IQ Collaboratory II: The Quiescent Fraction of Isolated, Low Mass Galaxies Across Simulations and Observations

TL;DR

This study compares three large-scale galaxy simulations with SDSS observations, revealing that none fully reproduce the observed absence of low-mass quiescent field galaxies, highlighting the need for improved feedback models.

Contribution

It introduces a framework for forward modeling simulations into observational space, enabling more accurate comparisons and revealing discrepancies in low-mass galaxy quenching.

Findings

All simulations show fewer quiescent, isolated galaxies than observed.

Simulations cannot reproduce the lack of quiescent galaxies below 10^9 M_sun.

Quenching timescales are consistent despite different star formation histories.

Abstract

We compare three major large-scale hydrodynamical galaxy simulations (EAGLE, Illustris-TNG, and SIMBA) by forward modeling simulated galaxies into observational space and computing the fraction of isolated and quiescent low mass galaxies as a function of stellar mass. Using SDSS as our observational template, we create mock surveys and synthetic spectroscopic and photometric observations of each simulation, adding realistic noise and observational limits. All three simulations show a decrease in the number of quiescent, isolated galaxies in the mass range $\mathrm{M}_* = 10^{9-10} \ \mathrm{M}_\odot$, in broad agreement with observations. However, even after accounting for observational and selection biases, none of the simulations reproduce the observed absence of quiescent field galaxies below $\mathrm{M}_*=10^{9} \ \mathrm{M}_\odot$. We find that the low mass quiescent populations selected via synthetic observations have consistent quenching timescales, despite apparent variation in the late time star formation histories. The effect of increased numerical resolution is not uniform across simulations and cannot fully mitigate the differences between the simulations and the observations. The framework presented here demonstrates a path towards more robust and accurate comparisons between theoretical simulations and galaxy survey observations, while the quenching threshold serves as a sensitive probe of feedback implementations.