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

TL;DR

This study confirms J154506 as a strongly lensed, ultra-luminous dusty galaxy at $z_{ m spec}=3.7515 \\pm 0.0005$, seen through the Lupus-I molecular cloud. By combining ALMA ACA data, LMT/RSR, APEX, and optical spectroscopy, the authors map the CO ladder and [CII], derive a lensing magnification of $\\mu = 6.0 \\pm 0.4$, and characterize the dust and gas properties with non-LTE radiative transfer. The joint dust+CO modelling indicates a relatively cold dust component with $T_{ m dust} \\\sim 38$ K, a diffuse molecular ISM with $n(H_2) \\\sim 10^{2}-10^{4}$ cm$^{-3}$, and a substantial gas reservoir of $M_{ m ISM} \\\sim (3.1-8.5) imes 10^{11}$ M$_\\odot$, consistent with an ULIRG-like high-$z$ DSFG rather than an AGN-dominated system. The [CII] emission aligns with active star formation without strong deficits, and the CO ladder peaks near CO(5-4), supporting moderate gas excitation; overall, the source offers a rare, detailed view of ISM conditions in a bright, lensed galaxy at cosmic noon.

Abstract

We present a detailed analysis of J154506, a strongly lensed submillimeter galaxy behind the Lupus-I molecular cloud, and 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 (SNR>5) emission lines at 97.0 and 145.5 GHz, corresponding to CO(4-3) and CO(6-5), respectively, in the spectral scans from the Atacama Compact Array and the Large Millimetre Telescope and the [CII] 158~$μ$m fine-structure line at 400~GHz using the Atacama Pathfinder Experiment. These detections yield a spectroscopic redshift of $z_{\rm{spec}}=3.7515\pm0.0005$. We also report the detection of [CI], HCN(4-3), and two H$_2\rm{O}^+$ transitions, further confirming the redshift and providing insights into J154506's physical properties. By modeling sub-arcsecond resolution (0.75) ALMA Band 6 and 7 continuum data in the uv-plane, we derive an average magnification factor of $6.0\pm0.4$ and our analysis reveals a relatively cold dust (38K) in a starburst ($\sim900~\rm{M}_{\odot}yr^{-1}$) galaxy with a high intrinsic dust mass ($\sim2.5\times10^{9}~\rm{M}_{\odot}$) and infrared (IR) luminosity ($\sim6\times10^{12}~\rm{L}_{\odot}$). The non-local thermodynamic equilibrium radiative transfer modelling of the joint dust SED and CO line excitation suggests the dust continuum emission is primarily associated with relatively diffuse regions with molecular gas densities of $10^2-10^4\rm{cm}^{-3}$, rather than compact, high-pressure environments typical of extreme starbursts or AGNs. This 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.

Figures (13)

• Figure 1: Far-infrared and submm colour-colour diagram (S$_{870\mu\rm{m}}$/S$_{500\mu\rm{m}}$ vs S$_{250\mu\rm{m}}$/S$_{500\mu\rm{m}}$), colour-coded by redshift. The brightest known sources from the Herschel, Planck, and SPT surveys are shown with circular, triangular, and rhomboidal symbols, respectively. J154506 is highlighted with a red star. Flux densities at 850 $\mu$m (rather than at 870 $\mu$m) are used for the Herschel and Planck sources. The extremely red S$_{250\mu\rm{m}}$/S$_{500\mu\rm{m}}$ colour of J154506 suggests a redshift of $z>3$.
• Figure 2: Left panel: Multi Unit Spectroscopic Explorer (MUSE) white-light (4750 to 9350 $\AA$) median stack image of a $50\times \ang{;;50}$ region centred on the lens coordinates, with the orange square representing a $3\times \ang{;;3}$ region. The 1, 2, 5, and 10 $\sigma$ Infrared Array Camera (IRAC) contours are shown in white. Central panel:$6\times \ang{;;6}$ MUSE zoom-in with the 1 and 2 $\sigma$ IRAC white contours overlaid. Both IRAC and MUSE images are aligned using Gaia-DR3 stars within a $\ang{;;30}$ radius circle around the source. The faint linear emission feature does not overlap with the ALMA main arc, but instead corresponds to the noise arising from the 'gaps' between the MUSE detectors. Right panel: Same as the central panel, but overlaid on the ALMA Band 7 image of the background galaxy at $\ang{;;0.75}$ resolution (robust=1) . The IRAC and MUSE 1 and 2 $\sigma$ contours are shown in white and grey, respectively, to illustrate the relative position of the lens with respect to the background galaxy in the image plane.
• Figure 3: Top panel: Large Millimetre Telescope (LMT) spectrum of J154506 obtained with the Redshift Search Receiver (RSR) instrument. Transitions with S/N>3 compatible with either the J154506 redshift or the local molecular clouds are marked with vertical dashed lines and labelled in teal and grey, respectively. Bottom: Velocity comparison of the continuum-subtracted line profile for the $^{12}$CO(4-3) line detected with ACA and LMT, respectively (left), and $^{12}$CO(4-3) versus $^{12}$CO(6-5) transitions from ALMA/ACA (right). The LMT spectrum in the bottom panel is binned to two-channel width to match the velocity resolution of the compared lines.
• Figure 4: Spectra from APEX nFLASH230 (top panel) and nFLASH460 (bottom panel), centred at the redshifted ($z=3.7515$) frequencies of the CO(9-8) and [CII] emission lines, respectively. The independent runs are colour-coded and labelled accordingly. The derived RMS (1$\sigma$ noise level) is indicated by light grey shading, while dark grey arises from the overlap region of the errors from distinct datasets. The CO(9-8) line is not detected, but [CII] is detected at $>2$ sigma in all three scans. For [CII], the median-averaged spectrum (S/N$>$4) is shown in dark grey, with the area shaded using diagonal grey hatching.
• Figure 5: Results from the lens modelling analysis in the uv-plane for B6 (top) and B7 (bottom). From left to right: the panels show the 'dirty' data image, 'dirty' model image, 'dirty' residuals, sky model (i.e. deconvolved), and source model. The white line represents the critical curve (first four panels) and the caustic curve (rightmost panel).