A NEW SCENARIO OF GALAXY EVOLUTION UNDER A UNIVERSAL INITIAL MASS FUNCTION

In this paper, basic observational properties of elliptical galaxies s uch as the integrated spectra, the chemical abundances and the enhance ment of alpha-elements inferred from broad-band colors and line streng th indices Mg-2 and (Fe) land their gradients), the color-magnitude re lation, the UV fluxes, and the mass-luminosity ratios, are examined in the light of current theoretical interpretations, and attention is ca lled on several points of internal contradiction. Indeed existing mode ls for the formation and evolution of elliptical galaxies are not able to simultaneously account for all of the above observational features . Specifically, in the context of standard star formation in the galac tic-wind driven models, that are at the base of present-day understand ing of the color-magnitude relation and UV fluxes, it is difficult to explain the slope of the M/L-B versus M-B relation (tilt of the Fundam ental Plane) and enhancement of the alpha-elements in the brightest el liptical galaxies. We suggest that the new initial mass function (IMF) by Padoan et al. (1997), which depends on the temperature, density, a nd velocity dispersion of the medium in which stars are formed, may al leviate some of the difficulties in question. Models of elliptical gal axies incorporating the new IMF (varying with time and position inside a galaxy) are presented and discussed at some extent. In brief, in a hot, rarefied medium the new IMF is more skewed toward the high mass e nd than in a cool, dense medium, a situation which is met passing from high to low mass galaxies or from the external regions to the center of a galaxy. As a result of the changing IMF, the enhancement of alpha -elements and tilt of the Fundamental Plane are easily explained leavi ng unaltered the interpretation of the remaining properties under exam ination. Finally, some implications concerning the relative proportion s of visible stars, collapsed remnants (baryonic dark matter), and gas left over by the star forming process are examined.