Near-Infrared Imaging of Early-Type Galaxies. III. The Near-Infrared Fundamental Plane

Near-infrared imaging data on 251 early-type galaxies in clusters and groups are used to construct the near-infrared fundamental plane (FP) r_eff ~ sigma^1.53+/-0.08_0<Sigma_K>^-0.79+/-0.03_eff. The slope of the FP therefore departs from the virial expectation of r_eff ~ sigma^2_0<Sigma>^-1_eff at all optical and near-infrared wavelengths, which could be a result of the variation of M/L along the elliptical galaxy sequence or a systematic breakdown of homology among the family of elliptical galaxies. The slope of the near-infrared FP excludes metallicity variations as the sole cause of the slope of the FP. Age effects, dynamical deviations from a homology, or any combination of these (with or without metallicity), however, are not excluded. The scatter of both the near-infrared and optical FP are nearly identical and substantially larger than the observational uncertainties, demonstrating small but significant intrinsic cosmological scatter for the FP at all wavelengths. The lack of a correlation of the residuals of the near-infrared FP and the residuals from the Mg_2-sigma_0 relation indicates that the thickness of these relations cannot be ascribed only to age or metallicity effects. Because of this metallicity independence, the small scatter of the near-infrared FP excludes a model in which age and metallicity effects ``conspire'' to keep the optical FP thin. All of these results suggest that the possible physical origins of the FP relations are complicated due to combined effects of variations of stellar populations and structural parameters among elliptical galaxies.