PREPRINT
A2DCB726-A44D-4CB1-BFE4-C8217827AE1E

# 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
arXiv:2207.00466

Submitted on 1 July 2022

## Abstract

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 $\gtrsim {10}^{11}\phantom{\rule{0.167em}{0ex}}{M}_{\odot }$ 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 $\gtrsim {10}^{3}\phantom{\rule{0.167em}{0ex}}{M}_{\odot }$ 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 $\sim {10}^{11}\phantom{\rule{0.167em}{0ex}}{M}_{\odot }$ and dust $\gtrsim {10}^{9}\phantom{\rule{0.167em}{0ex}}{M}_{\odot }$. The stellar content $\lesssim {10}^{11}\phantom{\rule{0.167em}{0ex}}{M}_{\odot }$ 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.

## Preprint

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

Subject: Astrophysics - Astrophysics of Galaxies