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 are investigated using large samples of early-type galaxies observed at optical and near-infrared wavelengths. The slope a in the FP relation r(eff) proportional to sigma(0)(a) [Sigma](eff)(b) is shown to increase systematically with wavelength from the U band (lambda similar to 0.35 mu m) through the K band (lambda similar to 2.2 mu m). A distance-independent construction of the observables is described that provides an accurate measurement of the change in the FP slope between any pair of bandpasses. The variation of the FP slope with wavelength is strong evidence of systematic variations in stellar content along the elliptical galaxy sequence, but is insufficient to discriminate between a number of simple models for possible physical origins of the FP. The intercept of the diagnostic relationship between log D-K/D-V and log sigma(0) shows no significant dependence on environment within the uncertainties of the Galactic extinction corrections, demonstrating the universality of the stellar population contributions at the level of Delta(V-K) = 0.03 mag to the zero point of the global scaling relations. Several other constraints on the properties of early-type galaxies-the slope of the Mg-2-sigma(0) relation, the slope of the FP in the K band, the effects of stellar population gradients, and the effects of deviations of early-type galaxies from a dynamically homologous family-are included to construct an empirical, self-consistent model that provides a complete picture of the underlying physical properties that are varying along the early-type galaxy sequence. The fundamental limitations to providing accurate constraints on the individual model parameters (variations in age and metallicity, and the size of the homology breaking) appear to be subtle variations between stellar population synthesis models and poorly constrained velocity dispersion aperture effects. This empirical approach nonetheless demonstrates that there are significant systematic variations in both age and metallicity along the elliptical galaxy sequence, and that a small, but systematic, breaking of dynamical homology (or a similar, wavelength-independent effect) is required. The intrinsic thickness of the FP can then be easily understood as small variations in age and metallicity, as well as deviations from a homology at any particular point along the FP. The model parameters will be better constrained by measurements of the change of the slope of the FP with redshift; predictions for this evolution with redshift are described. This model for the underlying physical properties that produce the FP scaling relations provides a comprehensive framework for future investigations of the global properties of early-type galaxies and their evolution.