The tilt of the Fundamental Plane of early-type galaxies: wavelength dependence

The photometric parameters R-e and mu(e) of 74 early-type (E+S0+S0a) galaxies within 2 degrees projected radius from the Coma cluster centre are derived for the first time in the near-infrared H band (1.65 mu m). These are used, coupled with measurements of the central velocity dispersion a found in the literature, to determine the II-band Fundamental Plane (FP) relation of this cluster: log R-e proportional to A log sigma + b mu(e). The same procedure is applied to previously available photometric data in the B, V, r, I and K bands, to perform a multiwavelength study of the FP relation. Because systematic uncertainties in the value of the FP parameters are introduced both by the choice of the fitting algorithm used to derive the FP template, and by the presence of statistical biases connected with the sample selection procedure, we emphasize the importance of deriving the FP parameters in the six different photometric bands using an identical fitting algorithm, and appropriate corrections to eliminate the effects of sample incompleteness. Once these corrections are applied, we find that the FP b coefficient is stable with wavelength (similar to 0.35 +/- 0.02), while the A coefficient increases significantly with increasing wavelength: from similar to 1.35 to similar to 1.70 (+/-0.1) from the optical to the infrared, in agreement with an earlier result presented recently by Pahre and Djorgovski, Therefore the slope of the FP relation, although changing with wavelength, never approaches the virial theorem expectation (A = 2.0) when the central velocity dispersion only is used to build the FP. We also find that the magnitude of the slope change can be entirely explained by the presence of the well-known relation between colour and magnitude among early-type galaxies. We conclude that the tilt of the Fundamental Plane is significant, and must be the result of some form of broken homology among early-type galaxies, while its wavelength dependence derives from whatever mechanism (currently the preferred one is the existence of a mass-metal content sequence) produces the colour-magnitude relation in those galaxies.