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

Abstract

Abstract We investigate a large sample of massive galaxies at z ∼ 1 with combined HST broad-band and grism observations to constrain the star-formation histories of these systems as they transition from a star-forming state to quiescence. Among our sample of massive ($M_*>10^{10}~\hbox{${\rm M}_{\odot }$}{}$) galaxies at 0.7 < z < 1.2, dust-corrected Hα and 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 SFR values 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 star-formation histories, 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.