  With its cored surface brightness profile, the elliptical galaxy NGC 5419
appears as a typical high-mass early-type galaxy (ETG). However, the galaxy
hosts two distinct nuclei in its center. We use high-signal MUSE (Multi-Unit
Spectroscopic Explorer) spectral observations and novel triaxial dynamical
orbit models to reveal a surprisingly isotropic central orbit distribution in
NGC 5419. Recent collisionless simulations of merging massive ETGs suggest a
two-phase core formation model, in which the low-density stellar core forms
rapidly by supermassive black holes (SMBHs) sinking into the center due to
dynamical friction. Only afterwards the SMBHs form a hard binary and the black
hole scouring process slowly changes the central orbit distribution from
isotropic to tangential. The observed cored density profile, the double nucleus
and the isotropic center of NGC 5419 together thus point to an intermediate
evolutionary state where the first phase of the core formation has taken place,
yet the scouring process is only beginning. This implies that the double
nucleus is a SMBH binary. Our triaxial dynamical models indicate a total mass
of the two SMBHs in the center of NGC 5419 of MBH = (1.0 +/- 0.08) 10^10 Msol.
Moreover, we find that NGC 5419's complex kinematically distinct core (KDC) can
be explained by a coherent flip of the orbital rotation direction of stars on
tube orbits at ~3kpc distance from the galaxy center together with projection
effects. This is also in agreement with merger simulations hosting SMBHs in the
same mass regime.

Astrophysics of Galaxies