Near-Infrared Imaging of Early-Type Galaxies. IV. The Physical Origins of the Fundamental Plane Scaling Relations

The physical origins of the Fundamental Plane (FP) scaling relations areinvestigated for early-type galaxies observed at optical and near-infraredwavelengths. The slope for the FP is shown to increase systematically withwavelength from the U-band through the K-band. A distance-independentconstruction of the observables is described which provides an accuratemeasurement of the change in the FP slope between any pair of bandpasses. Thevariation of the FP slope with wavelength is strong evidence of systematicvariations in stellar content along the elliptical galaxy sequence. Theintercept of the diagnostic relationship between log(D_K/D_V) and log(sigma_0)shows no significant dependence on environment within the uncertainties of theGalactic extinction corrections, demonstrating the universality of the stellarpopulations contributions at the level of Delta(V-K)=0.03 mag to the zero-pointof the global scaling relations. Several other constraints on the properties of early-type galaxies --- theslope of the Mg_2-sigma_0 relation, the effects of stellar populationsgradients, and deviations of early-type galaxies from a dynamically homologousfamily --- are included to construct an empirical, self-consistent model whichprovides a complete picture of the underlying physical properties which arevarying along the early-type galaxy sequence. This empirical approachdemonstrates that there are significant systematic variations in both age andmetallicity along the elliptical galaxy sequence, and that a small, butsystematic, breaking of dynamical homology (or a similar, wavelengthindependent effect) is required. Predictions for the evolution of the slope ofthe FP with redshift are described. [abriged]Comment: to appear in The Astronomical Journal; 40 pages, including 10 Postscript figures and 3 tables; uses AAS LaTeX style file