The ALMA REBELS Survey: discovery of a massive, highly star-forming, and morphologically complex ULIRG at z = 7.31
Inami, H.; Bowler, R. A. A.; Rybak, M.; De Looze, I.; Schneider, R.; Oesch, P. A.; Riechers, D. A.; van der Werf, P.; Bouwens, R. J.; Fudamoto, Y.; da Cunha, E.; Aravena, M.; Hodge, J. A.; Nanayakkara, T.; Ferrara, A.; Smit, R.; Stefanon, M.; Hygate, A. P. S.; Schouws, S.; Graziani, L.; Dayal, P.; Endsley, R.; Gonzalez, V.; Stark, D. P.; Algera, H. S. B.; Barrufet, L.; Hilhorst, J. H. A.; Pallottini, A.; Sommovigo, L.; Topping, M. W.
Netherlands, Australia, Japan, Spain, Italy, United Kingdom, United States, Chile, Switzerland, Denmark, Belgium, Germany
Abstract
We present Atacama Large Millimeter/Submillimeter Array (ALMA) [C II] and ~158 $\rm \mu m$ continuum observations of REBELS-25, a massive, morphologically complex ultra-luminous infrared galaxy (ULIRG; LIR = $1.5^{+0.8}_{-0.5} \times 10^{12}$ 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 $M_{*} = \mbox{$ 8^{+4}_{-2} \times 10^{9} $}{} ~\mbox{M$_\odot $}{}$. From dust-continuum and ultraviolet observations, we determine a total obscured + unobscured star formation rate of SFR $= \mbox{$199^{+101}_{-63}$}{} ~ \mbox{M$_\odot $}~ \mbox{${\rm 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 $M_{{\rm H}_{2}} = \mbox{$5.1^{+5.1}_{-2.6} \times 10^{10}$}{} ~\mbox{M$_\odot $}{}$, implying a molecular gas depletion time of $t_{\rm depl, {\rm H}_{2}} = \mbox{$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 $\mbox{M$_\odot $}~ \mbox{${\rm 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.