Flat Rotation Curves Found in Merging Dusty Starbursts at z = 2.3 through Tilted-ring Modeling

Abstract

The brightest 500 μ m 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 ] 3 P 2  →  3 P 1 , and p-H 2 O 2 11  → 2 02 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 ∼10 11 M ⊙ . The combination of the dynamical masses and the archival CO J  = 1 → 0 data places strong upper limits on the CO → H 2 conversion factor of α CO  ≲ 1.4–2.0 M ⊙ (K km s −1 pc 2 ) −1 . These limits are significantly below the Galactic inner disk α CO value of 4.3 M ⊙ (K km s −1 pc 2 ) −1 but are consistent with those of local starbursts. Therefore, the previously estimated short gas depletion timescale of ∼200 Myr remains unchanged.