A systematic search for dormant galaxies at z ∼ 5−7 from the JWST NIRSpec archive

¹ Department of Astronomy, University of Geneva Chemin Pegasi 51 1290 Versoix, Switzerland
² Cosmic Dawn Center (DAWN) Copenhagen, Denmark
³ Niels Bohr Insitute, University of Copenhagen Jagtvej 128 2200 Copenhagen, Denmark
⁴ Aix Marseille Univ, CNRS, CNES, LAM Marseille, France

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

Received: 26 June 2025
Accepted: 27 November 2025

Abstract

JWST has revealed a population of “dormant” galaxies at z > 5 that have recently halted their star formation and are characterized by weak emission lines and significant Balmer breaks. Until now, only four such galaxies have been reported at z > 5, three with low stellar masses, M_* < 10⁹ M_⊙ (so-called mini-quenched galaxies), and one massive quiescent galaxy with M_* = 10^10.2 M_⊙; no such galaxy had been reported at intermediate masses. Here, we present a systematic search for dormant galaxies at 5 < z < 7.4 that halted star formation at least 10 Myr before the time of observation. To do this, we made use of all the publicly available NIRSpec prism data in the DAWN JWST Archive (DJA) and select galaxies with low Hα equivalent widths (EW₀ < 50 Å) and strong Balmer breaks (F_ν, 4200/F_ν, 3500 > 1.4). We find 14 dormant galaxies with stellar masses ranging from 10^7.6 − 10^10.5, revealing an intermediate-mass population. By construction, these 14 sources are located about 1 dex below the star-forming main sequence. Their star formation histories suggest that they halted star formation between 10 and 25 Myr before the time of observation which, according to models, is comparable with the timescales of internally regulated bursts driving a “breathing” mode of star formation. Our results show that ∼1% of the galaxies in the DJA are in a dormant phase of their star formation histories, and they span a wide stellar mass range. These galaxies can be empirically selected using only their spectral features in NIRSpec prism data.