The effect of the Initial Mass Function (IMF) on the chemical evolution of elliptical galaxies.

We describe our work on the chemical evolution of elliptical galaxies, with a particular focus on the effect of the Initial Mass Function (IMF). Generally, ellipticals are systems characterized by old stellar populations, showing little or no sign of ongoing star formation. Due to the lack of gas and star formation, abundances in these objects can only be derived from the study of absorption lines in their integrated spectra. We adopted a multi-zone model of chemical evolution for elliptical galaxies, taking into account both Type Ia and II SN feedback, where galaxies are assumed to form after the initial infall of a gas cloud. After this gas accretion, the galaxy produces its stellar population through a strong burst, which is eventually quenched by the thermal energy produced by stellar winds and SNe feedback (galactic wind), which drives the remaining gas away from the galaxy. Detailed stellar nucleosynthesis allow to compute the formation and detailed evolution with time of 21 chemical elements, and we focused on reproducing the observed mass-metallicity and [α/Fe]-mass relations, with the aim to derive constraints on the formation and evolution of the galaxies. The IMF has a strong influence on the evolution of chemical abundances, and we tested the most recent suggested recipes for the IMFs for ellipticals. We present the comparison of our results with (1) the dataset by Thomas et al. (2010), containing information on the chemical abundance patterns for ≈3000 galaxies from the SDSS DR4, and (2) with a sample of early-type galaxies selected from the SPIDER-SDSS catalog, whose abundances have been derived from averaged spectra, obtained by a stacking procedure of individual spectra according to their central velocity dispersion (La Barbera et al., 2010; Rosani et al., 2018). In previous works, the considered abundance trends were reproduced by means of the so-called “inverse wind” model, namely assuming a larger efficiency of star formation in more massive ellipticals (also known as “downsizing” in star formation, see Matteucci, 1994), but the comparison with the first catalog showed the necessity to modify this scenario by adopting an IMF varying with galactic stellar mass. We found that the observed increase of total metallicity and the overabundance of α-elements with respect to Fe in more massive galaxies could only be reproduced by assuming a top-heavier IMF in galaxies with a higher total mass. We tested both an arbitrarily varying IMF and the Integrated Galactic IMF - IGIMF (both becoming top-heavier for higher galactic masses), and both IMFs were able to reproduce the trends. This is in contradiction with claims, diffused in literature, for the need of a bottom-heavy IMF for ellipticals (Ferreras et al., 2013). We repeated this analysis on the second catalog, and extended it by introducing new forms for the IMF; specifically, we tested a low-mass tapered (“bimodal”) IMF. The analysis of the second catalog, though not confirming the necessity of a top-heavier IMF (the trends in this catalog could be reproduced by assuming the inverse wind model only), strongly confirmed our rejection of bottom-heavier IMFs, which provided the worst agreement with data in our models. To reconcile our results with observational claim of bottom-heavy IMFs, we finally tested an explicitly time-dependent form of the bimodal IMF, switching from an initial top-heavy phase to a bottom-heavy one. This provided a good agreement with observed data as well, as long as the switching occurred in the early stages of the galactic evolution. Our results are new, since it was previously concluded that ellipticals should have either a top-heavy (Fontanot et al., 2017) or a bottom-heavy IMF, whereas we found that the best agreement is given by a combination of the two.