A population of faint, old, and massive quiescent galaxies at 3 < z < 4 revealed by JWST NIRSpec Spectroscopy

Abstract The confirmation of the presence of very massive quiescent galaxies at epochs only 1–2 Gyr after the Big Bang [1–8] has challenged models of cosmology and galaxy formation [9]. Producing sufficient numbers of these requires abundant numbers of the host dark matter halos to have been assembled and sufficient time for star formation to proceed extremely quickly and then cease just as rapidly. Ground-based spectroscopy has suggested ages of 200–300 Myr[3] at redshifts 3 < z < 4. The true number and ages of these objects have however been highly uncertain as ground-based spectra has been limited to the bright- est of them [e.g. 3, 5], at wavelengths ∼ 2μm, which introduces a signficant potential bias towards younger objects [7]. The launch of the James Webb Space Telescope (JWST) enables dramatically more sensitive and constraining spectroscopic observations due to the very low sky background, sharp image quality, and access to wavelengths beyond 2μm. Here we report JWST NIR- Spec [10] (0.6–5.3μm) observations of five new quiescent galaxy candidates that were beyond the limit of previous ground-based spectroscopy. The high signal:noise spectra of galaxies with continuum significantly fainter than ear- lier confirmations show that they are also at redshifts 3 < z 1 billion years pointing to a presence of substantially older and fainter galaxies than those revealed so far by ground-based spectroscopy. This suggests that some of the massive galaxies have very early formation epochs (during the epoch of reionization, z ∼> 6) pointing to a need for high conversion rates of baryons to stars in the first massive galaxy halos in the early Universe [11, 12].