Photometric Properties of Model Spherical Galaxies

Abstract

This paper presents the results of detailed calculations of the photometric properties of the dynamical models for the formation and evolution of spherical galaxies recently computed by Larson. The models that were found in Paper I to have radial density distributions closely matching those of elliptical galaxies are found to resemble elliptical galaxies also in their colors, mass-to-luminosity ratios, and narrow-band spectral energy distributions. Rowever, the residual star formation activity in the nuclei of these models makes the nuclear colors bluer than normal, suggesting that most elliptical galaxies have lost their residual gas and do not experience current star formation. The effect of the metal abundance gradient of Paper I on the colors has been estimated, and (in the absence of current star formation) it causes the nucleus of a galaxy to appear redder than the outer regions, as is generally observed. The models with expanding boundaries, in which star formation continues to be important at the present time, have colors and mass-to-luminosity ratios which resemble those of spiral galaxies, showing that differences in the photometric properties of different types of galaxies can be understood in terms of different dynamical histories. The evolution of the photometric properties of the models is discussed, and it is concluded that continuing star formation in elliptical galaxies is not likely to affect significantly the rate at which they evolve in color and luminosity. The predicted supernova rates in models whose UB V colors resemble those of different Rubble types of galaxies agree with the observed supernova rates in these types of galaxies. Subject headings: galactic structure - galaxies, photometry of - star formation - supernovae