Draft version July 4, 2022 Typeset using L A T E X twocolumn style in AASTeX631 ALMA resolves the first strongly-lensed Optical/NIR-dark galaxy Marika Giulietti , 1, 2 Andrea Lapi , 1, 3, 4, 5 Marcella Massardi , 5, 6, 1 Meriem Behiri , 1 Martina Torsello , 1 Quirino D’Amato , 1, 2 Tommaso Ronconi , 1, 7, 8 Francesca Perrotta , 1 and Alessandro Bressan 1 1 SISSA, Via Bonomea 265, I-34136 Trieste, Italy 2 INAF - Osservatorio di Astrofisica e Scienza dello Spazio, Via Gobetti 93/3, I-40129, Bologna, Italy 3 IFPU - Institute for fundamental physics of the Universe, Via Beirut 2, 34014 Trieste, Italy 4 INFN-Sezione di Trieste, via Valerio 2, 34127 Trieste, Italy 5 INAF/IRA, Istituto di Radioastronomia, Via Piero Gobetti 101, 40129 Bologna, Italy 6 INAF, Istituto di Radioastronomia - Italian ARC, Via Piero Gobetti 101, I-40129 Bologna, Italy 7 IFPU, Via Beirut 2, I-34014 Trieste, Italy 8 INFN-Sezione di Trieste, via Valerio 2, I-34127 Trieste, Italy ABSTRACT We present high-resolution ( 0.1arcsec) ALMA observations of the strongly-lensed galaxy HATLASJ113526.2-01460 at redshift z ∼ 3.1 discovered in the Gama 12 th field of the Herschel -ATLAS survey. The gravitationally lensed system is remarkably peculiar in that neither the background source nor the foreground lens show a clearly detected optical/NIR emission. We perform accurate lens mod- eling and source morphology reconstruction in three different (sub-)mm continuum bands, and in the C[II] and CO(8-7) spectral lines. The modeling indicates a foreground lensing (likely elliptical) galaxy with mass  10 11 M ⊙ at z  1.5, while the source (sub-)mm continuum and line emissions are am- plified 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 of ongoing merging events. We perform broadband SED-fitting and retrieve the intrinsic de-magnified physical properties of the source, which is found to feature a very high star-formation rate  10 3 M ⊙ yr −1 , that given the compact sizes is on the verge of the Eddington limit for starbursts; the radio luminosity at 6 cm from available EVLA observations is consistent with the star-formation activity. The galaxy is found to be extremely rich in gas ∼ 10 11 M ⊙ and dust  10 9 M ⊙ . The stellar content  10 11 M ⊙ places the source well above the main sequence of starforming galaxies, indicating that the starburst is rather young with estimated age ∼ 10 8 yr. Our results indicate that the overall properties of HATLASJ113526.2-01460 are consistently explained by in-situ galaxy formation and evolution scenarios. Keywords: Strong gravitational lensing (1643), High-redshift galaxies (734), Galaxy formation (595), Submillimeter astronomy (1647) 1. INTRODUCTION Sub-millimetre galaxies (SMGs) are the main protag- onists of the star formation at early cosmic times (Blain 1996, Casey et al. 2014). It is well established, that a substantial contribution at the peak of the cosmic Star Formation Rate (SFR) density comes from these heavily dust-obscured objects, featuring a sub-millimeter (sub- mm) flux density S 870μm  1 mJy and extremely high SFRs, up to ∼ 10 3 M ⊙ yr −1 (e.g. Simpson et al. 2020, Dudzeviˇ ci¯ ut˙ e et al. 2020). Because of their huge dust content these objects are heavily obscured in optical bands and extremely bright in far-infrared (FIR)/sub- mm bands where the light of newborn stars, reprocessed by dust, is re-emitted. Moreover, SMGs have been iden- tified as the progenitors of massive quiescent early-type galaxies and constitute the ideal laboratories to test galaxy evolutionary models. For example, in in-situ co- evolutionary scenarios (Lapi et al. 2014, 2018, Pantoni et al. 2019), the intense star formation activity is accom- panied by an exponential growth of the active nucleus, whose feedback will eventually sweep away the inter- stellar medium. The star formation is thus stopped on a relatively short timescale while the nucleus shines as an optical quasar. arXiv:2207.00466v1 [astro-ph.GA] 1 Jul 2022