ALMA Resolves the First Strongly Lensed Optical/Near-IR-dark Galaxy

Abstract

Abstract We present high-resolution (≲0.″1) Atacama Large Millimeter/submillimeter Array observations of the strongly lensed galaxy HATLASJ113526.2-01460 at redshift z ∼ 3.1, discovered in the GAMA 12th field of the Herschel-ATLAS survey. This gravitationally lensed system is remarkably peculiar, in that neither the background source nor the foreground lens show a clearly detected optical/near-IR Hubble Space Telescope-J band emission. We perform accurate lens modeling and source morphology reconstruction in three different (sub)millimeter continuum bands and in the C[ii] and CO(8−7) spectral lines. The modeling indicates a foreground lensing (likely elliptical) galaxy with mass ≳1011 M ⊙ at z ≳ 1.5, while the source (sub)millimeter continuum and line emissions are amplified by factors μ ∼ 6–13. We estimate extremely compact sizes—≲0.5 kpc for the star-forming region and ≲1 kpc for the gas component—with no clear evidence of rotation or ongoing merging events. We perform broadband SED fitting and retrieve the intrinsic demagnified physical properties of the source, which is found to feature a very high star formation rate, ≳103 M ⊙ yr−1, which, given the compact sizes, is on the verge of the Eddington limit for starbursts; the radio luminosity at 6 cm from the available EVLA observations is consistent with star formation activity. The galaxy is found to be extremely rich in gas ∼1011 M ⊙ and dust ≳109 M ⊙. The stellar content ≲1011 M ⊙ places the source well above the main sequence of star-forming galaxies, indicating that the starburst is rather young, with an estimated age ∼108 yr. Our results indicate that the overall properties of HATLASJ113526.2-01460 are consistently explained by in situ galaxy formation and evolution scenarios.