The ALMA REBELS Survey: discovery of a massive, highly star-forming, and morphologically complex ULIRG at z = 7.31

We present Atacama Large Millimeter/Submillimeter Array (ALMA) [C II] and ~158 μm continuum observations of REBELS-25, a massive, morphologically complex ultra-luminous infrared galaxy (ULIRG; LIR = 1.5+0.8−0.5×1012 L⊙) at z = 7.31, spectroscopically confirmed by the Reionization Era Bright Emission Line Survey (REBELS) ALMA Large Programme. REBELS-25 has a significant stellar mass of M∗=8+4−2×109 M⊙ . From dust-continuum and ultraviolet observations, we determine a total obscured + unobscured star formation rate of SFR =199+101−63 M⊙ yr−1 . This is about four times the SFR estimated from an extrapolated main sequence. We also infer a [C II]-based molecular gas mass of MH2=5.1+5.1−2.6×1010 M⊙ , implying a molecular gas depletion time of tdepl,H2=0.3+0.3−0.2 Gyr. We observe a [C II] velocity gradient consistent with disc rotation, but given the current resolution we cannot rule out a more complex velocity structure such as a merger. The spectrum exhibits excess [C II] emission at large positive velocities (~500 km s-1), which we interpret as either a merging companion or an outflow. In the outflow scenario, we derive a lower limit of the mass outflow rate of 200 M⊙ yr−1 , which is consistent with expectations for a star-formation-driven outflow. Given its large stellar mass, SFR, and molecular gas reservoir ~700 Myr after the big bang, we explore the future evolution of REBELS-25. Considering a simple, conservative model assuming an exponentially declining star formation history, constant star formation efficiency, and no additional gas inflow, we find that REBELS-25 has the potential to evolve into a galaxy consistent with the properties of high-mass quiescent galaxies recently observed at z ~ 4.