DESI Survey Validation Spectra Reveal an Increasing Fraction of Recently Quenched Galaxies at $z\sim 1$

Abstract

We utilize $\sim 17000$ bright Luminous Red Galaxies (LRGs) from the novel Dark Energy Spectroscopic Instrument Survey Validation spectroscopic sample, leveraging its deep ($\sim 2.5$ hour/galaxy exposure time) spectra to characterize the contribution of recently quenched galaxies to the massive galaxy population at $0.4<z<1.3$. We use Prospector to infer non-parametric star formation histories and identify a significant population of recently quenched galaxies that have joined the quiescent population within the past $\sim 1$ Gyr. The highest redshift subset (277 at $z>1$) of our sample of recently quenched galaxies represents the largest spectroscopic sample of post-starburst galaxies at that epoch. At $0.4<z<0.8$, we measure the number density of quiescent LRGs, finding that recently quenched galaxies constitute a growing fraction of the massive galaxy population with increasing lookback time. Finally, we quantify the importance of this population amongst massive (\logM$>11.2$) LRGs by measuring the fraction of stellar mass each galaxy formed in the Gyr before observation, $f_{1Gyr}$. Although galaxies with $f_{1Gyr}>0.1$ are rare at $z\sim 0.4$ ($\lesssim 0.5\%$ of the population), by $z\sim 0.8$ they constitute $\sim 3\%$ of massive galaxies. Relaxing this threshold, we find that galaxies with $f_{1Gyr}>5\%$ constitute $\sim 10\%$ of the massive galaxy population at $z\sim 0.8$. We also identify a small but significant sample of galaxies at $z=1.1-1.3$ that formed with $f_{1Gyr}>50\%$, implying that they may be analogues to high-redshift quiescent galaxies that formed on similar timescales. Future analysis of this unprecedented sample promises to illuminate the physical mechanisms that drive the quenching of massive galaxies after cosmic noon.

Re-uploaded after acceptance to the Astrophysical Journal Letters. 14 pages, 5 figures, comments welcome!

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