We present Atacama Large Millimeter/Submillimeter Array (ALMA) [C ii] and ∼158 continuum observations of REBELS-25, a massive, morphologically complex ultra-luminous infrared galaxy (ULIRG; LIR = 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. From dust-continuum and ultraviolet observations, we determine a total obscured + unobscured star formation rate of SFR. This is about four times the SFR estimated from an extrapolated main sequence. We also infer a [C ii]-based molecular gas mass of, implying a molecular gas depletion time of 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, 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.

All of the galaxies in the sample have imaging at ultraviolet, optical, near-infrared, and mid-infrared wavelengths, taken from GALEX, the Swift UV/Optical Monitor Telescope, the Sloan Digital Sky Survey III (SDSS-III), the Two Micron All Sky Survey (2MASS), the Spitzer Space Telescope, and/or the Wide-field Infrared Space Explorer (WISE).

Radio-continuum flux densities at 1.4GHz and 150MHz were determined using multiple sets of archival data. Our principal source of 1.4GHz flux densities is the NRAO VLA Sky Survey (NVSS, Condon+, 1998, VIII/65). Our principal source of 150MHz flux densities is the TIFR GMRT Sky Survey (TGSS), which has an angular resolution of ~25". We used the first alternative data release of the TGSS (TGSS ADR1; Intema+ 2017, J/A+A/598/A78).

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We present calibrations for star formation rate (SFR) indicators in the ultraviolet, mid-infrared, and radiocontinuumbands, including one of the first direct calibrations of 150 MHz as an SFR indicator. Our calibrationsutilize 66 nearby star-forming galaxies with Balmer-decrement-corrected Hα luminosities, which span five ordersof magnitude in SFR and have absolute magnitudes of -24 < Mr < -12. Most of our photometry andspectrophotometry are measured from the same region of each galaxy, and our spectrophotometry has beenvalidated with SDSS photometry, so our random and systematic errors are small relative to the intrinsic scatter seenin SFR indicator calibrations. We find that the Wide-field Infrared Space Explorer W4 (22.8 μm), Spitzer 24 μm,and 1.4 GHz bands have tight correlations with the Balmer-decrement-corrected Hα luminosity, with a scatter ofonly 0.2 dex. Our calibrations are comparable to those from the prior literature for L∗ galaxies, but for dwarfgalaxies, our calibrations can give SFRs that are far greater than those derived from most previous literature.