The JWST EXCELS survey: The ages and abundances of $3<z<5$ massive quiescent galaxies show that downsizing was already in place by $z\simeq4$

TL;DR

This study uses JWST/NIRSpec spectroscopy to analyze 14 massive quiescent galaxies at redshifts 3-5, confirming early downsizing and examining their stellar ages, metallicities, and chemical abundances, revealing challenges in detailed abundance measurements.

Contribution

First spectroscopic confirmation that downsizing was established by z~4, with detailed analysis of stellar populations and chemical abundances in early quiescent galaxies.

Findings

More massive galaxies assembled their stellar mass earlier.

No older, massive quiescent galaxies found at z=3-5, suggesting recent rejuvenation.

High stellar metallicities consistent with lower redshift counterparts.

Abstract

We present deep, medium-resolution $\lambda=1-5\,\mu$m JWST/NIRSpec spectroscopy for 14 quiescent galaxies at $3<z<5$ with $\log_{10}(M_*/\mathrm{M_\odot}){\,>\,}10$, obtained as part of the EXCELS survey. We perform a complete re-reduction of these data, including a custom optimal-extraction approach to combat the spectral "wiggles" that result from undersampling of the NIRSpec spatial PSF. We constrain the star-formation histories and stellar metallicities of these objects via full-spectral fitting, finding a clear stellar age vs stellar mass correlation, in which more massive galaxies assembled their stellar mass at earlier times. This confirms spectroscopically that the archaeological "downsizing" trend was already in place by $z\simeq4$. The slope of our measured relation ($\simeq2$ Gyr per dex in stellar mass) is consistent with literature results at $0 < z < 3$. We do not observe objects with $\log_{10}(M_*/\mathrm{M_\odot})\lesssim10.5$ and ages of more than a few hundred Myr at this epoch, suggesting that recently reported examples of higher-redshift quiescent galaxies at these masses are likely to soon rejuvenate. We measure relatively high stellar metallicities for the majority of our sample, consistent with similar objects at $0 < z < 3$. Finally, we explore evidence for $\alpha$-enhancement in six older and more luminous galaxies within our sample, finding considerable disagreements in the chemical abundances measured using different stellar population models, different fitted rest-frame wavelength ranges, star-formation history models and fitting codes. We therefore conclude that inferring detailed stellar chemical abundances for the earliest quiescent galaxies remains challenging, and higher signal-to-noise spectra are required (SNR per resolution element $>100$ for $R\simeq1000$).