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

Abstract

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\arcsec. From ${}^{12}$CO $J=4$--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, $z=3.386\pm 0.005$. This galaxy has a molecular mass $M_{\mathrm{mol}}\sim 10^{11}$ M${}_{\odot }$ and a black hole mass $M_{\mathrm{BH}}\sim 10^8$ M${}_{\odot }$, estimated from ${}^{12}$CO $J=4$--3 and archival {\it Chandra} X-ray data ($L_{2-10,\mathrm{keV}}\sim 4\times 10^{44}$\,erg\,s${}^{-1}$), respectively. We also estimate a total infrared luminosity of $L_{\mathrm{FIR}}=(2.8\pm 2.3)\times 10^{12}$ L${}_{\odot }$ and a stellar mass of $M_{\ast }\lesssim 10^{11}$ M${}_{\odot }$, 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.

Accepted to MNRAS. 9 pages, 3 figures