The Tilt of the Fundamental Plane: Three-Quarters Structural Nonhomology, One-Quarter Stellar Population

The variation of the mass-to-light ratios M/L of early type galaxies asfunction of their luminosities L is investigated. It is shown that the tiltbeta=0.27 (in the B--band) of the fundamental plane relation M/L ~ L^{beta} canbe understood as a combination of two effects: about one-quarter (i.e. dbeta=0.07) is a result of systematic variations of the stellar populationproperties with increasing luminosity. The remaining three-quarters (i.e. dbeta=0.2) can be completely attributed to nonhomology effects that lead to asystematic change of the surface brightness profiles with increasingluminosity. Consequently, the observed tilt in the K-band (beta=0.17) wherestellar population effects are negligible, is explained by nonhomology effectsalone. After correcting for nonhomology, the mean value of the mass-to-lightratio of elliptical galaxies (M/L_B) is 7.1+-2.8 (1 sigma scatter).Comment: 8 pages, 3 figures, ApJL, 600, 39, minor changes made to match the published versio