2018ApJ...864L..11X

The brightest 500 µm source in the XMM-LSS field, HXMM01, is a rare merger of luminous starburst galaxies at z = 2.3 with a dust-obscured star formation rate of 2000 M_☉ yr^–1. Here we present high-resolution spectroscopic observations of HXMM01 with the Atacama Large Millimeter/submillimeter Array (ALMA). We detect line emission from CO J = 7 - 6, [C I] ³P₂ - ³P₁, and p-H₂O 2₁₁ - 2₀₂ and continuum emission at 230 GHz. At a spatial resolution of 0.''2 and a spectral resolution of 40 km s^–1, the source is resolved into three distinct components, which are spatially and dynamically associated within a projected radius of 20 kpc and a radial velocity range of 2000 km s^–1. For two major components, our Bayesian-based tilted-ring modeling of the ALMA spectral cubes shows almost flat rotation curves peaking at ∼500 km s^–1 at galactocentric distances between 2 and 5 kpc. Each of them has a dynamical mass of ∼10¹¹ M_☉. The combination of the dynamical masses and the archival CO J = 1 - 0 data places strong upper limits on the CO - H₂ conversion factor of α_CO <= 1.4-2.0 M_☉(K km s^–1 pc²)^–1. These limits are significantly below the Galactic inner disk α_CO value of 4.3 M_☉(K km s^–1 pc²)^–1 but are consistent with those of local starbursts. Therefore, the previously estimated short gas depletion timescale of ∼200 Myr remains unchanged.