Two massive, compact, and dust-obscured candidate $z\sim 8$ galaxies discovered by JWST

Abstract

We present a search for extremely red, dust-obscured, $z>7$ galaxies with $\text{JWST}$/NIRCam+MIRI imaging over the first 20 arcmin${}^2$ of publicly-available Cycle 1 data from the COSMOS-Web, CEERS, and PRIMER surveys. Based on their red color in F277W$-$F444W ($\sim 2.5$ mag) and detection in MIRI/F770W ($\sim 25$ mag), we identify two galaxies$\unicode{x2014}$COS-z8M1 and CEERS-z7M1$\unicode{x2014}$which have best-fit photometric redshifts of $z=8.5_{-0.4}^{+0.3}$ and $z=7.6_{-0.1}^{+0.1}$, respectively. We perform SED 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 ($R_{\mathrm{eff}}\lesssim 200$ pc), highly obscured ($A_V\sim 1.5$$\unicode{x2013}$$2.5$), and, at our best-fit redshift estimates, likely have strong [OIII]+H$\beta$ emission contributing to their $4.4\mu$m photometry. We estimate stellar masses of $\sim 10^{10}{M}_{\odot }$ for both sources; by virtue of detection in MIRI at $7.7\mu$m, these measurements are robust to the inclusion of bright emission lines, for example, from an AGN. We identify a marginal (2.9$\sigma$) ALMA detection at 2 mm within $0.5^{\prime \prime }$ of COS-z8M1, which if real, would suggest a remarkably high IR luminosity of $\sim 10^{12}{L}_{\odot }$. These two galaxies, if confirmed at $z\sim 8$, would be extreme in their stellar and dust masses, and may be representative of a substantial population of modestly dust-obscured galaxies at cosmic dawn.

23 pages, 8 figures, submitted to ApJL, comments welcome