Evidence for Non-smooth Quenching in Massive Galaxies at $z\sim1$

TL;DR

This study analyzes massive galaxies at z~1, revealing that their star-formation histories are best explained by rapid, bursty quenching processes rather than smooth decline, with ongoing low-level star formation during quenching.

Contribution

It provides observational evidence supporting bursty quenching mechanisms in massive galaxies at moderate redshift, contrasting with smooth quenching models.

Findings

Massive galaxies at z~1 show significant scatter in star-formation indicators.

Hα emission persists in galaxies classified as quiescent, indicating ongoing low-level star formation.

Data favor quick, bursty quenching over smooth decline in star formation histories.

Abstract

We investigate a large sample of massive galaxies at $z\sim1$ 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 $(M_*>10^{10}~{\rm M_\odot})$ galaxies at $0.7<z<1.2$, dust-corrected H$\alpha$ and UV star-formation indicators agree with a small dispersion ($\sim0.2$~dex) for galaxies on the main sequence, but diverge and exhibit substantial scatter ($\sim0.7$~dex) once they drop significantly below the star-forming main sequence. Significant H$\alpha$ emission is present in galaxies with low dust-corrected UV SFR values as well as galaxies classified as quiescent using the $UVJ$ diagram. We compare the observed H$\alpha$ flux distribution to the expected distribution assuming bursty or smooth star-formation histories, and find that massive galaxies at $z\sim1$ 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.