2018ApJ...856..174V

We detect bright emission in the far-infrared (far-IR) fine structure [O III] 88 µm line from a strong lensing candidate galaxy, H-ATLAS J113526.3-014605, hereafter G12v2.43, at z = 3.127, using the second-generation Redshift (z) and Early Universe Spectrometer (ZEUS-2) at the Atacama Pathfinder Experiment Telescope (APEX). This is only the fifth detection of this far-IR line from a submillimeter galaxy at the epoch of galaxy assembly. The observed [O III] luminosity of 7.1 x 10⁹ (10/µ)L_☉ likely arises from H II regions around massive stars, and the amount of Lyman continuum photons required to support the ionization indicate the presence of (1.2-5.2) x 10⁶ (10/µ) equivalent O5.5 or higher stars, where µ would be the lensing magnification factor. The observed line luminosity also requires a minimum mass of ∼2 x 10⁸ (10/µ)M_☉ in ionized gas, that is 0.33% of the estimated total molecular gas mass of 6 x 10¹⁰ (10/µ)M_☉. We compile multi-band photometry tracing rest-frame ultraviolet to millimeter continuum emission to further constrain the properties of this dusty high-redshift, star-forming galaxy. Via SED modeling we find G12v2.43 is forming stars at a rate of 916 (10/µ)M_☉ yr^–1 and already has a stellar mass of 8 x 10¹⁰ (10/µ)M_☉. We also constrain the age of the current starburst to be ≤ 5 Myr, making G12v2.43 a gas-rich galaxy lying above the star-forming main sequence at z ∼ 3, undergoing a growth spurt, and it could be on the main sequence within the derived gas depletion timescale of ∼66 Myr.