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

TL;DR

This study reports the serendipitous discovery of an optically dark, ultra-luminous infrared galaxy at $z=3.386$ located $6''$ north of the Cloverleaf quasar. Three CO transitions detected with ALMA (J=4--3, 6--5, and 12--11 tracers) and complementary continuum data reveal a massive, gas-rich system with $M_{\rm mol}$ of order $10^{11}\,M_\odot$ and a luminous, obscured AGN with $M_{\rm BH}\sim10^{8}\,M_\odot$, along with $L_{\rm FIR}\sim(2.8\pm2.3)\times10^{12}\,L_\odot$. The double-peaked CO profiles suggest an ongoing gas-rich merger, placing the source in a possible transitional phase between starburst and quasar, with implications for galaxy–black hole co-evolution under extreme dust obscuration. The multi-wavelength detections (Spitzer, Chandra) alongside significant optical/NIR darkness underscore the importance of millimeter/submillimeter surveys for unveiling the obscured high-$z$ population and refining evolutionary scenarios for massive galaxies.

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

Figures (3)

• Figure 1: CO Spectra, submm continuum, and CO moment-0 maps of the newly discovered source at $6"$ north-east to the Cloverleaf QSO. Top: CO spectra $J$ = 4--3, 6--5, and 13--12 from the detected source at $\sim$40--45 km s$^{-1}$ resolution. We label the line emission velocity range in yellow. The vertical line represents $v_{\rm sys}$ = 0. The CO $J$=13-12 spectrum covers only part of the velocity range at the edge of the spectral window. Bottom: Moment-0 map of the CO $J$=4--3 line detected in ALMA Band 3, shown with grayscales. The green contours represent the Band 3 continuum data at levels 2, 3 and 4 $\sigma$. The red and cyan contours represent Band 4 and 7 continuum data, respectively, at 5, 10, and 20 $\sigma$ levels. The beam size for each data set is shown with its corresponding colour (272 $\times$ 156, 085 $\times$ 078, and 030 $\times$ 018, from top to bottom, respectively).
• Figure 2: NIR (Left and Middle) and X-ray (Right) snapshots of the new source. The red contours show the ALMA 850 $\mu$m continuum emission, at 5, 10, and 20 $\sigma$. The residual image from Spitzer 4.5 $\mu$m shows marginal detection, while the HST 1.6 $\mu$m band shows nondetection. The new source is detected in the archival Chandra X-ray data (integrated within in 0.8 -- 10 keV), which originally targeted the Cloverleaf.
• Figure 3: Photometry of the new source. The measurements are shown in red circles and blue triangles. (Left:) Blue dashed, light green dotted, orange, cyan, and brown curves represent the SED templates of high-redshift radio galaxy, DKB07 Dannerbauer2014, AGN torus model constructed from Compton-thick AGNs polletta2006, hot dust-obscured galaxies tsai2015, a Type-1 AGN richards2006, and an elliptical galaxy assef2010, respectively. The grey shaded region indicates the 1-$\sigma$ region for a large sample of SMGs from SCUBA-2 ivison2019Duzeviciute2020. None of the templates are compatible with the SED of the new source. (Right:) The best fit model from the SED fitting code X-CIGALE CigaleCigale2Xcigale. The best-fitting model well explains the upper limits in the optical wavebands (black solid curve). The dashed lines show the corresponding SED components.