Hiding behind a curtain of dust: Gas and dust properties of an ultra-luminous strongly-lensed

We present a detailed analysis of J154506, a strongly lensed submillimetre galaxy (SMG) behind the Lupus-I molecular cloud, and a characterisation of its physical properties using a combination of new and archival data, including VLT/MUSE and FORS2 optical data. We identify two high-significance (S/N > 5) emission lines at 97.0 and 145.5 GHz, corresponding to CO(4-3) and CO(6-5), respectively, in spectral scans from the Atacama Compact Array (ACA) and the Large Millimetre Telescope (LMT), as well as the [CII] 158 μm fine-structure line at 400 GHz observed with the Atacama Pathfinder Experiment (APEX). These detections yield a spectroscopic redshift of zspec = 3.7515 ± 0.0005. We also report the detection of [CI], HCN(4-3), and two H2O+ transitions, further confirming the redshift and providing insights into the physical properties of J154506. By modelling sub-arcsecond resolution (0.75″) ALMA Band 6 and 7 continuum data in the uv-plane, we derive an average magnification factor of 6.0 ± 0.4, and our analysis reveals a relatively cold dust (38 K) in a starburst galaxy (∼ 900 M⊙ yr−1) with a high intrinsic dust mass (∼ 2.5 × 109 M⊙) and infrared (IR) luminosity (∼ 6 × 1012 L⊙). Non-local thermodynamic equilibrium radiative transfer modelling of the joint dust spectral energy distribution (SED) and CO line excitation suggests the dust continuum emission is primarily associated with relatively diffuse regions with molecular gas densities of 102−104 cm−3, rather than compact, high-pressure environments typical of extreme starbursts or active galactic nuclei (AGNs). This interpretation is supported by the close-to-unity ratio between the dust and gas kinetic temperatures, which argues against highly energetic heating mechanisms. The CO excitation ladder peaks close to CO(5-4) and is dominated by slightly denser molecular gas. Our results underscore the unique power of far-IR and submillimetre observations to both uncover and characterise scarce, strongly lensed, high-redshift galaxies, even when they are obscured by foreground molecular clouds.