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A low mass-to-light (M/L) ratio for the stellar component of spiral galaxies (M/L 1 in the I band) is advocated by various dynamical arguments and by recent cosmological simulations of the formation of these systems. We discuss this possibility by means of chemo-photometric models for galactic discs, adopting different initial mass functions (IMFs). We show that a number of 'bottom-light' initial mass functions (namely, with less mass locked in low-mass stars than the standard Salpeter IMF), suggested independently in recent literature, do imply M/L ratios as low as mentioned above, at least for late-type spirals (Sbc/Sc). This conclusion still holds when the bulge contribution to mass and light is included. We also predict the typical stellar M/L ratio, and correspondingly the zero-point of the Tully-Fisher relation, to vary considerably with Hubble type (approximately 0.5-0.7 mag in the red bands, from Sa to Sc type).For some of the bottom-light IMFs considered, the efficiency of metal production tends to exceed what is typically estimated for spiral galaxies. Suitable tuning of the IMF mass limits, post-supernova fallback of metals on to black holes or metal outflows must then be invoked, to reproduce the observed chemical properties of disc galaxies.In the appendix we provide M/L-colour relations to estimate the stellar M/L ratio of a galaxy on the basis of its colours, for several IMFs.

Abstract. In this paper we present models for Single Stellar Populations (SSPs) of intermediate and old ages where dust enshrouded Asymptotic Giant Branch (AGB) stars are introduced. As long known AGB stars are surrounded by dust-rich shells of matter caused by their own stellar wind, which absorb the radiation coming from the central object and re-emit it in the far infrared (IR). To this aim, particular care is devoted to follow the evolution of the AGB stars throughout the quiet and thermally pulsing regimes, to evaluate the effect of self contamination in the outermost layers by the third dredge-up mechanism, to follow the transition from oxygen-rich to carbon-rich objects (as appropriate to their initial mass and chemical composition), and finally to estimate the efficiency of mass-loss by stellar winds, all aspects that concur to the formation and properties of the dusty shells around. In addition to this, accurate physical models of the dusty shells are presented in which the re-processing of radiation from the central stars is calculated by solving the radiative transfer equations in presence of dust particles of different chemical composition. The resulting spectral energy distribution (SED) is examined to show how important features, like the 10 µm Si−O stretching mode feature and the 11 µm SiC feature, evolve with time. The SEDs are then convolved with the IRAS filters to obtain the flux in various pass-bands, i.e. 12, 25 and 60 µm, for individual AGB stars of different mass, chemical composition, and age. The comparison is made by means of SSPs along which AGB stars of the same age but different initial masses are located. This allows us to explore the whole range of masses and ages spanned by AGB stars. The theoretical results are compared to the observational data for selected groups of stars. The same is made for the J, H, K, L pass-bands of the Johnson system. Finally, from the integrated SEDs of the SSPs, we derive the integrated Johnson J, H, K, L magnitudes and colors to be compared to infrared data for star clusters of the Magellanic Clouds. In general good agreement with the data is possible if the effects of the circumstellar shells of dust are taken into account.

The advent of modern infrared astronomy has brought into evidence the role played by the interstellar dust in galaxy formation and evolution. Therefore, to fully exploit modern data, realistic spectrophotometric models of galaxies must include this important component of the interstellar medium (ISM).In this paper, the first of a series of two devoted to modelling the spectra of galaxies of different morphological type in the presence of dust, we present our description of the dust both in the diffuse ISM and in the molecular clouds (MCs).Our galaxy model contains three interacting components: the diffuse ISM, made of gas and dust, the large complexes of MCs in which active star formation occurs and, finally, the populations of stars that are no longer embedded in the dusty environment of their parental MCs.Our model for the dust takes into account three components, i.e. graphite, silicates and polycyclic aromatic hydrocarbons (PAHs). We consider and adapt to our aims two prescriptions for the size distribution of the dust grains and two models for the emission of the dusty ISM. We cross-check the emission and extinction models of the ISM by calculating the extinction curves and the emission for the typical environments of the Milky Way (MW) and the Large and Small Magellanic Clouds (LMC and SMC) and by comparing the results with the observational data. The final model we have adopted is a hybrid one which stems from combining the analysis of Guhathakurta & Draine for the emission of graphite and silicates and Puget, Leger & Boulanger for the PAH emission, and using the distribution law of Weingartner & Draine and the ionization model for PAHs of Weingartner & Draine.We apply the model to calculate the spectral energy distribution (SED) of single stellar populations (SSPs) of different age and chemical composition, which may be severely affected by dust at least in two types of stars: the young, massive stars while they are still embedded in their parental MCs and the intermediate-and low-mass asymptotic giant branch (AGB) stars when they form their own dust shell around.We use the 'ray-tracing' method to solve the problem of radiative transfer and to calculate extended libraries of SSP SEDs. Particular care is taken to model the contribution from PAHs, introducing different abundances of C in the population of very small carbonaceous grains (VSGs) and different ionization states in PAHs. The SEDs of young SSPs are then compared with observational data of star-forming regions of four local galaxies successfully reproducing their SEDs from the ultraviolet (UV)-optical regions to the mid-and far-infrared region (MIR and FIR, respectively).