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