The JWST EXCELS Survey: Insights into the Nature of Quenching at Cosmic Noon

Abstract We study 24 massive quiescent galaxies with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi mathvariant="italic">M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi mathvariant="italic">M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo>&gt;</mml:mo> <mml:mn>10</mml:mn> </mml:math> at 1 &lt; z &lt; 3 with JWST/NIRSpec medium-resolution observations from the Early Extragalactic Continuum and Emission Line Survey (EXCELS). We reconstruct their star formation histories and find that they have large bursts (100–1000 M ⊙ yr −1 ), followed by a rapid truncation of star formation. The number densities of the quenched galaxies in our sample that we predict underwent a submillimeter phase are consistent with submillimeter galaxies being the progenitors of our quenched population. The median post-starburst visibility time is ∼600 Myr, with more massive galaxies ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi mathvariant="italic">M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi mathvariant="italic">M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo>&gt;</mml:mo> <mml:mn>10.7</mml:mn> </mml:math> ) exhibiting shorter visibility times than lower-mass galaxies. The range of quenching times—defined as the time from the peak starburst to the time of quiescence—found in this sample (0.06–1.75 Gyr) suggests multiple quenching pathways, consistent with previous studies. We do not see evidence for quenching mechanisms varying with redshift between 1 &lt; z &lt; 3. We detect evidence for weak active galactic nucleus activity in four out of the eight galaxies with robust emission line detections, based on line ratio diagnostics. Our findings suggest that there is a diverse range of quenching mechanisms at Cosmic Noon, and support a scenario in which the primary quenching mechanisms are rapid (&lt;500 Myr) following a starburst.