Hiding behind a curtain of dust: Gas and dust properties of an ultra-luminous strongly-lensed z = 3.75 galaxy behind the Milky Way disc

¹ ESO Vitacura, Alonso de Córdova 3107 Vitacura Casilla 19001 Santiago de Chile, Chile
² Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales Av. Ejército Libertador 441 Santiago, Chile
³ SKA Observatory, Jodrell Bank SK11 9FT Macclesfield, UK
⁴ Joint ALMA Observatory Alonso de Córdova 3107 Vitacura 763-0355 Santiago, Chile
⁵ National Astronomical Observatory of Japan Los Abedules 3085 Oficina 701 Vitacura 763 0414 Santiago, Chile
⁶ Centre for Extragalactic Astronomy, Durham University, Department of Physics South Road Durham DH1 3LE, UK
⁷ Millenium Nucleus for Galaxies (MINGAL) Concepción, Chile
⁸ Department of Astronomy, University of Massachusetts Amherst MA 01003, USA
⁹ Centro de Estudios de Física del Cosmos de Aragón (CEFCA) Plaza San Juan 1 E-44001 Teruel, Spain
¹⁰ Leiden Observatory, Leiden University P.O. Box 9513 2300 RA Leiden, The Netherlands
¹¹ Max-Planck-Institut für Radioastronomie Auf dem Hügel 69 53121 Bonn, Germany
¹² Centre for Extragalactic Astronomy, Department of Physics, Durham University South Road Durham DH1 3LE, UK
¹³ Institute for Computational Cosmology, Durham University South Road Durham DH1 3LE, UK
¹⁴ Departamento de Astronomía, Universidad de Chile Camino El Observatorio 1515 Las Condes Santiago, Chile
¹⁵ Institut de Radioastonomie Millimétrique (IRAM), Av. Divina Pastora 7 Núcleo Central 18012 Granada, Spain
¹⁶ Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, Antigua Carretera a Pátzcuaro 8701 Ex-Hda. San José de la Huerta Morelia Michoacán C.P. 58089, México
¹⁷ Department of Astronomy, Smith College Northampton MA 01063, USA
¹⁸ School of Earth and Space Exploration, Arizona State University PO Box 871404 Tempe AZ 85287-1404, USA
¹⁹ Department of Space, Earth & Environment, Chalmers University of Technology SE-412 96 Gothenburg, Sweden
²⁰ Cornell Centre for Astrophysics and Planetary Science, Cornell University Space Sciences Building Ithaca NY 14853, USA
²¹ Steward Observatory, University of Arizona 933 N Cherry Ave Tucson AZ 85721-0009, USA

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Received: 25 June 2025
Accepted: 5 December 2025

Abstract

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 z_spec = 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.