(Star)bursts of FIRE: observational signatures of bursty star formation in galaxies

TL;DR

This paper uses FIRE simulations to show that bursty star formation causes observable differences in SFR indicators, providing new constraints on galaxy formation feedback models.

Contribution

It demonstrates that short-term SFR variability affects observable indicators and compares simulation results with observations to constrain feedback physics.

Findings

FIRE galaxies exhibit order-of-magnitude SFR variations over a few Myr.

The difference in scatter between Hα and FUV SFR relations matches observational data.

Simulated low-mass galaxies show lower Hα/FUV ratios than observed, indicating areas for model improvement.

Abstract

Galaxy formation models are now able to reproduce observed relations such as the relation between galaxies' star formation rates (SFRs) and stellar masses ($M_*$) and the stellar mass--halo mass relation. We demonstrate that comparisons of the short-timescale variability in galaxy SFRs with observational data provide an additional useful constraint on the physics of galaxy formation feedback. We apply SFR indicators with different sensitivity timescales to galaxies from the Feedback in Realistic Environments (FIRE) simulations. We find that the SFR--$M_*$ relation has a significantly greater scatter when the H$\alpha$-derived SFR is considered compared with when the far-ultraviolet (FUV)-based SFR is used. This difference is a direct consequence of bursty star formation because the FIRE galaxies exhibit order-of-magnitude SFR variations over timescales of a few Myr. We show that the difference in the scatter between the simulated H$\alpha$- and FUV-derived SFR--$M_*$ relations at $z=2$ is consistent with observational constraints. We also find that the H$\alpha$/FUV ratios predicted by the simulations at $z=0$ are similar to those observed for local galaxies except for a population of low-mass ($M_* \lesssim 10^{9.5} {\rm M}_\odot$) simulated galaxies with lower H$\alpha$/FUV ratios than observed. We suggest that future cosmological simulations should compare the H$\alpha$/FUV ratios of their galaxies with observations to constrain the feedback models employed.