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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).

This paper represents a collective effort to provide an extensive electronic database useful for the interpretation of the spectra and evolution of galaxies. A broad variety of empirical and theoretical data are discussed here, and the data are made fully available in the AAS CD-ROM Series, Vo. 7. Several empirical stellar libraries are part of this database. They cover the ultraviolet spectral range observed with IUE, optical data from different ground-based telescopes, and ground-based infrared data. Spectral type coverage depends on the wavelength, but it is mostly complete for types O and M and luminosity classes V to I. A large metallicity range is covered as well. Theoretical libraries of selected spectral indices of cool stars and of stellar continuum fluxes in the temperature range 2000 K to 50,000 K, as well as Wolf-Rayet energy distributions are presented. Several libraries of star clusters and early-type galaxies have been selected for this database. We discuss an extensive set of empirical spectra templates covering the wavelength region from 1200 - 9800 A, as well as narrow-band line indices in a large number of passbands. Bench-mark spectra of nearby galaxies for model tests are included as well. We compiled numerous evolutionary models and isochrones for stars of all mass ranges of interest, wide metallicity range, and for all evolutionary phases, including the pre-main-sequence phase. The majority of the models have been computed by the Geneva and Padova groups. Evolutionary synthesis models computed by several independent groups are made available. They can be applied to old and young systems, and are optimized with respect to different aspects of input physics. The model predictions include stellar (colors, magnitudes, absorption features) and nebular (emission-line fluxes) properties. Finally, we present models of ionized gas to be used for the interpretation of active galactic nuclei and young star-forming galaxies. The community is encouraged to make use of this electronic database and to perform a critical comparison between the individual datasets

We present the Galaxy Evolution Explorer (GALEX) far‐ultraviolet (FUV) and near‐ultraviolet (NUV) imaging of three nearby shell galaxies, namely NGC 2865, NGC 5018 and NGC 7135 located in low‐density environments. The system of shells and fine structures visible in the optical is detected in the NUV image of NGC 2865 and in both NUV and FUV images of NGC 7135. The NUV image of NGC 5018 does not present shell structures. We detect absorption features in the nuclear region of all three galaxies. NGC 2865 has a nearly flat colour profile with (FUV−NUV) ≈ 2 throughout the whole galaxy. NGC 7135 is blue in the centre (FUV−NUV) ≈ 0 and as red as (FUV−NUV) ≈ 1.5 in the outskirts, including the faint shell‐like feature. The three shell galaxies are members of poor groups of galaxies. We compare GALEX NUV observations with available H i large‐scale measurements, and determine the UV magnitudes of likely companions. Most of the known (and possible) companions are gas‐rich late‐type galaxies, suggesting that our shell galaxies inhabit the ideal environment for hosting rejuvenating episodes driven by accretion events. We investigate the ability of the nuclearGALEX (FUV−NUV) colour to provide information about rejuvenation phenomena in the stellar populations of the shell galaxies. To this aim, we derive from theory the relationship between the Mg2, Hβ, HγA, HδA Lick line‐strength indices and the (FUV−NUV) colour. We extend the study to a sample of early‐type galaxies in low‐density environments which includes shell galaxies and/or galaxies with emission lines in their optical spectra. In the index versus (FUV−NUV) colour diagrams, most of the galaxies are well explained by passively evolving single stellar populations. On the average, ages and metallicities of the galaxies in our sample estimated from optical line‐strength indices are consistent with those inferred from the (FUV−NUV) colour. We note that the GALEX (FUV−NUV) and (UV−V) colours have different response to age and metallicity. In general, all the colours but for (FUV−NUV) and (FUV−V), become nearly age‐insensitive when 1–2 Gyr have elapsed from the last star‐forming event. Finally, considering composite stellar population models with a recent burst of star formation, we suggest that the positions of the NGC 7135 and NGC 2865 nuclei in the (FUV−NUV)–Hβ plane could be explained in terms of a recent rejuvenation episode.

In this study we present detailed calculations of absorption‐line indices on the Lick system based on the new stellar models by Salasnich et al. incorporating the enhancement of α‐elements, both in opacity and in chemical abundances. The models span large ranges of initial masses, chemical compositions and ages, and are calculated for both solar and enhanced abundance ratios [Xel/Fe] of α‐elements. With these models and the so‐called response functions of Tripicco & Bell, we calculate the indices for single stellar populations (SSPs) of different age, metallicity and degree of enhancement. Starting from the widely accepted conviction that Hβ is a good age indicator, that [MgFe] is most sensitive to metallicity and indices such as Mgb, Mg2 and others are most sensitive to metallicity and degree of enhancement, we made use of the triplet Hβ, Mgb and 〈Fe〉 and minimum‐distance method proposed by Trager et al. to estimate the age, the metallicity and the enhancement degree for the galaxies of the González sample and compare the results with those of Trager et al. and Thomas, Maraston & Bender. Because very large differences are found – in particular, as far as age is concerned, ours are systematically older than those of Trager et al. and Thomas et al. – we analyse in a great detail all possible sources of disagreement, going from the stellar models and SSPs to many technical details of the procedure to calculate the indices and finally the pattern of chemical elements (especially when α‐enhanced mixtures are adopted). Each of the above aspects of the problem bears on the final result: amazingly enough, at increasing complexity of the underlying stellar models and SSPs, the uncertainty increases. However, the key issue of the analysis is that at given metallicity Z and enhancement factor, the specific abundance ratios [Xel/Fe] adopted for some elements (e.g. O, Mg, Ti and probably others) dominate the scene because with the Tripicco & Bell response functions they may strongly affect indices such as Hβ and age in turn. In brief, with the ratio [Ti/Fe]= 0.63 adopted by Salasnich et al., Hβ at old ages turned out to be larger than the mean observational value and therefore the age was forced to very old values in order to recover the observations. In contrast, the results by Trager et al. and Thomas et al. are immediately recovered if their [Ti/Fe] ratios are adopted, i.e. [Ti/Fe]= 0.0 or 0.3, respectively. We have also analysed how the galaxy ages, metallicities and degrees of enhancement vary with the triplets of indices in usage. With this aim in mind we turn to the Trager ‘IDS pristine’ sample, which contains many more galaxies and a much wider list of indices than the González sample. The solution is not unique in reflecting that the poor ability of most indices to be disentangled among the three parameters. Finally, in light of the above results and points of uncertainty, we have made some remarks on the interpretation of the distribution of early‐type galaxies in popular two‐indices planes, such as Hβ ver...

Abstract. In this paper we present a quantitative analysis of the chemical abundances in "monolithic" N-bodyTree-SPH models of elliptical galaxies to determine whether the ratio [α/Fe] increases with the galaxy mass as suggested by observational data.