The galaxies missed by Hubble and ALMA: the contribution of extremely red galaxies to the cosmic census at 3<z<8

TL;DR

This study leverages JWST's deep NIRCam and MIRI observations, in concert with HST and ALMA data, to identify and characterize extremely red, optically faint galaxies at $z>3$ that were largely missed in previous census efforts. Through joint photometry, forced measurements across mid-IR and submillimeter wavelengths, and SED fitting with EAZY and Prospector, the authors show that these galaxies can harbor dusty or post-starburst-like stellar populations down to $M_\ast \sim 10^8\,M_\odot$ and can substantially alter the inferred cosmic budget of star formation and stellar mass density, especially at $4<z<6$. The inclusion of long-wavelength data reduces extreme mass ($\sim0.6$ dex) and SFR (up to a factor of $\sim10$) estimates from NIRCam alone, underscoring the risk of mischaracterizing high-$z$ populations without JWST's full SED coverage. The work also highlights a population of dust-reddened AGN candidates (LRDs) and discusses five sources with Balmer breaks at $5.5<z<7.7$, emphasizing the need for spectroscopy to confirm their nature and refine their role in early black hole and galaxy growth. Overall, the paper demonstrates that JWST reveals a substantial, previously hidden component of the high-$z$ galaxy population, potentially doubling the obscured part of the star-formation-rate density in certain epochs and reshaping our understanding of early galaxy evolution.

Abstract

Using deep JWST imaging from JADES, JEMS and SMILES, we characterize optically-faint and extremely red galaxies at $z>3$ that were previously missing from galaxy census estimates. The data indicate the existence of abundant, dusty and post-starburst-like galaxies down to $10^8$M$_\odot$, below the sensitivity limit of Spitzer and ALMA. Modeling the NIRCam and HST photometry of these red sources can result in extreme, high values for both stellar mass and star formation rate (SFR); however, including 7 MIRI filters out to 21$μ$m results in decreased mass (median 0.6 dex for log$_{10}$M$^*$/M$_{\odot}>$10), and SFR (median 10$\times$ for SFR$>$100 M$_{\odot}$/yr). At $z>6$, our sample includes a high fraction of little red dots (LRDs; NIRCam-selected dust-reddened AGN candidates). We significantly measure older stellar populations in the LRDs out to rest-frame 3$μ$m (the stellar bump) and rule out a dominant contribution from hot dust emission, a signature of AGN contamination to stellar population measurements. This allows us to measure their contribution to the cosmic census at $z>3$, below the typical detection limits of ALMA ($L_{\rm IR}<10^{12}L_\odot$). We find that these sources, which are overwhelmingly missed by HST and ALMA, could effectively double the obscured fraction of the star formation rate density at $4<z<6$ compared to some estimates, showing that prior to JWST, the obscured contribution from fainter sources could be underestimated. Finally, we identify five sources with evidence for Balmer breaks and high stellar masses at $5.5<z<7.7$. While spectroscopy is required to determine their nature, we discuss possible measurement systematics to explore with future data.

Figures (2)

• Figure 1: F150W--F444W color vs F444W magnitude illustrating our selection from the JADES catalog with S/N $>$ 20 in F444W (black points). We identify very red sources by their F150W--F444W$> 2.2$ color (all colored points) color coded by their EAZY photometric redshift. We further identify the optically fainter sample whose colors and magnitudes are consistent with $z>3$ massive galaxies that were absent from earlier HST selected samples (i.e. have F150W $<$ 27 ABmag: the black diagonal line). In this work, we focus on the optically faint sources behind the black line marked as colored squares. Existing samples of optically faint objects (maroon open circles) are collected from the literature using pre-JWST datasets Wang2016Wang2019Franco2018AP2019Xiao2023. We note the detection limit of the Spitzer GREATS program (dotted line) indicating newly discovered sources by JWST.
• Figure 2: For sources with MIRI and ALMA coverage, we compare inferred photometric redshift, stellar mass and SFR from our prospector modeling of HST+NIRCam+MIRI+ALMA vs just the HST+NIRCam photometry. Left panel shows the inferred photometric redshift (sources with spectroscopic redshifts are excluded from this panel). Middle panel shows the stellar mass, and right panel shows the SFR. At the very high mass and SFR end, we find that including the MIRI+ALMA data lowers the extreme masses and SFRs that are otherwise inferred (median decrease of 0.6 dex for sources above $\rm Log_{10} \rm M^*/\rm{M}_\odot$$>$10, and median 10$\times$ less for SFR $>$ 100 M$_{\odot}$/yr). This figure shows that stellar masses and SFRs can be wrong for extreme and red sources using NIRCam data without longer wavelength constraints.