First we will summarize evidence that there actually is a variation of with L. van der Marel (1991) used a dynamical model to predict the kinematics of an elliptical galaxy on the basis of observed surface photometry. The predictions were compared with actual kinematical data along both the major and the minor axes of 37 bright E galaxies. From this, accurate mass-to-light ratios were derived that were corrected for the effects of rotation and radial anisotropy. Note, that these mass-to-light ratios were not based on the assumption of structural homology. van der Marel found, that the mass-to-light ratio correlated with luminosity as .
Second we note, that part of the FP slope is due to a metallicity effect. Higher luminosity galaxies have higher metallicity than fainter galaxies, and because of the line-blanketing effect, brighter galaxies will emit more of their light at longer wavelengths than fainter galaxies. Therefore, the bolometric mass-to-light ratio could be constant with bolometric luminosity, while at the same time the blue mass-to-light ratio could increase with blue luminosity. Djorgovski & Santiago (1993) found the FP coefficient to increase monotonically with the effective wavelength of the bandpass, from at U ( ) to at K ( ). remained constant at . The increase in with wavelength is indeed a sign of line blanketing, since it implies a decrease in , the coefficient in . However, at line blanketing should be negligible, and still Djorgovski & Santiago find . Recillas-Cruz et al. (1990) found the same trend, namely at B, at V, and at K. At K this implies . Dressler et al. (1987b) found that the metallicity effect only explains a minor part of the FP slope. They found , and after applying bolometric correction, the dependence on L was only reduced to .
Renzini & Ciotti (1993) explored whether a systematic variation in either the IMF slope below or the minimum stellar mass could produce the FP slope. They found that a major change of either of these parameters along the FP was required to reproduce the tilt. At the same time, an extremely small dispersion was required to reproduce the small constant thickness of the FP. In other words, fine tuning was needed, making this explanation unattractive.
Another possible explanation could be that the dark matter fraction increased with luminosity, while remained constant. This was explored by Renzini & Ciotti (1993) and Ciotti, Lanzoni, & Renzini (1996), and also here it was found that a fine tuning was needed.
Part of the FP slope could also be due to a systematic variation of mean age along the FP (Faber et al. 1995), with the stellar populations of high luminosity galaxies having higher mean ages than for low luminosity galaxies.
Properties of E and S0 Galaxies in the Clusters HydraI and Coma
Master's Thesis, University of Copenhagen, July 1997
Bo Milvang-Jensen (email@example.com)