Chemical evolution of local galaxies in a hierarchical model

Abstract

We investigate the chemical properties of local galaxies within a cosmological framework in the hierarchical picture of galaxy formation. To this aim, we use a hierarchical semi-analytic model which includes the contribution from (i) low and intermediate mass stars, (ii) type Ia Supernovae (SNe) and (iii) massive stars. - Abridged - We compare our predictions with available observations in the Milky Way (MW), in local dwarf galaxies and in local ellipticals. For Milky-Way-like galaxies, we can successfully reproduce the [O-Fe] vs [Fe/H] relation observed in disc stars and the stellar metallicity distribution (SMD). For dwarf galaxies, the stellar metallicity vs mass relation is reproduced by assuming that a substantial fraction of the heavy elements is lost through metal-enhanced outflows and a type Ia SN realization probability lower than the one of MW-like galaxies. - Abridged - In ellipticals, the observations indicate higher [alpha/Fe] values in larger galaxies. - Abridged - Our results computed with a standard Salpeter initial mass function (IMF) indicate a flat [alpha/Fe]-sigma relation. However, we suggest a possible solution to this problem and show how, by assuming a star formation-dependent IMF with a slope x=1.35 in systems with star formation rates < 100 M_sun/yr and slightly flatter (i.e. with x=1) in object with stronger star formation, the observed correlation between [alpha/Fe] and sigma can be accounted for on a large velocity dispersion range. Fundamental roles are played also by interaction-triggered starbursts and AGN.Comment: MNRAS, accepted, 27 pages, 22 figure

Chemoarchaeological downsizing in a hierarchical universe: impact of a top-heavy IGIMF

“…Among the most recent ones, Arrigoni et al (2010) find that with a top-heavier slope of the IMF, they can achieve a positive slope in the [α/Fe]-stellar mass relation, close to the observed one. On the other hand, Calura & Menci (2009) use another SAM and show that simply changing the slope of the IMF seems to be insufficient to obtain the right slope in the observed relation. They argue that the assumption of a constant IMF flatter than Salpeter only lift the zero-point of the relation.…”

“…They find a better agreement with the observed [α/Fe]-σ relation when taking into account AGN feedback in their models, although still marginal. Calura & Menci (2009) implement a chemical model on top of the star formation histories extracted from a semi-analytic model, and evaluate the impact on the [α/Fe]-σ relation of the AGN feedback and an adhoc variable IMF that becomes top heavy in starbursts, finding that they are both needed to obtain a slope in the relation similar to the observed one. This was the first attempt to include a variable IMF in a semi-analytic model to study the [α/Fe] ratios in ellipticals.…”

The metal enrichment of passive galaxies in cosmological simulations of galaxy formation

“…Simultaneously explaining the increasing α-to-Fe ratio with velocity dispersion (or stellar mass) and increasing stellar metallicity with stellar mass is therefore fundamentally difficult within the current framework of galaxy formation. One possible solution would be the introduction of a variable IMF which becomes more top-heavy for a higher SFR as suggested by Calura & Menci (2009) and Gargiulo et al (2015) and previously suggested on independent grounds by Baugh et al (2005) and Lacey et al (2016).…”

“…This assumption, together with quasar-mode AGN feedback and interaction-triggered starbursts, yields a better agreement with the [α/Fe]-σ and mass-metallicity relations than the case where the Salpeter IMF is assumed for all star formation activities. Calura & Menci (2011) employed the same model as Calura & Menci (2009), but computed chemical evolution using individual star formation histories of all progenitors of a galaxy considered. They claimed that inclusion of quasar-mode AGN feedback which quenches late star formation in large haloes and starbursts triggered by galaxy harassment, which boosts star formation activity at high redshift, is crucial and the variable IMF used in Calura & Menci (2009) is not necessary to explain the observed [α/Fe]-σ relation, although they did not show the mass-metallicity relation predicted by their model.…”

“…Calura & Menci (2011) employed the same model as Calura & Menci (2009), but computed chemical evolution using individual star formation histories of all progenitors of a galaxy considered. They claimed that inclusion of quasar-mode AGN feedback which quenches late star formation in large haloes and starbursts triggered by galaxy harassment, which boosts star formation activity at high redshift, is crucial and the variable IMF used in Calura & Menci (2009) is not necessary to explain the observed [α/Fe]-σ relation, although they did not show the mass-metallicity relation predicted by their model. Arrigoni et al (2010) also investigated the [α/Fe]-mass and mass-metallicity relations by using a semi-analytic model which includes both quasar-and radio-mode AGN feedback and a detailed chemical-evolution model in which they relax the instantaneous recycling approximation.…”