Detailed study of a rare hyperluminous rotating disk in an Einstein ring 10 billion years ago

Hyperluminous infrared galaxies (HyLIRGs) are the rarest and most extreme starbursts and found only in the distant Universe (z ≳ 1). They have intrinsic infrared (IR) luminosities L_IR ≥ 10¹³ L_⊙ and are commonly found to be major mergers. Recently, the Planck All-Sky Survey to Analyze Gravitationally-lensed Extreme Starbursts project (PASSAGES) searched ~10⁴ deg² of the sky and found ~20 HyLIRGs. We describe a detailed study of PJ0116-24, the brightest (μL_IR ≈ 2.6 × 10¹⁴ L_⊙, magnified with μ ≈ 17) Einstein-ring HyLIRG in the southern sky, at z = 2.125, with observations from the near-IR integral-field spectrograph VLT/ERIS and the submillimetre interferometer ALMA. We detected Hα, Hβ, [N II] and [S II] lines and obtained an extreme Balmer decrement (Hα/Hβ ≈ 8.73 ± 1.14). We modelled the molecular-gas and ionized-gas kinematics with CO(3-2) and Hα data at ~100-300 pc and (sub)kiloparsec delensed scales, respectively, finding consistent regular rotation. We found PJ0116-24 to be highly rotationally supported (v_rot/σ_{0, mol. gas} ≈ 9.4) with a richer gaseous substructure than other known HyLIRGs. Our results imply that PJ0116-24 is an intrinsically massive (M_baryon ≈ 10^11.3 M_⊙) and rare starbursty disk (star-formation rate, SFR = 1,490 M_⊙ yr⁻¹) probably undergoing secular evolution. This indicates that the maximal SFR (≳1,000 M_⊙ yr⁻¹) predicted by simulations could occur during a galaxy's secular evolution, away from major mergers.