Normal, Dust-Obscured Galaxies in the Epoch of Reionization

Y. Fudamoto, P. A. Oesch, S. Schouws, M. Stefanon, R. Smit, R. J. Bouwens, R. A. A. Bowler, R. Endsley, V. Gonzalez, H. Inami, I. Labbe, D. Stark, M. Aravena, L. Barrufet, E. da Cunha, P. Dayal, A. Ferrara, L. Graziani, J. Hodge, A. Hutter, Y. Li, I. De Looze, T. Nanayakkara, A. Pallottini, D. Riechers, R. Schneider, G. Ucci, P. van der Werf, C. White

Published 2021-09-21Version 1

Over the past decades, rest-frame ultraviolet (UV) observations have provided large samples of UV luminous galaxies at redshift (z) greater than 6, during the so-called epoch of reionization. While a few of these UV identified galaxies revealed significant dust reservoirs, very heavily dust-obscured sources at these early times have remained elusive. They are limited to a rare population of extreme starburst galaxies, and companions of rare quasars. These studies conclude that the contribution of dust-obscured galaxies to the cosmic star formation rate density at $z>6$ is sub-dominant. Recent ALMA and Spitzer observations have identified a more abundant, less extreme population of obscured galaxies at $z=3-6$. However, this population has not been confirmed in the reionization epoch so far. Here, we report the discovery of two dust-obscured star forming galaxies at $z=6.6813\pm0.0005$ and $z=7.3521\pm0.0005$. These objects are not detected in existing rest-frame UV data, and were only discovered through their far-infrared [CII] lines and dust continuum emission as companions to typical UV-luminous galaxies at the same redshift. The two galaxies exhibit lower infrared luminosities and star-formation rates than extreme starbursts, in line with typical star-forming galaxies at $z\sim7$. This population of heavily dust-obscured galaxies appears to contribute 10-25 per cent to the $z>6$ cosmic star formation rate density.