GALICS. II: the [ α/Fe] -mass relation in elliptical galaxies

Abstract

Aims. We test whether the mass-and σ-[α/Fe] relations in the stellar populations of early-type galaxies can be reproduced by a cosmologically motivated assembly history for spheroids. Methods. We implement a detailed treatment for the chemical evolution of H, He, O, and Fe in GalICS, a semi-analytical model for galaxy formation that successfully reproduces basic low-and high-redshift galaxy properties. We take the contribution of supernovae into account (both type Ia and II), as well as low-and intermediate-mass stars, to chemical feedback. The model predictions are compared with the most recent observational results. Results. We find that the model shows significant improvement at the highest masses with respect to previous work, where the most massive galaxies were also the most α-depleted. In fact the predicted [α/Fe] ratios in this regime are now marginally consistent with observed values. We show that this result comes from the implementation of AGN quenching of star formation in massive haloes. However, this does not help with the creation of the mass-metallicity relation. Instead, at intermediate masses, the scatter in the predicted [α/Fe] ratios is much larger than the observed dispersion. This problem is related to inadequacies of the model in treating satellite galaxies. In particular, we find an excess of low-mass strongly α-enhanced satellites. Conclusions. The final stellar [α/Fe] of a single galaxy is determined by the star formation history summed over all the progenitors. In particular, a longer duration the integrated star formation history leads to a lower α-enhancement, as might be expected from the results of closed box chemical evolution models. However, non-negligible differences between closed box and hierarchical model predictions are found, due to processes such as dry mergers and hot gas-phase metal recycling in the latter case. These processes help to build up the galactic mass while keeping the α element abundance in the stars at a super-solar level. To match the observed mass-[α/Fe] relation at low and intermediate masses, we suggest that the next generation of semi-analytical model should feature either stellar or AGN feedback schemes that allow galaxies to self-regulate their own star formation history, rather than being crudely linked to the halo mass. At the same time, mechanisms that allow the very old and passively evolving satellite galaxies that do not merge to accrete fresh gas and form stars at later times should be implemented.

The dilution peak, metallicity evolution, and dating of galaxy interactions and mergers

“…It is well known that the bulge of the Milky Way is both metalrich (typically about [Fe/H] ∼ −0.3 to 0.3 with a mean of about 0) (Zoccali et al 2008) and is α enhanced (Lecureur et al 2007;Zoccali et al 2008;Ryde et al 2010). Both of these results are consistent with our simulations modulo the uncertainty in the delay times of type Ia SNe, and the mass and characteristics of preexisting bulges in the merging galaxies (see Pipino et al 2009). This general result appears to hold generally for bulges, namely they are metal-rich and α enhanced (Thomas et al 2005) and normalizing for their mass differences, very similar to early type galaxies (Thomas et al 2005).…”

AGN feedback and the origin of the α enhancement in early-type galaxies – insights from the GAEA model

“…This has allowed the problem of the α enhancements of elliptical galaxies to be revisited in greater details. Pipino et al (2009) showed that the inclusion of radio-mode feedback in their semi-analytic model was able to invert the slope of the galaxy stellar mass-[α/Fe] relation for massive ellipticals. The predicted slope was, however, still relatively shallow and the scatter too large, particularly for intermediate mass galaxies.…”

Elemental abundances in Milky Way-like galaxies from a hierarchical galaxy formation model

“…The situation has improved significantly in recent years with detailed chemical schemes being incorporated both in hydro-dynamical simulations (Tornatore et al 2004;Scannapieco et al 2005;Wiersma et al 2009;Few et al 2012) and in semi-analytic models of galaxy formation (Nagashima et al 2005;Cora 2006;Pipino et al 2009;Arrigoni et al 2010;Benson 2012;Yates et al 2013).…”