Evidence for non-smooth quenching in massive galaxies at z ∼ 1

We investigate a large sample of massive galaxies at z ∼ 1 with combined Hubble Space Telescope (HST) broad-band and grism observations to constrain the star formation histories (SFHs) of these systems as they transition from a star-forming state to quiescence. Among our sample of massive (M_*> 10^{10} M_{⊙}) galaxies at 0.7 < z < 1.2, dust-corrected Hα and ultraviolet (UV) star formation indicators agree with a small dispersion (∼0.2 dex) for galaxies on the main sequence, but diverge and exhibit substantial scatter (∼0.7 dex) once they drop significantly below the star-forming main sequence. Significant Hα emission is present in galaxies with low dust-corrected UV star formation rates (SFRs) as well as galaxies classified as quiescent using the UVJ-diagram. We compare the observed Hα flux distribution to the expected distribution assuming bursty or smooth SFHs, and find that massive galaxies at z ∼ 1 are most consistent with a quick, bursty quenching process. This suggests that mechanisms such as feedback, stochastic gas flows, and minor mergers continue to induce low-level bursty star formation in massive galaxies at moderate redshift, even as they quench.