EVOLUTIONARY POPULATION SYNTHESIS IN STELLAR-SYSTEMS .2. EARLY-TYPE GALAXIES

In this work we present an articulated analysis addressing the general problem of the present evolutionary status of early-type galaxies (ET Gs). This relies on the original approach to stellar evolutionary popu lation synthesis developed in Buzzoni (1989). Optical and infrared syn thetic colors and spectral indices are analyzed in some detail explori ng their selective sensitivity to the distinctive parameters of the st ellar populations. We demonstrate that, among broadband colors, B - V is the best tracer of age while V - K is the most effective tracer of metallicity. Also the infrared indices of the CO and H2O features are implemented supplying a grid of 78 theoretical models for simple stell ar populations (SSPs). It is shown that they have a direct impact both on the study of the stellar mass loss modulating post-main-sequence e volution and internal reddening, as well as on the determination of th e initial mass function (IMF) and the M/L ratio of galaxy stellar popu lations through the study of the dwarf star contribution. The relevant case of the template galaxy M32 is discussed in detail, dealing with the well-known ''age-metallicity dilemma'' for a confident discriminat ion of the combined effects on the galaxy spectral energy distribution . ETGs are found to be fully consistent with old (15 Gyr) stellar popu lations, confirming that metallicity is the main parameter governing t he observed color distribution. The typical value of [Fe/H] is about o r slightly higher than the solar value ([Fe/H] = +0.10 +/- 0.35). Clus ter ellipticals are found to be coeval within a +/-20% uncertainty, wh ile a larger spread in age is found for held ETGs with a mean age of 1 1.5 Gyr. About three-fourths of the ETG population in the field should be older than 10 Gyr, and only 2% younger than 5 Gyr. Stellar populat ions in ETGs seem consistent with a Salpeter or with a slightly natter IMF (s less than or equal to 2.35) with a spread in the power-law ind ex as Delta s less than or equal to +/-1. Dwarf-dominated populations are definitely ruled out, at least in the core of the galaxies. The ca se of composite stellar populations to fit real galaxies is accounted for, deriving a contribution from the metal-poor stellar component to the galactic spectral energy distribution of typically 5%. Luminosity evolution of ETGs follows a power law such as partial derivative M(ups ilon)/partial derivative ln t = 1.30 - 0.27 (s - 1) which exactly matc hes Tinsley and Gunn's (1976) former results. Evolutionary (e + k)-cor rections are given in the Johnson B, V, and K bands and in the Gunn g, r, and i system, allowing us to track the expected apparent evolution in magnitude and color for six different cosmological models.