The ALMA REBELS Survey: discovery of a massive, highly star-forming, and morphologically complex ULIRG at <i>z</i> = 7.31
A. P. S. Hygate, Jacqueline Hodge, E da Cunha, Matus Rybak, Sander Schouws, Hanae Inami, Mauro Stefanon, Luca Graziani, Raffaella Schneider, Pratika Dayal, R. J. Bouwens, Renske Smit, R. A. A. Bowler, Ryan Endsley, Valentino González, Pascal A. Oesch, Daniel P. Stark, Hiddo Algera, Manuel Aravena, Laia Barrufet, Andrea Ferrara, Yoshinobu Fudamoto, Juliëtte Hilhorst, I De Looze, Themiya Nanayakkara, A. Pallottini, Dominik A. Riechers, Laura Sommovigo, Michael W. Topping, P. van der Werf
Abstract
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.