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 1011Me 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, 103Me 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 ∼1011Me and dust 109Me. The
stellar content 1011Me 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.
