A systematic search for dormant galaxies at z~5-7 from the JWST NIRSpec archive

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, $M_*<10^9M_\odot$ (so-called mini-quenched galaxies), and one massive quiescent galaxy with $M_*=10^{10.2}M_\odot$; 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 selected galaxies with low H$α$ equivalent widths ($EW_{0}<50$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 $\sim1%$ 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.

💡 Research Summary

The authors present a systematic search for “dormant” galaxies at redshifts 5 < z < 7.4 using the publicly available JWST NIRSpec prism spectra compiled in the DAWN JWST Archive (DJA). After applying strict quality cuts—requiring robust spectroscopic redshifts (grade 3), a clear Lyα break, and the exclusion of broad‑line AGN—they assemble a parent sample of 1,598 high‑z galaxies. For each object they perform a simultaneous fit of the NIRSpec spectrum and available NIRCam/HST photometry with the Bagpipes SED‑fitting code, employing BPASS stellar population models, a non‑parametric continuity prior for the star‑formation history (SFH), and a flexible dust attenuation law (Salim et al. 2018). The redshift is fixed to the spectroscopic value, while stellar mass, metallicity, visual attenuation (A_V), and the detailed SFH are left as free parameters.

To identify galaxies that have recently quenched their star formation, the authors adopt two physically motivated spectral diagnostics. First, the rest‑frame Hα equivalent width (EW₀) is measured by fitting a three‑Gaussian model (Hα plus the