2019ApJ...871...85L

We present CO (J = 1 - 0; 3 - 2; 5 - 4; 10 - 9) and 1.2 kpc resolution [C II] line observations of the dusty star-forming galaxy (SFG) HXMM05-carried out with the Karl G. Jansky Very Large Array, the Combined Array for Research in Millimeter-wave Astronomy, the Plateau de Bure Interferometer, and the Atacama Large Millimeter/submillimeter Array, measuring an unambiguous redshift of z = 2.9850 ± 0.0009. We find that HXMM05 is a hyperluminous infrared galaxy (L_IR = (4 ± 1) x 10¹³ L_☉) with a total molecular gas mass of (2.1 ± 0.7) x 10¹¹(α_CO/0.8) M_☉. The CO (J = 1 - 0) and [C II] emission are extended over ∼9 kpc in diameter, and the CO line FWHM exceeds 1100 km s^–1. The [C II] emission shows a monotonic velocity gradient consistent with a disk, with a maximum rotation velocity of v_c = 616 ± 100 km s^–1 and a dynamical mass of (7.7 ± 3.1) x 10¹¹ M_☉. We find a star formation rate of 2900_–595⁺⁷⁵⁰M_☉ yr^–1. HXMM05 is thus among the most intensely SFGs known at high redshift. Photodissociation region modeling suggests physical conditions similar to nearby SFGs, showing extended star formation, which is consistent with our finding that the gas emission and dust emission are cospatial. Its molecular gas excitation resembles the local major merger Arp 220. The broad CO and [C II] lines and a pair of compact dust nuclei suggest the presence of a late-stage major merger at the center of the extended disk, again reminiscent of Arp 220. The observed gas kinematics and conditions, together with the presence of a companion and the pair of nuclei, suggest that HXMM05 is experiencing multiple mergers as a part of the evolution.