2025MNRAS.537.3453B

The advent of the JWST has revolutionized our understanding of high-redshift galaxies. In particular, the NIRCam instrument on-board JWST has revealed a population of red galaxies that had largely evaded detection with Hubble Space Telescope (HST), potentially due to significant dust obscuration, quiescence, or extreme redshift. Here, we present the first NIRSpec spectra of 23 red, HST faint or dark galaxies ($\mathrm{H-F444W\gt 1.75}$), unveiling their nature and physical properties. This sample includes both dusty and quiescent galaxies with spectroscopic data from NIRSpec/PRISM, providing accurate spectroscopic redshifts with $\mathrm{\overline{z}_{spec} = 4.1 \pm 0.7}$. The spectral features demonstrate that, while the majority of red galaxies are dusty, a substantial fraction, $\mathrm{13^{+9}_{-6} \%}$, are quiescent. For the dusty galaxies, we have quantified the dust attenuation using the Balmer decrement ($\mathrm{H\,\alpha / H\,\beta }$), finding attenuations $\mathrm{A_{V} \gt 2\ mag}$. We find that red dusty galaxies are $\mathrm{H\,\alpha }$ emitters with equivalent widths spanning the range $\mathrm{ 68 \mathring{\rm A}\lt EW_{H\alpha } \lt 550 \mathring{\rm A}}$, indicative of a wide range of recent star-formation activity. Whether dusty or quiescent, we find that red galaxies are predominantly massive, with 85 per cent of the galaxies in the sample having masses $\mathrm{log({\it M}_{*}/{\rm M}_{\odot }) \gt 9.8}$. This pilot NIRSpec programme reveals the diverse nature of HST-dark galaxies and highlights the effectiveness of NIRSpec/PRISM spectroscopic follow-up in distinguishing between dusty and quiescent galaxies and properly quantifying their physical properties. Upcoming research utilizing higher-resolution NIRSpec data and combining JWST with ALMA observations will enhance our understanding of these enigmatic and challenging sources.