Observational Constraints on the Origins of the Fundamental Plane

We review the concept of the Fundamental Plane (FP), and present new results on the near-infrared FP. We show that the K-band FP differs both from the optical form and the virial expectation under the assumption of constant (M/L) and homology. Systematic variations of only the stellar populations parameters (age, metallicity, IMF) cannot reproduce the slopes of the FP simultaneously at both the optical and near-infrared wavelengths. There appears to be an additional effect which could be due to a systematic departure of the velocity distributions of elliptical galaxies from a homologous family. In order to distinguish between the different possible stellar populations parameters, the FP and its projections can be observed at a range of redshifts. We describe several such evolutionary tests: in the intercept of the color-magnitude relation; in the intercept of the Kormendy surface brightness-radius correlation; and in the intercept of the FP itself. All three tests are fully consistent with the population of cluster elliptical galaxies having formed at high redshift; this contradicts large, systematic age variations as a significant contributor to the slope of the FP.