Double trouble: Two spectroscopically confirmed low-mass quiescent galaxies at z > 5 in overdensities

¹ DARK, Niels Bohr Institute, University of Copenhagen Jagtvej 155A DK-2200 Copenhagen, Denmark
² Cosmic Dawn Center (DAWN), Denmark
³ DTU Space, Technical University of Denmark, Elektrovej 327 DK-2800 Kgs. Lyngby, Denmark
⁴ INAF-Osservatorio Astrofisico di Arcetri Largo Enrico Fermi 5 I-50125 Firenze, Italy
⁵ Niels Bohr Institute, University of Copenhagen Jagtvej 128 DK-2200 Copenhagen North, Denmark
⁶ Institute for Astronomy, University of Edinburgh, Royal Observatory Edinburgh EH9 3HJ, UK

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 11 September 2025
Accepted: 9 December 2025

Abstract

We present the discovery of two low-mass, high-redshift quiescent galaxies, GS-z5-Q1 and COS-z5-Q1, using JWST NIRSpec spectroscopy alongside NIRCam and MIRI photometry. Observed at a redshift of z = 5.39 and z = 5.11, respectively, and with stellar masses of 10^9.6 M_⊙ and 10^9.5 M_⊙, GS-z5-Q1 and COS-z5-Q1 are two of the most distant quiescent galaxies spectroscopically confirmed to date. They are also by far the least massive (∼10× lower mass). Full spectrophotometric modelling has revealed that COS-z5-Q1 appears to have quenched more than 300 Myr prior to observation (z ∼ 7) and has a formation redshift of around z ∼ 11, whilst GS-z5-Q1 formed and quenched in a single burst around 150 Myr prior to observation (z ∼ 6). GS-z5-Q1 is found to lie near the centre of a known high-z overdensity in GOODS-S, as would be expected by galaxy formation models, while COS-z5-Q1 lies towards the outskirts of an overdense region. This highlights the role that environment could play in accelerating galaxy evolutionary processes and could possibly be linked to the galaxies’ quiescent nature. By modelling their stellar populations, we show that these types of low-mass quiescent galaxies could potentially be descendants of the higher z ‘mini-quenched’ galaxies. The discovery of these two low-mass, z > 5 quiescent galaxies illuminates a previously undiscovered galaxy population and motivates further dedicated follow-up surveys to investigate the overall population.