SPECTROPHOTOMETRIC EVOLUTION OF ELLIPTIC GALAXIES .1. ULTRAVIOLET EXCESS AND COLOR-MAGNITUDE-REDSHIFT RELATIONS

We present new chemical-spectrophotometric models of population synthe sis to predict the spectral evolution of galaxies, in which stars span wide ranges of ages and chemical compositions. The library of stellar models assembled here supersedes other existing libraries for the num ber of tracks and the coverage of stellar phases and chemical paramete rs. All evolutionary phases, from the main sequence until the white dw arf or C-ignition stage, as appropriate for the star mass, are include d. The range of metallicity and helium content goes from Z = 0.0004 an d Y = 0.230 to Z = 0.1 and Y = 0.475. Six combinations of Z and Y are considered, however, scaled according to the law DELTAY/DELTAZ = 2.5. The inclusion of stellar models of both low and high metallicity allow s us to follow in a great detail the evolution of spectral properties as function of the chemical enrichment and age. The library of stellar spectra stands on the most recent release of the Kurucz library, howe ver, implemented at the low effective temperatures by means of observe d spectra for stars of the latest spectral types. The population synth esis technique stems from the calculation of accurate isochrones in th e color-magnitude diagram. Detailed results and useful calibrations ar e presented for single stellar populations aimed at providing, first, the building blocks of galaxy models and, second, the basic tools for studies of star clusters. The chemical-spectrophotometric model presen ted here is particularly designed for elliptical galaxies. It includes the presence of dark matter and galactic winds triggered by supernova explosion and energy injection by stellar winds from massive stars an d allows for different laws of star formation. This model naturally fo llows the gradual enrichment in metallicity as a function of time both for the gas and the stars, thus providing the distribution in metalli city among the various stellar populations of a galaxy. The chemical m odel is the entry point for the model of spectral synthesis both for t he detailed spectra and the broadband colors of the Johnson system. Th e results are used to study the color-magnitude relation for nearby ga laxies and the origin of the UV excess in elliptical galaxies together with its dependence on metallicity and Mg2 index. We confirm and exte nd previous predictions that the main sources of the UV excess are the old, hot horizontal-branch (HB) and asymptotic giant branch (AGB) man que stars of high metallicity present in varying percentages in the st ellar content of a galaxy. Since in our model the mean and maximum met allicity are ultimately driven by the mass of the galaxy, this provide s a natural explanation for the observed correlation between UV excess and metallicity. We seek to disentangle the effects of metallicity an d age on the global properties of a galaxy and address the question wh ether the prototype galaxy M32 suffered from a recent episode of star formation. With the aid of the new calibrations for the magnitudes and colors of single stellar populations in particular stages of evolutio n and the new chemical-spectrophotometric model, we find that spectrum , turnoff color, integrated colors, brightest red giant branch (RGB) a nd AGB stars are all compatible with the notion that M32 is an old sys tem that underwent a prominent initial episode of star formation follo wed by later activity at about 5 X 10(9) yr ago. Finally, we examine t he photometric evolution of the models as a function of the redshift, and, in particular, we address the question whether there are prominen t features of the evolving spectra of galaxies that might be used as a ge probes. We suggest that the color (1550-V) measuring the intensity of the UV emission of a galaxy is a good age indicator. Specifically, this color suffers from a sudden variation from red to blue at the ons et of the hot HB and AGB manque stages of the old, metal-rich stars, m ost likely present in massive galaxies. In the framework of the stella r models in use, this transition occurs at about 7.6 X 10(9) yr. Equiv alentIy, it is expected to be observed at redshifts perhaps accessible to present-day instrumentation. The detection of this feature at a ce rtain redshift would impose firm constraints on the underlying cosmolo gical model of the universe. Finally, we present the preliminary compa rison of the magnitudes and colors as a function of the redshift, with the observational ones limited to a sample of radio galaxies. The imp lications of the present models for the study of galaxy evolution are shortly summarized in the concluding remarks.