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
Journal article, 2026
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 mu m 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 (similar to 900 M-circle dot yr(-1)) with a high intrinsic dust mass (similar to 2.5 x 10(9) M-circle dot) and infrared (IR) luminosity (similar to 6 x 10(12) L-circle dot). 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 10(2)-10(4) 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.
galaxies: starburst
gravitational lensing: strong
submillimeter: galaxies
galaxies: high-redshift
Author
Belen Alcalde Pampliega
ESO Vitacura
Square Kilometre Array, UK
Diego Portales University
Kevin C. Harrington
Diego Portales University
National Astronomical Observatory of Japan
ESO Vitacura
Atacama Large Millimeter-submillimeter Array (ALMA)
Aristeidis Amvrosiadis
Durham University
Manuel Aravena
Millenium Nucleus for Galaxies (MINGAL)
Diego Portales University
Min S. Yun
University of Massachusetts
Hugo Messias
ESO Vitacura
Atacama Large Millimeter-submillimeter Array (ALMA)
Antonio Hernan-Caballero
CEFCA
Leindert Boogaard
Leiden University
Axel Weiss
Max Planck Society
Benjamin Beauchesne
Durham University
Alejandro Santamaria-Miranda
University of Chile (UCH)
Monica Ivette Rodriguez
Inst Radioastonomie Millimetr IRAM
Eric Jimenez-Andrade
Universidad Nacional Autónoma de México
Manuel Solimano
Diego Portales University
James Lowenthal
Smith College
Patrick Kamieneski
Arizona State University
Q. Daniel Wang
University of Massachusetts
Chentao Yang
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
Amit Vishwas
Cornell University
Pascale Hibon
ESO Vitacura
Brenda Frye
University of Arizona
Jorge Gonzalez-Lopez
Diego Portales University
Yiqing Song
ESO Vitacura
Meghana Killi
Diego Portales University
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 706 A174Subject Categories (SSIF 2025)
Atom and Molecular Physics and Optics
Astronomy, Astrophysics, and Cosmology
DOI
10.1051/0004-6361/202556106