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

Abstract

We take advantage of our recently published model for Galaxy Evolution and Assembly (GAEA) to study the origin of the observed correlation between [α/Fe] and galaxy stellar mass. In particular, we analyse the role of radio mode AGN feedback, that recent work has identified as a crucial ingredient to reproduce observations. In GAEA, this process introduces the observed trend of star formation histories extending over shorter time-scales for more massive galaxies, but does not provide a sufficient condition to reproduce the observed α enhancements of massive galaxies. In the framework of our model, this is possible only assuming that any residual star formation is truncated for galaxies more massive than 10 10.5 M ⊙ . This results, however, in even shorter star formation time-scales for the most massive galaxies, that translate in total stellar metallicities significantly lower than observed. Our results demonstrate that (i) trends of [α/Fe] ratios cannot be simply converted in relative time-scale indicators, and (ii) AGN feedback cannot explain alone the positive correlation between [α/Fe] and galaxy mass/velocity dispersion. Reproducing simultaneously the mass-metallicity relation and the α enhancements observed poses a challenge for hierarchical models, unless more exotic solutions are adopted such as metal-rich winds or a variable IMF.

Resolved Multi-element Stellar Chemical Abundances in the Brightest Quiescent Galaxy at z ∼ 2

“…Moreover, for the case of MRG-M0138, the high αenhancement is observed simultaneously with a high [Fe/H]. It is a long-standing problem to reproduce both high metallicities and high [α/Fe] simultaneously in models, as simply truncating the star formation (e.g., with AGN feedback) produces a high [α/Fe] but a low [Fe/H] (e.g., Okamoto et al 2017;De Lucia et al 2017). MRG-M0138 makes this problem even worse, as it has even higher [Fe/H] and [α/Fe] than local galaxies.…”

Galaxy evolution across environments as probed by the ages, stellar metallicities and [alpha/Fe] of central and satellite galaxies

“…Segers et al (2016) have shown that EAGLE cosmological hydrodynamical simulations that reproduce the main scaling relations observed for local early-type galaxies (including the mass-metallicity relation - Schaye et al 2015) can also reproduce the trend between [𝛼/Fe] and stellar mass for masses above 10 10.5 M as a result of AGN feedback quenching star formation in massive galaxies. However, De Lucia et al (2017) discuss, in the framework of the semi-analytic GAEA model, that quenching induced by AGN feedback is not enough to reach the observed levels of [𝛼/Fe]. Imposing a truncation of star-formation in galaxies more massive than 10 10.5 M appears necessary to reproduce the [𝛼/Fe]-M * relation, but would violate the stellar mass-metallicity relation.…”

Chemical evolution of elliptical galaxies with a variable IMF

“…If the IMF is universal and invariant, the observed αenhancement of massive ellipticals indicate that they should have a short star formation timescale (e.g., Matteucci 1994;Thomas et al 2005;de La Rosa et al 2011). However, in the case of the most massive ellipticals, the required timescale for reproducing the α-enhancement is too short such that the galaxy does not have enough time to recycle the metal-rich gas and enrich its stellar population (De Lucia et al 2017;Okamoto et al 2017). The difficulty in simultaneously reproducing the total metallicity and α-enhancement of massive elliptical galaxies indicates the necessity of introducing at least one new degree of freedom, e.g., a varying and top-heavy gwIMF (Arrigoni et al 2010;Martín-Navarro et al 2018).…”