2023ApJ...943..151G

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 ≳10¹¹M_⊙ 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, ≳10³M_⊙ 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 ∼10¹¹M_⊙ and dust ≳10⁹M_⊙. The stellar content ≲10¹¹M_⊙ 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.