passages. Minor mergers (q ≲ 1/3), on the other hand, form non-rotating cores and the orbit reversal becomes less important. Low-density stellar cores scoured in (multiple) minor mergers are less tangentially biased. This implies that the nuclear stellar properties of massive ETGs can be solely explained by stellar dynamical processes during their final assembly without any need for “feedback” from accreting black holes. We predict a strong correlation between decoupled cores, central anisotropy, and merger history: decoupled cores form in binary mergers and we predict them to occur in elliptical galaxies with the strongest central anisotropy. Measurements of the central orbital structure are the key to understanding the number of mergers that a given galaxy has experienced.