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

M. Giulietti, A. Lapi, M. Massardi, M. Behiri, M. Torsello, Q. D'Amato, T. Ronconi, F. Perrotta, A. Bressan

Submitted on 1 July 2022


We present high-resolution (0.1arcsec) ALMA observations of the strongly-lensed galaxy HATLASJ113526.2-01460 at redshift z3.1 discovered in the Gama 12th 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 1011M at z1.5, while the source (sub-)mm continuum and line emissions are amplified by factors μ613. 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 103M yr1, 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 1011M and dust 109M. The stellar content 1011M places the source well above the main sequence of starforming galaxies, indicating that the starburst is rather young with 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.


Comment: 17 Pages, 12 Figures, 7 Tables. Submitted to ApJ

Subject: Astrophysics - Astrophysics of Galaxies