Evolutionary Population Synthesis in Stellar Systems. II. Early-Type Galaxies

Abstract

In this work we present an articulated analysis addressing the general problem of the present evolutionary status of early-type galaxies (ETGs). This relies on the original approach to stellar evolutionary population synthesis developed in Buzzoni(1989). Optical and infrared synthetic colors and spectral indices are analyzed in some detail exploring their selective sensitivity to the distinctive parameters of the stellar 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 H_2_O features are implemented supplying a grid of 78 theoretical models for simple stellar populations (SSPs). It is shown that they have a direct impact both on the study of the stellar mass loss modulating post-main-sequence evolution and internal reddening, as well as on the determination of the initial mass function (IMF) and the M/L ratio of galaxy stellar populations 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 discrimination of the combined effects on the galaxy spectral energy distribution. ETGs are found to be fully consistent with old (15 Gyr) stellar populations, confirming that metallicity is the main parameter governing the observed color distribution. The typical value of [Fe/H] is about or slightly higher than the solar value([Fe/H] = +0.10+/-0.35). Cluster ellipticals are found to be coeval within a +/-20% uncertainty, while a larger spread in age is found for field ETGs with a mean age of 11.5 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 populations in ETGs seem consistent with a Salpeter or with a slightly flatter IMF (s <= 2.35) with a spread in the power-law index {DELTA}S <= +/-1. Dwarf-dominated populations are definitely ruled out, at least in the core of the galaxies. The case 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 of (M_v_) with respect to (lnt) = 1.30 - 0.27 (s - 1) which exactly matches Tinsley & Gunn's (1976) former results. Evolutionary (e + k)-corrections 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.