Evidence for Non-smooth Quenching in Massive Galaxies at z ∼ 1

We investigate a large sample of massive galaxies at z similar to 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-circle dot) galaxies at 0.7 < z < 1.2, dust-corrected Ha and ultraviolet (UV) star formation indicators agree with a small dispersion (similar to 0.2 dex) for galaxies on the main sequence, but diverge and exhibit substantial scatter (similar to 0.7 dex) once they drop significantly below the star-forming main sequence. Significant H alpha 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 Ha flux distribution to the expected distribution assuming bursty or smooth SFHs, and find that massive galaxies at z similar to 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.