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

Martin Sparre, Christopher C. Hayward, Robert Feldmann, Claude-André Faucher-Giguère, Alexander L. Muratov, Dušan Kereš, Philip F. Hopkins

Published 2015-10-13Version 1

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.