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

We utilize ~17,000 bright luminous red galaxies (LRGs) from the novel Dark Energy Spectroscopic Instrument Survey Validation spectroscopic sample, leveraging its deep (~2.5 hr galaxy<SUP>-1</SUP> exposure time) spectra to characterize the contribution of recently quenched galaxies to the massive galaxy population at 0.4 &lt; z &lt; 1.3. We use Prospector to infer nonparametric star formation histories and identify a significant population of recently quenched galaxies that have joined the quiescent population within the past ~1 Gyr. The highest-redshift subset (277 at z &gt; 1) of our sample of recently quenched galaxies represents the largest spectroscopic sample of post-starburst galaxies at that epoch. At 0.4 &lt; z &lt; 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 look-back time. Finally, we quantify the importance of this population among massive ( $\mathrm{log}({M}_{\star }/{M}_{\odot })$ &gt; 11.2) LRGs by measuring the fraction of stellar mass each galaxy formed in the gigayear before observation, f <SUB>1 Gyr</SUB>. Although galaxies with f <SUB>1 Gyr</SUB> &gt; 0.1 are rare at z ~ 0.4 (≲0.5% of the population), by z ~ 0.8, they constitute ~3% of massive galaxies. Relaxing this threshold, we find that galaxies with f <SUB>1 Gyr</SUB> &gt; 5% constitute ~10% of the massive galaxy population at z ~ 0.8. We also identify a small but significant sample of galaxies at z = 1.1-1.3 that formed with f <SUB>1 Gyr</SUB> &gt; 50%, implying that they may be analogs 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.