2019ApJ...879...95R

We report Northern Extended Millimeter Array CO(J = 3 - 2) observations of the dusty star-forming galaxy ACT-S J020941+001557 at z = 2.5528, which was detected as an unresolved source in the Atacama Cosmology Telescope (ACT) equatorial survey. Our spatially resolved spectral line data support the derivation of a gravitational lens model from 37 independent velocity channel maps using a pixel-based algorithm, from which we infer a velocity-dependent magnification factor µ ≃ 7-22 with a luminosity-weighted mean <µ>~13. The resulting source-plane reconstruction is consistent with a rotating disk, although other scenarios cannot be ruled out by our data. After correction for lensing, we derive a line luminosity L_CO(3–2)^′^=(5.53±0.69)×10¹⁰Kkms^–1pc², a cold gas mass M_gas=(3.86±0.33)×10¹⁰M_☉, a dynamical mass M_dynsin²i=3.9_–1.5^+1.8×10¹⁰M_☉, and a gas mass fraction f_gascsc²i=1.0_–0.4^+0.8. The line brightness temperature ratio of r_3,1 ≃ 1.6 relative to a Green Bank Telescope CO(J = 1 - 0) detection may be elevated by a combination of external heating of molecular clouds, differential lensing, and/or pointing errors.