Dissipation and Extra Light in Galactic Nuclei. IV. Evolution in the Scaling Relations of Spheroids

We develop a model for the physical origin and redshift evolution of spheroid scaling relations. We consider spheroid sizes, velocity dispersions, dynamical masses, profile shapes (Sérsic indices), stellar and supermassive black hole (BH) masses, and their related scalings. Our approach combines advantages of prior observational constraints in halo occupation models and libraries of high-resolution hydrodynamic simulations of galaxy mergers. This allows us to separate the relative roles of dissipation, dry mergers, formation time, and evolution in progenitor properties, and identify their impact on observed scalings at each redshift. We show that, at all redshifts, dissipation is the most important factor determining spheroid sizes and fundamental plane scalings, and can account (at z = 0) for the observed fundamental plane tilt and differences between observed disk and spheroid scaling relations. Because disks (spheroid progenitors) at high redshift have characteristically larger gas fractions, this predicts more dissipation in mergers, yielding systematically more compact, smaller spheroids. In detail, this gives rise to a mass-dependent evolution in the sizes of spheroids of a given mass, which agrees well with observations. This relates to a subtle weakening of the tilt of the early-type fundamental plane with redshift, important for a number of studies that assume a nonevolving stellar mass fundamental plane. This also predicts evolution in the BH-host mass relations, toward more massive BHs at higher redshifts. Dry mergers are also significant, but only for large systems which form early—they originate as compact systems, but undergo a number of dry mergers (consistent with observations) such that they have sizes at any later observed redshift similar to systems of the same mass formed more recently. Most of the observed, compact high-redshift ellipticals will become the cores of present brightest cluster galaxies, and we show how their sizes, velocity dispersions, and BH masses evolve to become consistent with observations. We also predict what fraction might survive intact from early formation and identify their characteristic z = 0 properties. We make predictions for residual correlations as well, e.g., the correlation of size and fundamental plane residuals with formation time of a given elliptical, that can be used as additional tests of these models.