Unveiling the Nature of Infrared Bright, Optically Dark Galaxies with Early JWST Data


Over the last few years, both ALMA and Spitzer/IRAC observations have revealed a population of likely massive galaxies at $z>3$ that was too faint to be detected in HST rest-frame ultraviolet imaging. However, due to the very limited photometry for individual galaxies, the true nature of these so-called HST-dark galaxies has remained elusive. Here, we present the first sample of such galaxies observed with very deep, high-resolution NIRCam imaging from the Early Release Science Program CEERS. 33 HST- dark sources are selected based on their red colours across 1.6 $μ$m to 4.4 $μ$m. Their physical properties are derived from 12-band multi-wavelength photometry, including ancillary HST imaging. We find that these galaxies are generally heavily dust- obscured ($A_V∼2$ mag), massive ($łog (M/M_ødot) ∼10$), star-forming sources at $z∼2-8$ with an observed surface density of $∼0.8$ arcmin$^-2$. This suggests that an important fraction of massive galaxies may have been missing from our cosmic census at $z>3$ all the way into the reionization epoch. The HST-dark sources lie on the main- sequence of galaxies and add an obscured star formation rate density (SFRD) of $mathrm1.3^+1.6_-1.0 times 10^-3 M_ødot/yr/Mpc^3$ at $z∼6$, similar to previous estimates. Our analysis shows the unique power of JWST to reveal this previously missing galaxy population and to provide a complete census of galaxies at $z=2-8$ based on rest-frame optical imaging.

arXiv e-prints