A Census of Optically Dark Massive Galaxies in the Early Universe from Magnification by Lensing Galaxy Clusters

• Published in: The Astrophysical journal Vol. 926; no. 2; pp. 155 - 169
• Main Authors: Shu, Xinwen, Yang, Lei, Liu, Daizhong, Wang, Wei-Hao, Wang, Tao, Han, Yunkun, Huang, Xingxing, Lim, Chen-Fatt, Chang, Yu-Yen, Zheng, Wei, Zheng, XianZhong, Wang, Junxian, Kong, Xu
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.02.2022; IOP Publishing
• Subjects: Astrophysics; Confidence intervals; Cosmic dust; Density; Dust; Dust continuum emission; Dust emission; Galactic clusters; Galaxies; Gravitational lenses; High-redshift galaxies; Massive stars; Median (statistics); Red shift; Spectroscopic observation; Star & galaxy formation; Star formation; Star formation rate; Stars & galaxies; Stellar mass; Universe
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ac3de5

Abstract We present ALMA 870 μ m and JCMT/SCUBA2 850 μ m dust continuum observations of a sample of optically dark and strongly lensed galaxies in cluster fields. The ALMA and SCUBA2 observations reach a median rms of ∼0.11 mJy and 0.44 mJy, respectively, with the latter close to the confusion limit of the data at 850 μ m. This represents one of the most sensitive searches for dust emission in optically dark galaxies. We detect the dust emission in 12 out of 15 galaxies at >3.8 σ , corresponding to a detection rate of 80%. Thanks to the gravitational lensing, we reach a deeper limiting flux than previous surveys in blank fields by a factor of ∼3. We estimate delensed infrared luminosities in the range 2.9 × 10 11 –4.9 × 10 12 L ⊙ , which correspond to dust-obscured star formation rates of ∼30–520 M ⊙ yr −1 . Stellar population fits to the optical-to-NIR photometric data yield a median redshift z = 4.26 and delensed stellar mass 6.0 × 10 10 M ⊙ . They contribute a lensing-corrected star formation rate density at least an order of magnitude higher than that of equivalently massive UV-selected galaxies at z > 3. The results suggest that there is a missing population of massive star-forming galaxies in the early Universe, which may dominate the SFR density at the massive end ( M ⋆ > 10 10.3 M ⊙ ). Five optically dark galaxies are located within r < 50″ in one cluster field, representing a potential overdensity structure that has a physical origin at a confidence level >99.974% from Poisson statistics. Follow-up spectroscopic observations with ALMA and/or JWST are crucial to confirm whether it is associated with a protocluster at similar redshifts.