ALMA resolves the first strongly-lensed Optical/NIR-dark galaxy

We present high-resolution ($\lesssim 0.1$arcsec) ALMA observations of the strongly-lensed galaxy HATLASJ113526.2-01460 at redshift $z\sim 3.1$ discovered in the Gama 12${}^{\mathrm{t}\mathrm{h}}$ 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 modeling 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 $\gtrsim {10}^{11}{M}_{\odot }$ at $z\gtrsim 1.5$, while the source (sub-)mm continuum and line emissions are amplified by factors $\mu \sim 6-13$. We estimate extremely compact sizes $\lesssim 0.5$ kpc for the star-forming region and $\lesssim 1$ 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 $\gtrsim {10}^3{M}_{\odot }$ 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 $\sim {10}^{11}{M}_{\odot }$ and dust $\gtrsim {10}^9{M}_{\odot }$. The stellar content $\lesssim {10}^{11}{M}_{\odot }$ places the source well above the main sequence of starforming galaxies, indicating that the starburst is rather young with estimated age $\sim {10}^8$ yr. Our results indicate that the overall properties of HATLASJ113526.2-01460 are consistently explained by in-situ galaxy formation and evolution scenarios.