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