Dissipation and the Fundamental Plane: Observational Tests

Abstract

We develop observational tests of the idea that dissipation in gas-rich mergers produces the fundamental plane (FP) and related correlations obeyed by ellipticals. The FP 'tilt' implies lower-mass ellipticals have a higher ratio of stellar to dark matter within their stellar effective radii. Models argue that mergers between more gas-rich (typically lower-mass) disks yield larger mass fractions formed in compact starbursts, giving a smaller stellar R_e and higher M_stellar/M_tot within that R_e. Such starbursts leave a characteristic imprint in the surface brightness profile: a central excess above an outer profile established by the dissipationless violent relaxation of disk stars. In previous work, we developed empirical methods to decompose the observed profiles of ellipticals and robustly estimate the amount of dissipation in the original spheroid-forming merger(s). Applying this to a large sample of observed ellipticals, we test whether or not their location on the FP and its tilt are driven by dissipation. At fixed mass, ellipticals formed in more dissipational events are smaller and have higher M_stellar/M_tot. At fixed degree of dissipation, there is no tilt in the FP. We show that the dynamical mass estimator R_e*sigma^2/G is a good estimator of the true mass: the observed FP tilt cannot primarily owe to other forms of non-homology. Removing the effects of dissipation, observed ellipticals obey the same FP correlations as disks: unusual progenitors are not required to make typical ellipticals. Dissipation appears to be both necessary and sufficient to explain the FP tilt.