A Census of Optically Dark Massive Galaxies in the Early Universe from Magnification by Lensing Galaxy Clusters
Xinwen Shu, Lei Yang, Daizhong Liu, Wei-Hao Wang, Tao Wang, Yunkun Han, Xingxing Huang, Chen-Fatt Lim, Yu-Yen Chang, Wei Zheng, Xianzhong Zheng, Junxian Wang, Xu Kong
Abstract
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.