SPECTRO-PHOTOMETRIC EVOLUTION OF ELLIPTIC GALAXIES .2. MODELS WITH INFALL

In this paper we present new chemo-spectrophotometric models of ellipt ical galaxies in which infall of primordial gas is allowed to occur. T hey aim to simulate the collapse of a galaxy made of two components, i .e. luminous material and dark matter. The mass of the dark component is assumed to be constant in time, whereas that of the luminous materi al is supposed to accrete at a suitable rate. They also include the ef fect of galactic winds powered by supernova explosions and stellar win ds from massive, early-type stars. The models are constrained to match a number of properties of elliptical galaxies, i.e. the slope and mea n colours of the colour-magnitude relation (CMR), V versus (V-K), the UV excess as measured by the colour (1550-V) together with the overall shape of the integrated spectral energy distribution (ISED) in the ul traviolet, the relation between the Mg-2 index and (1550-V), the mass to blue luminosity ratio M/L(B) as a function of the B luminosity, and finally the broad-band colours (U-B), (B-V), (V-I), (V-K), etc. The C MR is interpreted as a mass-metallicity sequence of old, nearly coeval objects, whose mean age is 15 Gyr. Assuming the law of star formation to be proportional to M(g)(k)(t) with k = 1, the rate of star formati on as function of time starts small, grows to a maximum, and then decl ines thus easily avoiding the excess of metal-poor stars found by BCF with the closed-box scheme (the analog of the G-Dwarf Problem in the s olar vicinity). Owing to their stellar content, infall models can easi ly reproduce all the basic data of the galaxies under examination. As far as the UV excess is concerned, the same sources proposed by BCF ar e found to hold also with the infall scheme. H-HB and AGB manque stars of high metallicity play the dominant role, and provide a robust expl anation of the correlation between the (1550-V) colour and the luminos ity, mass and metallicity of the galaxies. Furthermore, these models c onfirm the potential of the (1550-V) colour as an age indicator in cos mology as already suggested by BCF. In the rest frame of a massive and metal-rich elliptical galaxy, this colour suffers from one major vari ation: at the onset of the so-called H-HB and AGB-manque stars (age ab out 5.6 Gyr). This transition occurs at reasonably small red-shifts an d therefore could be detected with the present-day instrumentation.