Evidence for Non-smooth Quenching in Massive Galaxies at $z\sim 1$

Abstract

We investigate a large sample of massive galaxies at $z\sim 1$ 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 $(M_{\ast }>10^{10}{\mathrm{M}}_{\odot })$ galaxies at $0.7<z<1.2$, dust-corrected H$\hat{I}\pm$ and UV star-formation indicators agree with a small dispersion ($\sim 0.2$~dex) for galaxies on the main sequence, but diverge and exhibit substantial scatter ($\sim 0.7$~dex) once they drop significantly below the star-forming main sequence. Significant H$\hat{I}\pm$ 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 H$\hat{I}\pm$ flux distribution to the expected distribution assuming bursty or smooth star-formation histories, and find that massive galaxies at $z\sim 1$ 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.

Submitted to MNRAS after revision. Comments welcome!