Hyperluminous infrared galaxies (HyLIRGs) represent the most extreme starburst galaxies, characterized by infrared luminosities exceeding 10¹³ solar luminosities (L_☉). Their rarity confines their presence to the distant universe (redshift z ≥ 1). The prevailing theory attributes their formation to major mergers, a hypothesis supported by the frequent observation of merging structures in HyLIRGs. However, alternative models propose that HyLIRGs might represent young, primordial galaxies undergoing periods of maximal star formation fueled by the rapid infall of cold gas during secular evolution. The limited availability of high-resolution, high-quality multi-tracer observations has impeded robust testing of these competing models. This study leverages the unique opportunity provided by PJ0116-24, a gravitationally lensed HyLIRG forming an Einstein ring, to conduct an in-depth investigation of its properties and test the existing theoretical frameworks. The strong lensing magnifies the light from the galaxy significantly, allowing for a detailed study of its properties that would otherwise be impossible with current technology for this rare type of object.

All-sky surveys using IR satellites like IRAS, WISE, and Planck have significantly advanced the discovery of HyLIRGs. The brightest HyLIRGs often exhibit gravitational lensing. High-resolution imaging using telescopes like HST, VLA, NOEMA, and ALMA reveals that HyLIRGs are predominantly gas-rich mergers, frequently hosting active galactic nuclei (AGNs). The prevailing model posits HyLIRGs as the high-luminosity counterparts of local ultra-luminous IR galaxies (ULIRGs), with extreme starburst activity triggered by mergers. Conversely, another theoretical model suggests that HyLIRGs might be young galaxies in their peak star-formation phases, an idea supported by hydrodynamical simulations demonstrating that exceptionally high star formation rates (SFR ≥ 1000 M_☉ yr⁻¹) could be achieved during the secular evolution of massive, turbulent, gas-rich galaxies.

This research employed a multi-faceted approach, combining data from multiple telescopes and sophisticated analysis techniques. High-resolution CO(3-2) observations were obtained using ALMA to trace the distribution and kinematics of cold gas at a resolution of ~100-300 pc. Simultaneously, near-IR integral field unit (IFU) observations were conducted with the VLT/ERIS instrument to map the distribution and kinematics of ionized gas at (sub)kiloparsec scales. The high spectral resolution of ERIS allowed for the detection and measurement of key emission lines (Hα, Hβ, [N II], [S II]) to determine the Balmer decrement, dust attenuation, and star formation rate (SFR). The data were complemented by HST near-infrared observations, providing high resolution structural information, allowing for accurate lens modelling. Advanced lens modelling techniques, utilizing software like glafic, were implemented to reconstruct the intrinsic structure and kinematics of the galaxy. The kinematics of both the molecular and ionized gas were meticulously analyzed using state-of-the-art dynamical modeling based on direct fits to high-resolution CO data in the image plane. This approach incorporated lensing effects, beam-smearing, line instrumental broadening, and three-dimensional projection effects, providing a robust characterization of PJ0116-24's rotation and turbulence. Spectral energy distribution (SED) modeling, using code like MiChi², further characterized the stellar and dust properties of the galaxy, improving on previous results. Multiple diagnostics for determining the ionization source and gas-phase metallicity were employed, including the BPT diagram and strong line ratios, to shed light on the processes driving the observed properties. A novel approach to ISM mass estimation from radiative transfer modelling of CO and [CI] lines enhanced the accuracy of the results. The various techniques and data sets were integrated to build a complete picture of this unique galaxy.

The analysis of PJ0116-24 yielded several key findings. Firstly, the galaxy exhibits a near-perfect Einstein ring structure, revealing a highly ordered, regular rotation in both molecular and ionized gas. The high rotation-to-dispersion ratio (v_rot/σ_{0, mol. gas} ≈ 9.4) is uncommon for HyLIRGs and exceptionally well constrained in this study. This indicates that PJ0116-24 is a rotationally supported disk. The galaxy's intrinsic properties, derived from lensing corrections, reveal a massive baryonic mass (M_baryon ≈ 10^11.3 M_☉), a high star formation rate (SFR = 1490 M_☉ yr⁻¹), and a rich gaseous substructure. The detailed kinematic modeling confirmed PJ0116-24's massive disk structure with high rotational support, intrinsic turbulence, and a relatively small bulge. The high Balmer decrement (Hα/Hβ ≈ 8.73) indicates significant dust attenuation, necessitating careful corrections in SFR estimates. The study unveiled a central deficit of cold gas within ~500 pc, yet the gas is abundant at larger radii (~1-2 kpc), extending out to ~3-4 kpc in a stream-like pattern. A giant star-forming clump, located ~4 kpc from the center, is also detected in all tracers. The clump’s mass contribution to the overall galaxy is small. The super-solar metallicity determined from strong-line diagnostic indicates that the star formation in the disk is responsible for the high metallicity of the interstellar medium. The analysis using the BPT diagram confirms that star formation is the dominant ionization source, with only a low AGN contribution. Overall, the findings suggest that PJ0116-24 is not the product of a recent major merger, but instead is an example of a massive star-forming disk galaxy in the process of secular evolution. This suggests that the extremely high SFRs predicted by simulations are plausible during secular evolution, rather than solely during major mergers. Detailed comparisons with other HyLIRGs and studies of galaxy evolution emphasize the unique nature of this discovery.

The findings from this study have significant implications for our understanding of galaxy evolution. The discovery of PJ0116-24, a massive, rotationally supported HyLIRG with a high SFR, undergoing secular evolution, challenges the long-held assumption that extreme starbursts in the distant universe are solely caused by major mergers. The central cold gas deficit and the surrounding abundant gas with arm-like features strongly suggest that the high SFR is likely driven by gas accretion and transport processes rather than merger-driven bursts. The coexistence of high SFR and a relatively quiescent central region provides strong evidence for the ‘wet disc compaction’ mechanism, where inner regions are quenched while the outer regions continue to form stars. The super-solar metallicity supports this picture. These results provide compelling observational support for the theoretical models predicting that high SFRs can be achieved during secular evolution in massive, gas-rich galaxies and complements the long-standing model of merger-driven ULIRGs. This discovery highlights the importance of considering alternative evolutionary paths for understanding the formation and evolution of massive galaxies in the early universe.

This study presents a comprehensive analysis of PJ0116-24, a unique hyperluminous rotating disk galaxy, offering strong evidence for secular evolution as a significant driver of maximal star formation in the distant universe. The combination of high-resolution multi-tracer observations, advanced lens modeling, and detailed kinematic modeling provides robust support for this conclusion. The discovery significantly expands our understanding of galaxy evolution, highlighting the need for further detailed studies of lensed HyLIRGs to refine theoretical models and explore the diversity of evolutionary pathways. Future studies using high-resolution near-IR IFU observations with adaptive optics could refine the properties of the giant clump, the central molecular gas properties and probe for any inflow/outflow signal, providing further insights into the galaxy’s evolution.

While this study provides a detailed analysis of PJ0116-24, several limitations exist. The sample size is limited to a single galaxy. Although the strong lensing allows for extremely high resolution, the methodology relies heavily on the accuracy of the lens model. While efforts were made to address these effects, uncertainties in the lens model might impact the interpretation of some results. The idealized mass model adopted in the dynamical modeling might not fully capture the complexities of the galaxy's internal structure. Finally, the analysis assumes a standard Chabrier IMF. Deviations from this assumption could affect the derived stellar masses and SFRs. Future studies should aim to expand the sample size, further refine the lens model, and investigate the impact of variations in the IMF.