Quiescent or dusty? Unveiling the nature of extremely red galaxies at $z>3$

Abstract

The advent of the JWST has revolutionised our understanding of high-redshift galaxies. In particular, the NIRCam instrument on-board JWST has revealed a population of Hubble Space Telescope (HST)-dark galaxies that had previously evaded optical detection, potentially due to significant dust obscuration, quiescence, or simply extreme redshift. Here, we present the first NIRSpec spectra of 23 HST-dark galaxies ($\mathrm{H}-F444W>1.75$), 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 ${\overline{\mathrm{z}}}_{\mathrm{spec}}=4.1\pm 0.7$. The spectral features demonstrate that, while the majority of HST-dark galaxies are dusty, a substantial fraction, $13_{-6}^{+9}\%$, are quiescent. For the dusty galaxies, we have quantified the dust attenuation using the Balmer decrement ($\mathrm{H}\alpha /H\beta$), finding attenuations ${\mathrm{A}}_{\mathrm{V}}>2\mathrm{mag}$. We find that HST-dark dusty galaxies are $\mathrm{H}\alpha$ emitters with equivalent widths spanning the range $68A<\mathrm{E}{\mathrm{W}}_{\mathrm{H}\alpha }<550A$, indicative of a wide range of recent star-formation activity. Whether dusty or quiescent, we find that HST-dark galaxies are predominantly massive, with 85\% of the galaxies in the sample having masses $\log ({\mathrm{M}}_{\ast }/{\mathrm{M}}_{\odot })>9.8$. This pilot NIRSpec program 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 utilising higher-resolution NIRSpec data and combining JWST with ALMA observations will enhance our understanding of these enigmatic and challenging sources.

13 pages, 11 figures

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