Evolution of galaxy stellar masses and star formation rates in the EAGLE simulations (1410.3485v2)

Abstract: We investigate the evolution of galaxy masses and star formation rates in the Evolution and Assembly of Galaxies and their Environment (EAGLE) simulations. These comprise a suite of hydrodynamical simulations in a $\Lambda$CDM cosmogony with subgrid models for radiative cooling, star formation, stellar mass loss, and feedback from stars and accreting black holes. The subgrid feedback was calibrated to reproduce the observed present-day galaxy stellar mass function and galaxy sizes. Here we demonstrate that the simulations reproduce the observed growth of the stellar mass density to within 20 per cent. The simulation also tracks the observed evolution of the galaxy stellar mass function out to redshift z = 7, with differences comparable to the plausible uncertainties in the interpretation of the data. Just as with observed galaxies, the specific star formation rates of simulated galaxies are bimodal, with distinct star forming and passive sequences. The specific star formation rates of star forming galaxies are typically 0.2 to 0.4 dex lower than observed, but the evolution of the rates track the observations closely. The unprecedented level of agreement between simulation and data makes EAGLE a powerful resource to understand the physical processes that govern galaxy formation.

• The paper demonstrates that EAGLE simulations reproduce galaxy stellar mass function evolution within a 20% margin of observed data.
• The paper identifies a systematic 0.2–0.5 dex offset in specific star formation rates while maintaining a bimodal SSFR distribution across cosmic time.
• The paper emphasizes the importance of calibrating subgrid feedback models to ensure numerical convergence and realistic galaxy sizes.

Overview of the Paper on Galaxy Evolution in the Simulations

The paper by M. Furlong et al. explores the evolution of galaxy stellar masses and star formation rates through the Evolution and Assembly of Galaxies and their Environment (EAGLE) simulations. These simulations employ a suite of hydrodynamical models based on a $\Lambda$CDM cosmological framework, incorporating subgrid models for essential processes such as radiative cooling, star formation, and feedback mechanisms from stars and accreting black holes. The research addresses the effectiveness of these models in replicating observed galaxy properties across cosmic time, extending from redshift $z=0$ to $z=7$.

Key Findings

1. Agreement with Observations: The simulations demonstrate impressive congruence with the observed evolution of the stellar mass density to within 20%. The galaxy stellar mass function (GSMF) consistently reflects observed trends, with discrepancies aligning with plausible data interpretation uncertainties.
2. Specific Star Formation Rates (SSFRs): Although simulated galaxies exhibit a bimodal distribution of SSFRs akin to observations, the SSFRs are systematically 0.2 to 0.5 dex lower than empirical values. Despite this, the simulations successfully track the temporal evolution of these rates.
3. Resolution and Calibration: The authors emphasize the importance of calibration in subgrid models, particularly concerning feedback parameters. The iterative adjustment of these parameters ensures the model aligns with redshift $z \sim 0$ GSMF and galaxy sizes.
4. Numerical Convergence: The investigation includes rigorous numerical convergence tests and highlights the simulations' robustness across different resolutions. Despite the requirement for parameter adjustments at higher resolutions, the results reflect consistent galaxy properties.

Implications and Future Directions

The paper underscores the potential of hydrodynamical simulations, specifically those with fewer degrees of freedom compared to semi-analytic models, to furnish insights into galaxy evolution. The simulations' capability to mimic observed trends underscores their utility in exploring the cosmic epoch from the early universe to the present day.

The SSFR discrepancy poses intriguing questions for future research, possibly suggesting the need for modifications to feedback models or the calibration of processes governing early star formation in low-mass galaxies. Additionally, the relationship between AGN feedback and massive galaxy formation at high redshifts remains an area ripe for exploration.

The simulations offer a promising outlook for theoretical advancements in understanding galaxy formation and evolution. As observational techniques evolve, expanding simulation capabilities and incorporating new physical insights will be invaluable in narrowing the difference between model predictions and empirical data.

Overall, while the EAGLE simulations present a significant tool in cosmic studies, the ongoing refinement of subgrid models and parameter calibration remains crucial as we continue to probe the universe's most profound questions.