We present Atacama Large Millimeter/Submillimeter Array (ALMA) [CII] and $\sim158$ $\rm\mu m$ continuum observations of REBELS-25, a massive, morphologically complex ultra-luminous infrared galaxy (ULIRG; $L_{ \rm IR}=1.5^{ +0.8}_{ -0.5}\times10^{ 12}$ L$_\odot$) 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 $M_{ *}=8^{ +4}_{ -2}\times10^{ 9}$ M$_\odot$. From dust-continuum and ultraviolet observations, we determine a total obscured + unobscured star formation rate of SFR $=199^{ +101}_{ -63}$ M$_\odot$ yr$^{ -1}$. This is about four times the SFR estimated from an extrapolated main-sequence. We also infer a [CII]-based molecular gas mass of $M_{ \rm H_2}=5.1^{ +5.1}_{ -2.6}\times10^{ 10}$ $M_\odot$, implying a molecular gas depletion time of $ t_{ \rm depl, H_2}=0.3^{ +0.3}_{ -0.2}$ Gyr. We observe a [CII] 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 [CII] emission at large positive velocities ($\sim500$ 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$_\odot$ yr$^{ -1}$, which is consistent with expectations for a star formation-driven outflow. Given its large stellar mass, SFR and molecular gas reservoir $\sim700$ 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 $z\sim4$.
Categories: astro-ph.GA