Stacked star formation rate profiles of bursty galaxies exhibit 'coherent' star formation

TL;DR

This study compares observed stacked star formation rate profiles with simulated galaxies, revealing that individual galaxy profiles are complex and bursty, challenging the interpretation of smooth galaxy evolution along the star formation main sequence.

Contribution

It demonstrates that the observed coherent star formation profiles are compatible with highly bursty, clumpy star formation in individual galaxies, contrasting with previous interpretations of smooth evolution.

Findings

Individual galaxy SFR profiles are highly complex and variable.

Stacked profiles can mask the bursty nature of star formation.

Bursty star formation explains the observed coherent profiles.

Abstract

In a recent work based on 3200 stacked H$\alpha$ maps of galaxies at $z \sim 1$, Nelson et al.~find evidence for `coherent star formation': the stacked SFR profiles of galaxies above (below) the 'star formation main sequence' (MS) are above (below) that of galaxies on the MS at all radii. One might interpret this result as inconsistent with highly bursty star formation and evidence that galaxies evolve smoothly along the MS rather than crossing it many times. We analyze six simulated galaxies at $z\sim1$ from the Feedback in Realistic Environments (FIRE) project in a manner analogous to the observations to test whether the above interpretations are correct. The trends in stacked SFR profiles are qualitatively consistent with those observed. However, SFR profiles of individual galaxies are much more complex than the stacked profiles: the former can be flat or even peak at large radii because of the highly clustered nature of star formation in the simulations. Moreover, the SFR profiles of individual galaxies above (below) the MS are not systematically above (below) those of MS galaxies at all radii. We conclude that the time-averaged coherent star formation evident stacks of observed galaxies is consistent with highly bursty, clumpy star formation of individual galaxies and is not evidence that galaxies evolve smoothly along the MS.