Zusammenfassung
Galaxy formation models exhibit remarkable success in reproducing observed
relations such as the relation between galaxies' star formation rates (SFRs)
and stellar masses 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--stellar mass relation has a significantly
greater scatter when the Halpha-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. Consequently,
low-mass galaxies can go through both quenched (in terms of the 10-Myr averaged
SFR) and starburst phases within a 200-Myr period. We also find that the
Halpha/FUV ratios are very similar to those observed for local galaxies,
although there is a population of simulated galaxies with lower Halpha/FUV
ratios than observed at stellar masses smaller than 10^9.5 solar masses. The
interpretation is that our sample of FIRE galaxies is slightly more bursty than
the observed sample of galaxies in the vicinity of the Galaxy. A possible
explanation is that despite the very high resolution of the simulations, the
SFR variability and thus Halpha/FUV ratios may not be fully converged. We
suggest that future cosmological simulations should compare the Halpha/FUV
ratios of their galaxies with observations to constrain the feedback models
employed.
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