AGN feedback and the origin of the $\alpha$ enhancement in early type galaxies - insights from the GAEA model

TL;DR

This study uses the GAEA galaxy evolution model to investigate the role of AGN feedback in the observed alpha enhancement in early-type galaxies, highlighting the complexity of reproducing observed chemical properties.

Contribution

The paper demonstrates that AGN feedback alone cannot fully explain the alpha enhancement trend, emphasizing the need for additional mechanisms in hierarchical galaxy formation models.

Findings

AGN feedback influences star formation timescales but is insufficient alone to produce observed alpha enhancements.

Shorter star formation timescales in massive galaxies lead to lower metallicities than observed.

Reproducing both alpha enhancements and metallicity relations requires additional processes like metal-rich winds or a variable IMF.

Abstract

We take advantage of our recently published model for Galaxy Evolution and Assembly (GAEA) to study the origin of the observed correlation between [$\alpha$/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 $\alpha$ 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 $10^{10.5}\,{\rm M}_{\odot}$. 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 [$\alpha$/Fe] ratios cannot be simply converted in relative time-scale indicators, and (ii) AGN feedback cannot explain alone the positive correlation between [$\alpha$/Fe] and galaxy mass/velocity dispersion. Reproducing simultaneously the mass-metallicity relation and the $\alpha$ enhancements observed poses a challenge for hierarchical models, unless more exotic solutions are adopted such as metal-rich winds or a variable IMF.