AGN feedback and the origin of the $\alpha$ 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 [$\alpha$/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 $10^{10.5}{\mathrm{M}}_{\odot }$. 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.

5 pages, 3 figures, accepted for publication MNRAS Letters

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• The star formation history and the nature of the mass-metallicity relation of passive galaxies at 1.0<z<1.4 from VANDELS
• Variation of the stellar initial mass function in semi-analytical models III: testing the cosmic ray regulated integrated galaxy-wide initial mass function
• CASCO: Cosmological and AStrophysical parameters from Cosmological simulations and Observations -- II. Constraining cosmology and astrophysical processes with early- and late-type galaxies
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• Downsizing revised: Star formation timescales for elliptical galaxies with an environment-dependent IMF and number of SNIa