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It turns out that the heavier the galaxy, the earlier its stellar construction ceased. A tenfold difference in mass shifts the age by about 2 billion years. It's as if in one city houses stopped being built in 1900, while in a poorer one, only in 2000. This pattern was known for nearby galaxies, but now it has been found in the early universe as well.
The lightest of the studied galaxies turned out to be young: their stars are only a few hundred million years old. So, the recently discovered 'old' galaxies of the same mass are likely to start birthing stars again soon.
The chemical composition of stars in these ancient galaxies resembles that of later ones. But when scientists tried to measure the exact amounts of individual elements, different methods gave different answers. To sort this out, sharper spectra are needed, like trying to count windows in a city through a fogged-up glass.
We present deep, medium-resolution $$\lambda=1-5\,\mu$$m JWST/NIRSpec spectroscopy for 14 quiescent galaxies at $$3\,}10$$, 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 $$R\simeq1000$$).
Light from these galaxies has been traveling to us for 11–12 billion years — almost the age of the universe. We see them as they were when the Sun did not yet exist.
