The JWST EXCELS survey: the ages and abundances of 3 < z < 5 massive quiescent galaxies show that downsizing was already in place by z ≃ 4

ABSTRACT We present deep, medium-resolution $\lambda =1{\text{-}}5\, \mu$m JWST/NIRSpec spectroscopy for 14 quiescent galaxies at $3\,\lt\,z\,\lt\, 5$ with $\log _{10}(M_*/\mathrm{M_\odot }){\, \gt\, }10$, 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 versus 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 $z\simeq 4$. The slope of our measured relation (${\simeq}2$ Gyr per dex in stellar mass) is consistent with literature results at $0 \,\lt\, z \,\lt\, 3$. We do not observe objects with $\log _{10}(M_*/\mathrm{M_\odot })\lesssim 10.5$ 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 $0 \,\lt\, z \,\lt\, 3$. 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 (observed frame SNR per resolution element ${\simeq}100$ for $R\simeq 1000$).