Two Massive, Compact, and Dust-obscured Candidate z ≃ 8 Galaxies Discovered by JWST

We present a search for extremely red, dust-obscured, z > 7 galaxies with JWST/NIRCam+MIRI imaging over the first 20 arcmin² of publicly available Cycle 1 data from the COSMOS-Web, CEERS, and PRIMER surveys. Based on their red color in F277W-F444W (~2.5 mag) and detection in MIRI/F770W (~25 mag), we identify two galaxies, COS-z8M1 and CEERS-z7M1, that have best-fit photometric redshifts of $z={8.4}_{-0.4}^{+0.3}$ and ${7.6}_{-0.1}^{+0.1}$ , respectively. We perform spectral energy distribution fitting with a variety of codes (including BAGPIPES, PROSPECTOR, BEAGLE, and CIGALE) and find a >95% probability that these indeed lie at z > 7. Both sources are compact (R _eff ≲ 200 pc) and highly obscured (A _V ~ 1.5-2.5) and, at our best-fit redshift estimates, likely have strong [O III]+Hβ emission contributing to their 4.4 μm photometry. We estimate stellar masses of ~10¹⁰ M _⊙ for both sources; by virtue of detection in MIRI at 7.7 μm, these measurements are robust to the inclusion of bright emission lines, for example, from an active galactic nucleus. We identify a marginal (2.9σ) Atacama Large Millimeter/submillimeter Array detection at 2 mm within 0.″5 of COS-z8M1, which, if real, would suggest a remarkably high IR luminosity of ~10¹² L _⊙. These two galaxies, if confirmed at z ~ 8, would be extreme in their stellar and dust masses and may be representative of a substantial population of highly dust-obscured galaxies at cosmic dawn.