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

Abstract

We present Atacama Large Millimeter/Submillimeter Array (ALMA) [CII] and $\sim 158$ $\hat{\mathrm{I}}¼\mathrm{m}$ continuum observations of REBELS-25, a massive, morphologically complex ultra-luminous infrared galaxy (ULIRG; $L_{\mathrm{IR}}=1.5_{-0.5}^{+0.8}\times 10^{12}$ 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 $M_{\ast }=8_{-2}^{+4}\times 10^9$ M${}_{\odot }$. From dust-continuum and ultraviolet observations, we determine a total obscured + unobscured star formation rate of SFR $=199_{-63}^{+101}$ 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 $M_{{\mathrm{H}}_2}=5.1_{-2.6}^{+5.1}\times 10^{10}$ $M_{\odot }$, implying a molecular gas depletion time of $t_{\mathrm{depl},{\mathrm{H}}_2}=0.3_{-0.2}^{+0.3}$ 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 ($\sim 500$ 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${}_{\odot }$ yr${}^{-1}$, which is consistent with expectations for a star formation-driven outflow. Given its large stellar mass, SFR and molecular gas reservoir $\sim 700$ 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 $z\sim 4$.

Accepted for publication in MNRAS. 21 pages, 8 figures