Over the past decades, rest-frame ultraviolet (UV) observations have provided large samples of UV luminous galaxies at redshift (z) greater than 6 (refs. ^1,2,3), during the so-called epoch of reionization. While a few of these UV-identified galaxies revealed substantial dust reservoirs^4,5,6,7, very heavily dust-obscured sources at these early times have remained elusive. They are limited to a rare population of extreme starburst galaxies^8,9,10,11,12 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 ≈ 7. This population of heavily dust-obscured galaxies appears to contribute 10–25% to the z > 6 cosmic star formation rate density.

在过去的几十年中，静止框架紫外 (UV) 观测提供了在所谓的再电离时期红移 ( z ) 大于 6（参考文献^1,2,3）的紫外发光星系的大量样本。虽然这些紫外线识别的星系中的一些揭示了大量的尘埃储存^4,5,6,7，但在这些早期非常严重的尘埃遮蔽的来源仍然难以捉摸。它们仅限于极少数的极端星暴星系^8,9,10,11,12和稀有类星体的伴星^13,14。这些研究得出结论，尘埃遮蔽的星系对z处的宇宙恒星形成率密度的贡献 > 6 次占优势。最近的 ALMA 和斯皮策观测已经确定了在z  = 3−6 处更丰富、不那么极端的模糊星系群（参考文献^15,16）。然而，到目前为止，这一群体尚未在再电离时期得到证实。在这里，我们报告在z  = 6.6813 ± 0.0005 和z  = 7.3521 ± 0.0005 处发现了两个被尘埃遮挡的恒星形成星系。这些物体在现有的静止帧 UV 数据中没有被检测到，仅通过它们的远红外 [C  ii] 线和尘埃连续辐射作为典型的紫外发光星系的伴星在相同的红移处。这两个星系表现出比极端星暴更低的红外光度和恒星形成率，这与z ≈ 7 的典型恒星形成星系一致。这群被严重尘埃遮挡的星系似乎对z  > 6 的宇宙 贡献了 10-25%恒星形成率密度。