Normal, dust-obscured galaxies in the epoch of reionization

Two serendipitously detected dust-obscured galaxies are reported at z = 6.7 and 7.4, with estimates that such galaxies provide an additional 10-25% contribution to the total star formation rate density at z > 6. Over the past decades, rest-frame ultraviolet (UV) observations have provided large samples of UV luminous galaxies at redshift (z) greater than 6 (refs. (1-3)), during the so-called epoch of reionization. While a few of these UV-identified galaxies revealed substantial dust reservoirs(4-7), very heavily dust-obscured sources at these early times have remained elusive. They are limited to a rare population of extreme starburst galaxies(8-12) and companions of rare quasars(13,14). 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 (refs. (15,16)). 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 +/- 0.0005 and z = 7.3521 +/- 0.0005. These objects are not detected in existing rest-frame UV data and were discovered only through their far-infrared [C ii] 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 approximate to 7. This population of heavily dust-obscured galaxies appears to contribute 10-25% to the z > 6 cosmic star formation rate density.