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Astron. Astrophys. 363, 517-525 (2000)

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3. The evolutionary population synthesis models

Synthetic SEDs are constructed following a self consistent procedure, incorporating stellar emission, internal extinction and re-emission by dust. These chemo-photometric EPS models, providing the SEDs extending over four decades in wavelength (ie. from 0.05[FORMULA] to 1 mm) (Mazzei et al. 1992, hereafter MXD92), allow us to investigate the local properties and the evolution with galactic age of the SEDs of different galaxy types. In particular, we use realistic EPS models for disc galaxies (MXD92), early-type galaxies (Mazzei et al. 1994, hereafter MDX94; Mazzei & De Zotti 1996) and the far-infrared luminous starburst population (Mazzei et al. 1995).

In order to match the photometric properties of the galaxy types, expected to dominate deep surveys, we consider 4 different templates, consisting of; elliptical, spiral, irregular and starburst. Details of the SED models for these types and their evolution with cosmic time are given in the above references. In this section, an overview of the main parameters (to be optimised using observed SEDs) will be presented.

The EPS models, used to generate synthetic SEDs for different types of galaxies, have been linked to their chemical evolution so that the increased metallicity of successive stellar generations is taken into account. We adopt a parametrization for the time evolution of the star-formation rate as


where [FORMULA] is the fractional mass of gas which takes part in the star formation ([FORMULA]) and [FORMULA] is the initial star-formation rate (i.e. SFR at [FORMULA]; where [FORMULA] is the formation redshift). We assume that initially [FORMULA] with [FORMULA] [FORMULA].

The synthetic spectra for the stellar generations with different metallicities which contribute to the galaxy SED are estimated at any given time, using the recent set of isochrones constructed by Bertelli et al. (1994). These incorporate the results of new stellar evolutionary calculations, based on six metallicity values: 0.0004,0.001,0.004,0.008,0.02 and 0.05. The isochrones have a fine coverage of masses and ages and include almost all the evolutionary phases from the main sequence to the stage of planetary nebulae ejection or carbon ignition, as appropriate for a given initial mass. This allows us to link the photometric and chemical evolution in galaxies. Further, at any given time and wavelength, the models incorporate both the average correction due to internal extinction by the enriched ISM, assuming a dust-to-gas ratio proportional to the gas metallicity and the dust re-radiation from 3 µm up to 1 mm. We assume that gas and stars have the same distribution. In particular, for spiral, starburst and irregular galaxies we follow the same approach as described in MXD92 (i.e. an exponential function of the galacto-centric radius- see their Eq. (11)) whereas for ellipticals we follow MDX94 (i.e a density profile given by the King (1966) model). Results are not strongly dependent on the adopted geometry. The evolution of the optical depth, [FORMULA], follows directly from that of the gas metallicity, [FORMULA] and [FORMULA] since [FORMULA] (as a consequence of the assumption of a gas-to-dust ratio being proportional to the gas metallicity; MXD92, MDX94). This results in very different histories for the effective optical depths (where [FORMULA])) in the early phases of the evolution of ellipticals in the model, which is based on the adopted IMF parameters.

The IMF and the SFR are the main parameters, controling the e-folding star formation time scale, the chemical evolution and, as a direct consequence, the optical depth of the system. Thus, the far-infrared data, where available, are expected to constrain the galaxy metallicity. Therefore, by comparing the local observed SEDs of different galaxy types with models, we constrain both [FORMULA] and the IMF. The observed SEDs for nearby spiral and elliptical galaxies with known metallicities and far-IR (IRAS) observations are used to constrain the IMF parameters so that these models successfully reproduce the local chemo-photometric properties of both these types over four decades in wavelength. However, the parameter which is most sensitive to the local properties (i.e. the gas content and optical colours) of galaxies along the Hubble sequence, is the initial star formation rate, [FORMULA] (Sandage 1986), which will be constrained for different types of galaxies, using the observed SEDs (Sect. 5.1).

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© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000