The Dissipative Merger Progenitors of Elliptical Galaxies

We address the deviations of the scaling relations of elliptical galaxies from the expectations based on the virial theorem and homology, including the “tilt” of the “fundamental plane” and the steep decline of density with mass. We show that such tilts result from dissipative major mergers once the gas fraction available for dissipation declines with progenitor mass, and derive the scaling properties of the progenitors. We use hydrodynamical simulations to quantify the effects of major mergers with different gas fractions on the structural properties of galaxies. The tilts are driven by the differential shrinkage of the effective stellar radius as a function of dissipation in the merger, while the correlated smaller enhancements in internal velocity and stellar mass keep the slope of the velocity-stellar mass relation near V ∝ M. The progenitors match a straightforward model of disc formation in ΛCDM haloes. Their total-to-stellar mass ratio within the effective radius varies as M/M ∝ M, consistent with the effect of supernova feedback. They are nearly homologous in the sense that the dark-matter fractions within the effective and virial radii scale with mass in a similar way, with only a weak decline of density with mass. The progenitor radius is roughly R ∝ M, compatible with today’s intermediate late-type galaxies. This may indicate that the latest dissipative mergers in the history of ellipticals involved relatively big discs. The dissipative gas-to-stellar mass ratio is predicted to decline ∝ M, similar to the observed trend in today’s blue-sequence galaxies. Such a trend should be observed in relatively massive gaseous galaxies at z ∼ 1 − 4. The corresponding “baryonic” relations are consistent with homology and spherical virial equilibrium, V ∝ R ∝ M 1/3 bar .