Two Distinct Classes of Quiescent Galaxies at Cosmic Noon Revealed by JWST PRIMER and UNCOVER

TL;DR

This study uses JWST data to reveal two distinct classes of quiescent galaxies at Cosmic Noon, differentiated by mass, age, and morphology, indicating different quenching mechanisms and growth processes.

Contribution

It uncovers a low-mass quiescent size-mass relation flattening and identifies two separate galaxy populations with different properties and formation pathways.

Findings

Low-mass quiescent galaxies are younger and disky.

Massive quiescent galaxies are spheroidal and older.

A transition mass around 10^10.3 solar masses marks a shift in galaxy properties.

Abstract

We present a measurement of the low-mass quiescent size-mass relation at Cosmic Noon (1<z<3) from the JWST PRIMER and UNCOVER treasury surveys, which highlights two distinct classes of quiescent galaxies. While the massive population is well studied at these redshifts, the low-mass end has been previously under-explored due to a lack of observing facilities with sufficient sensitivity and spatial resolution. We select a conservative sample of low-mass quiescent galaxy candidates using rest-frame UVJ colors and specific star formation rate criteria and measure galaxy morphology in both rest-frame UV/optical wavelengths (F150W) and rest-frame near-infrared (F444W). We confirm an unambiguous flattening of the low-mass quiescent size-mass relation, which results from the separation of the quiescent galaxy sample into two distinct populations at $\log(M_\star/M_\odot)\sim10.3$: low-mass quiescent galaxies that are notably younger and have disky structures, and massive galaxies consistent with spheroidal morphologies and older median stellar ages. These separate populations imply mass quenching dominates at the massive end while other mechanisms, such as environmental or feedback-driven quenching, form the low-mass end. This stellar mass dependent slope of the quiescent size-mass relation could also indicate a shift from size growth due to star formation (low masses) to growth via mergers (massive galaxies). The transition mass between these two populations also corresponds with other dramatic changes and characteristic masses in several galaxy evolution scaling relations (e.g. star-formation efficiency, dust obscuration, and stellar-halo mass ratios), further highlighting the stark dichotomy between low-mass and massive galaxy formation.