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Spectrophotometric evolution of elliptical galaxies. 1: Ultraviolet excess and color-magnitude-redshift relations
Bressan, Alessandro; Chiosi, Cesare; Fagotto, Franco
AA(Padua Astronomical Observatory, Padova, Italy), AB(Padua Astronomical Observatory, Padova, Italy), AC(Padua Univ., Padova, Italy)
The Astrophysical Journal Supplement Series, vol. 94, no. 1, p. 63-115 (ApJS Homepage)
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NASA/STI Keywords:
Elliptical Galaxies, Galactic Evolution, Stellar Composition, Stellar Evolution, Stellar Models, Stellar Spectra, Stellar Spectrophotometry, Ultraviolet Astronomy, Ultraviolet Emission, Color-Magnitude Diagram, Cosmology, Metallicity, Red Shift, Spectral Energy Distribution, Star Formation Rate, Stellar Color, Stellar Magnitude, Stellar Winds
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We present new chemical-spectrophotometric models of population synthesis 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 number of tracks and the coverage of stellar phases and chemical parameters. All evolutionary phases, from the main sequence until the white dwarf or C-ignition stage, as appropriate for the star mass, are included. The range of metallicity and helium content goes from Z = 0.0004 and Y = 0.230 to Z = 0.1 and Y = 0.475. The inclusion of stellar models of both low and high metallicity allows us to follow in a great detail the evolution of spectral properties as function of the chemical enrichment and age. Detailed results and useful calibrations are presented for single stellar populations aimed at providing, first, the building blocks of galazy models, and second, the basic tools for studies of star clusters. The chemical-spectrophotometric model presented 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 and allows for different laws of star formation. This model naturally follows the gradual enrichment in metallicity as a function of time both for the gas and the stars, thus providing the distribution in metallicity among the various stellar populations of a galaxy. We confirm and extend previous predictions that the main sources of the UV excess are the old, hot horizontal-branch (HB) and asymptotic giant branch (AGB) manque stars of high metallicity present in varying percentages in the stellar content of a galaxy. Since in our model the mean and maximum metallicity are ultimately driven by the mass of the galaxy, this provides a natural explanation for the observed correlation between UV excess and metallicity. We seek to disentangle the effects of metallicity and age on the global properties of a galaxy and address the question whether the prototype galaxy M32 suffered from a recent episode of star formation. Finally, we examine the photometric evolution of the models as a function of the redshift, and in particular, we address the question whether there are prominent features of the evolving spectra of galaxies that might be used as age probes. We suggest that the color (1550 - V) measuring the intensity of the UV emission of a galaxy is a good age indicator. Finally, we present the preliminary comparison of the magnitudes and colors as a function of the redshift, with the observational ones limited to a sample of radio galaxies. The implications of the present models for the study of galaxy evolution are shortly summarized in the concluding remarks.

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