The Atacama Cosmology Telescope: CO(J = 3 - 2) mapping and lens modeling of an ACT-selected dusty star-forming galaxy

We report Northern Extended Millimeter Array (NOEMA) 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 $\mu \approx 7-22$ with a luminosity-weighted mean $<\mu >\approx 13$. 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 $L^{\prime}{\rm CO(3-2)}= (5.53\pm 0.69) \times 10^{10},{\rm ,K,km,s^{-1},pc^{2}}$,acoldgasmass$M{{\rm gas}}= (3.86 \pm 0.33) \times 10^{10},M_{\odot}$,adynamicalmass$M_{\rm dyn},{\rm sin}^2,i = 3.9^{+1.8}{-1.5} \times 10^{10},M{\odot}$, and a gas mass fraction $f_{\mathrm{g}\mathrm{a}\mathrm{s}}{\mathrm{c}\mathrm{s}\mathrm{c}}^{2}i=1.0_{-0.4}^{+0.8}$. The line brightness temperature ratio of $r_{3,1}\approx 1.6$ 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.