CO, H₂O, H₂O⁺ line and dust emission in a z = 3.63 strongly lensed starburst merger at sub-kiloparsec scales

Abstract

Using the Atacama Large Millimeter/submillimeter Array (ALMA), we report high angular-resolution observations of the redshift z = 3.63 galaxy H-ATLAS J083051.0+013224 (G09v1.97), one of the most luminous strongly lensed galaxies discovered by the Herschel-Astrophysical Terahertz Large Area Survey (H-ATLAS). We present 0.″2-0.″4 resolution images of the rest-frame 188 and 419 μm dust continuum and the CO(6-5), H₂O(2₁₁-2₀₂), and J_up = 2 H₂O⁺ line emission. We also report the detection of H₂O(2₁₁-2₀₂) in this source. The dust continuum and molecular gas emission are resolved into a nearly complete ∼1.^″5 diameter Einstein ring plus a weaker image in the center, which is caused by a special dual deflector lensing configuration. The observed line profiles of the CO(6-5), H₂O(2₁₁-2₀₂), and J_up = 2 H₂O⁺ lines are strikingly similar. In the source plane, we reconstruct the dust continuum images and the spectral cubes of the CO, H₂O, and H₂O⁺ line emission at sub-kiloparsec scales. The reconstructed dust emission in the source plane is dominated by a compact disk with an effective radius of 0.7 ± 0.1 kpc plus an overlapping extended disk with a radius twice as large. While the average magnification for the dust continuum is μ ∼ 10-11, the magnification of the line emission varies from 5 to 22 across different velocity components. The line emission of CO(6-5), H₂O(2₁₁-2₀₂), and H₂O⁺ have similar spatial and kinematic distributions. The molecular gas and dust content reveal that G09v1.97 is a gas-rich major merger in its pre-coalescence phase, with a total molecular gas mass of ∼10¹¹ M_⊙. Both of the merging companions are intrinsically ultra-luminous infrared galaxies (ULIRGs) with infrared luminosities L_IR reaching ≳4 × 10¹² L_⊙, and the total L_IR of G09v1.97 is (1.4 ± 0.7)×10¹³ L_⊙. The approaching southern galaxy (dominating from V = -400 to -150 km s^-1 relative to the systemic velocity) shows no obvious kinematic structure with a semi-major half-light radius of a_s = 0.4 kpc, while the receding galaxy (0 to 350 km s^-1) resembles an a_s = 1.2 kpc rotating disk. The two galaxies are separated by a projected distance of 1.3 kpc, bridged by weak line emission (-150 to 0 km s^-1) that is co-spatially located with the cold dust emission peak, suggesting a large amount of cold interstellar medium (ISM) in the interacting region. As one of the most luminous star-forming dusty high-redshift galaxies, G09v1.97 is an exceptional source for understanding the ISM in gas-rich starbursting major merging systems at high redshift.

Reduced images and datacubes are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/624/A138