Gravitationally Lensed View of DSFG-1 in PLCK G165.7+67.0: Strong Dust Emission and Spatially Resolved Stellar Population Analysis with JWST and SMA

Abstract: We present a detailed stellar population analysis of the strongly lensed dusty star-forming galaxy (DSFG) PLCK G165.7+67.0 DSFG-1 at $z = 2.236$, combining JWST NIRCam imaging with new Submillimeter Array (SMA) observations. This source is multiply imaged into two lensed components: image 1a, with a moderate magnification factor of $μ\sim 5$, and image 1bc, with an extreme magnification factor of $μ\sim 40$. The new SMA observations detect significant dust continuum emission at 225GHz and 273GHz, with combined flux densities of $S_{\rm cont}=(1.19\pm0.38)$ mJy in image 1a and $S_{\rm cont}=(10.02\pm0.85)$ mJy in image 1bc, indicating active star formation at sub-kpc scale. Based on the integrated SED modeling, DSFG-1 exhibits a lensing amplification-corrected stellar mass of $$M_{\star} = (1.2 \pm 0.4) \times 10^{10} M_{\odot}$$, and a star-formation rate (SFR) of $(103 \pm 14) M_{\odot}\,\mathrm{yr<sup>{-1}}$, similar to previous $Hα$-based results, placing it four times above the star-forming main sequence at this redshift. Its location on the size-mass plane and its morphological properties suggest that the system occupies a transitional phase between star-forming late-type galaxies and compact early-type systems. Together with its elevated star-formation activity, this is consistent with a rapidly evolving galaxy observed during Cosmic Noon. We further investigate the spatially resolved stellar population properties, and found significant spatial variations in stellar age and dust attenuation. These results point to a non-uniform star-formation history and highlight the complex interplay between dust geometry, stellar growth, and gravitational lensing, consistent with a merger scenario.

• The paper demonstrates how gravitational lensing with JWST and SMA data reveals DSFG-1’s stellar populations and dust structure at sub-kpc scales.
• It employs adaptive Voronoi binning and forward modeling to precisely map variations in SFR, stellar mass, and dust attenuation.
• The analysis supports a merger-driven starburst scenario and underscores the need to account for lensing and beam effects in high-z DSFG studies.

Gravitationally Lensed DSFG-1 in PLCK G165.7+67.0: Stellar Populations and Dust Structure at Sub-Kiloparsec Scales

Scientific Context and Motivation

DSFGs at $z \gtrsim 2$ are significant contributors to cosmic star formation, yet high angular resolution studies of their internal structure are hampered by their dust-obscured nature. Strong gravitational lensing enables spatially resolved investigations of these systems at sub-kpc scales, providing unique insight into the interplay between dust distributions, stellar populations, and star formation processes. The lensing cluster PLCK G165.7+67.0 is an archetype for such studies, with the background DSFG-1 at $z = 2.236$ split into two lensed images: 1a (moderate magnification, $\mu \approx 5$) and 1bc (extreme magnification, $\mu \approx 40$). The combination of new SMA submillimeter continuum imaging and JWST NIRCam data enables an integrated and spatially resolved analysis of the stellar and dust properties of DSFG-1.

Observational Data and Methodology

The study employs SMA continuum observations at 225 and 273 GHz, together with JWST NIRCam imaging spanning F090W to F444W, supplemented by archival ALMA 2 mm and 3 mm continuum data. Photometric measurements were performed in the reconstructed source plane for accuracy in lensing correction. Integrated SED models and spatially resolved fits were produced using CIGALE, with adaptive Voronoi binning applied to the JWST data for spatial SED analysis. All reconstructions adopted the state-of-the-art gravitational lens model of the G165 cluster, enabling accurate mapping of image-plane features to the source plane and robust estimation of magnification factors.

Global Stellar Populations and Star Formation

DSFG-1 exhibits a lensing-corrected stellar mass of $(1.2 \pm 0.4) \times 10^{10} \, M_{\odot}$ and an SFR of $(103 \pm 14) \, M_{\odot}\,\text{yr}^{-1}$, consistent with H$\alpha$-based estimates and placing it $\sim4\times$ above the star-forming main sequence at $z\sim2.2$ (Figure 1).

Figure 1: Offset from the star-forming main sequence versus stellar mass, showing DSFG-1’s elevated SFR compared to the Tan et al. comparison sample.

The system’s IR luminosity of $$L_{\mathrm{IR}} \approx 9.4 \times 10^{11} \, L_{\odot}$$ places it in the ULIRG regime, substantiated by strong dust continuum emission. The best-fit dust SED parameters indicate exposure to an intense ISRF and significant PAH content, typical of active starburst galaxies. Notably, DSFG-1 occupies a transitional region in the size-mass plane: with an effective dust-continuum radius of $z = 2.236$0 kpc, it lies between star-forming late-type disks and compact early-types at this epoch (Figure 1, right panel).

Spatially Resolved Stellar Population Properties

Spatially resolved SED fits with Voronoi binning reveal pronounced internal variations in SFR, stellar mass surface density, dust attenuation, and stellar age across both lensed images. The resolved maps show that dust continuum emission correlates with regions of high attenuation and star formation (Figure 2 and Figure 3), but also uncover significant spatial asymmetries.

Figure 2: Spatially resolved SFR, stellar mass surface density, dust attenuation, and mass-weighted stellar age for image 1a.

Figure 3: Spatially resolved SED properties in image 1bc, with the foreground source masked out.

The global SFR is systematically lower than the aggregate of spatially resolved SFRs (e.g., $z = 2.236$1 versus $z = 2.236$2 in 1a), a direct consequence of missing long-wavelength constraints in spatial fits—highlighting the degeneracy between age, dust, and SFR when millimeter data are omitted. Conversely, resolved stellar mass estimates are consistent with global values, indicating reliable mass recovery.

Dust Morphology and Differential Lensing Effects

Comparisons of SMA and ALMA continuum morphologies elucidate the impact of beam shape and lensing. SMA observations show pronounced flux asymmetry in 1bc, not reflected in ALMA maps (Figure 4). Forward modeling demonstrates that the SMA asymmetry can be entirely attributed to the synthesised beam shape and lensing geometry, not intrinsic source structure (Figure 5).

Figure 4: SMA and ALMA continuum contours overlaid on the attenuation ($z = 2.236$3) map for Arc 1bc, with critical line and foreground source.

Figure 5: SMA continuum best-fit forward model and residuals, confirming flux asymmetry arises from lensing/differential magnification and beam effects, not intrinsic in the source.

This rigorous approach validates the lens model and underscores the necessity of accounting for instrumental and lensing effects in interpreting resolved submillimeter emission.

Source-Plane Morphology and Merger Scenario

Source-plane reconstructions, utilizing both 1a and 1bc, consistently recover two distinct stellar mass concentrations separated by $z = 2.236$4 kpc (Figure 6), independently evident in both images. The SFR surface density maps, though sensitive to reconstruction, also show multi-component morphologies. Combined with previous NIRSpec velocity data (showing $z = 2.236$5 km s$z = 2.236$6 separation and $z = 2.236$7 kpc projected offset), this spatial evidence supports a merger-driven scenario for DSFG-1.

Figure 6: Source-plane reconstructions of SFR and stellar mass surface density reveal robust bimodal structure, indicative of a merger.

Additional interaction with DSFG-3, visible in SMA data and located $z = 2.236$8 kpc away at same redshift, enhances the likelihood of ongoing or imminent mergers.

Evolutionary Status and Comparison with High-Redshift SMGs

Forward-modeling of SMA data places DSFG-1 at the compact end of the SMG distribution, confirming its position as a lower-mass, highly star-forming DSFG with compact dust-emitting geometry. Compared to Tan et al. (Tan et al., 2024), DSFG-1 is undergoing intense, concentrated burst activity possibly associated with spheroid formation, while retaining a less centrally concentrated structure than local ULIRGs at comparable IR luminosity. This is indicative of an evolutionary phase preceding or transitional to the formation of compact early-type systems.

Implications and Future Directions

The synergy of high-resolution JWST imaging, submillimeter interferometry, and strong gravitational lensing enables unprecedented spatial dissection of high-redshift DSFGs. The results demonstrate the complexity of internal dust geometry, stellar population gradients, and the non-uniformity of star formation histories on sub-kiloparsec scales. DSFG-1 exemplifies the potential for gravitationally lensed systems to advance our understanding of galaxy assembly, merger-driven starbursts, and the routes by which compact spheroids form in the early Universe.

Further studies leveraging additional spectroscopic and high-frequency submillimeter data will refine mass, age, and dust estimates, clarify the merger dynamics, and trace the direct link between dusty starbursts and the emergence of compact early-type systems across cosmic time.

Conclusion

The spatially resolved stellar population and dust continuum analysis of G165 DSFG-1 reveals a system undergoing rapid, obscured star formation, structurally distinct from typical main-sequence disks and consistent with a merger-driven starburst. The compactness and pronounced internal variation underscore the importance of spatially resolved, multi-wavelength studies in disentangling galaxy evolution during Cosmic Noon. Instrumental and lensing effects must be rigorously accounted for in interpreting resolved morphologies. DSFG-1 serves as a benchmark for the physical processes governing star formation, dust attenuation, and stellar mass assembly at high redshift.

Figure 7: JWST NIRCam RGB images overlaid with ALMA continuum contours for both image-plane and source-plane reconstructions, showing consistent dust-obscured star formation in highly reddened regions.

Figure 8: Posterior distributions from the SMA forward-modeling MCMC analysis, illustrating the effective radius and other structural parameters of the dust continuum emission.

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Cited arXiv paper: "Gravitationally Lensed View of DSFG-1 in PLCK G165.7+67.0: Strong Dust Emission and Spatially Resolved Stellar Population Analysis with JWST and SMA" (2607.11049)