Calibrated, cosmological hydrodynamical simulations with variable IMFs I: Method and effect on global galaxy scaling relations

TL;DR

This paper introduces new cosmological hydrodynamical simulations with variable IMFs that depend on environmental pressure, affecting galaxy properties and scaling relations, and calibrated to match observed galaxy trends.

Contribution

The study develops pressure-dependent IMF models within cosmological simulations, calibrated to reproduce observed galaxy scaling relations and stellar mass-to-light ratios.

Findings

IMF variations influence stellar mass-to-light ratios and galaxy properties.

Bottom-heavy IMFs have minimal impact on metallicity and sizes.

Top-heavy IMFs increase galaxy metallicity, sizes, and star formation rates, conflicting with observations.

Abstract

The recently inferred variations in the stellar initial mass function (IMF) among local high-mass early-type galaxies may require a reinterpretation of observations of galaxy populations and may have important consequences for the predictions of models of galaxy formation and evolution. We present a new pair of cosmological, hydrodynamical simulations based on the EAGLE model that self-consistently adopt an IMF that respectively becomes bottom- or top-heavy in high-pressure environments for individual star-forming gas particles. In such models, the excess stellar mass-to-light ($M/L$) ratio with respect to a reference IMF is increased due to an overabundance of low-mass dwarf stars or stellar remnants, respectively. Crucially, both pressure-dependent IMFs have been calibrated to reproduce the observed trends of increasing excess $M/L$ with central stellar velocity dispersion ($\sigma_e$) in early-type galaxies, while maintaining agreement with the observables used to calibrate the EAGLE model, namely the galaxy luminosity function, half-light radii of late-type galaxies, and black hole masses. We find that while the $M/L$ excess is a good measure of the IMF for low-mass slope variations, it depends strongly on the age of the stellar population for high-mass slope variations. The normalization of the [Mg/Fe]$-\sigma_e$ relation is decreased (increased) for bottom- (top-)heavy IMF variations, while the slope is not strongly affected. Bottom-heavy variations have little impact on galaxy metallicities, half-light radii of early-type galaxies, or star formation rates, while top-heavy variations significantly increase these quantities for high-mass galaxies, leading to tension with observations.