Serendipitous Discovery of an Optically-Dark Ultra-Luminous Infrared Galaxy at z = 3.4

Dusty, submillimeter-selected galaxies without optical counterparts contribute a non-negligible fraction of the star formation in the early universe. However, such a population is difficult to detect through classical optical/UV-based surveys. We report the serendipitous discovery of such an optically dark galaxy, behind the quadruply-lensed z = 2.56 quasar, H1413+117, offset to the north by 6”. From 12CO J = 4–3, J = 6–5, and part of the J = 13–12 transitions, which all spatially coincide with a compact submillimeter continuum emission, we determine an unambiguous spectroscopic redshift, z = 3.386 ± 0.005. This galaxy has a molecular mass Mmol ∼ 1011 M⊙ and a black hole mass MBH ∼ 108 M⊙, estimated from 12CO J = 4–3 and archival Chandra X-ray data (L2-10, keV ∼ 4 × 1044 erg s−1), respectively. We also estimate a total infrared luminosity of LFIR = (2.8 ± 2.3) × 1012 L⊙ and a stellar mass of M* ≲ 1011 M⊙, from spectral energy distribution fitting. According to these simple mass estimations, this gas-rich and X-ray bright galaxy might be in a transition phase from starburst to quasar offering a unique case for studying galaxy-black hole co-evolution under extremely dusty conditions.