Resolving the dusty star-forming galaxy GN20 at z=4.055 with NOEMA and JWST: A similar distribution of stars, gas and dust despite distinct apparent profiles

TL;DR

GN20 is a prototypical high-$z$ dusty starburst whose dust, gas, and stellar components are spatially extensive. By combining NOEMA 1.1 mm imaging with JWST NIRCam/MIRI data, the study shows that dust, gas, and stars share a common mass distribution, and that the observed differences in their radial profiles arise from radiative-transfer effects rather than intrinsic structural separations. Self-consistent radiative transfer modeling (TUNER/TED) yields a molecular gas mass of about $M_{\rm mol}\approx(2.9-3.4)\times10^{11}\,M_\odot$, a CO-to-H$_2$ conversion factor around $\alpha_{\rm CO}\sim 2.8$–$3.6$, and a dust-to-gas ratio of $\delta_{\rm GDR}\approx50$, with dust mass $M_{\rm dust}\approx(5.9\pm0.6)\times10^{9}\,M_\odot$. The extended dust emission reaches roughly $14$ kpc in diameter and, together with kinematic evidence, supports a recent interaction or merger as the starburst trigger. The findings underscore the necessity of radiative-transfer-aware comparisons of gas and dust tracers in dusty, high-$z$ galaxies and demonstrate the power of JWST–NOEMA synergy for resolving the gas–dust–star interplay in the early universe.

Abstract

We present high-resolution (0.13"-0.23") NOEMA observations of the dust continuum emission at 1.1 mm (rest-frame 220 micron) and JWST/NIRCam and MIRI imaging of the z=4.055 starburst galaxy GN20. The sensitive NOEMA imaging at 1.6 kpc resolution reveals extended dust emission, ~14 kpc in diameter (r_e~2.5 kpc, b/a=0.5), that is centrally asymmetric and clumpy. The dust emission is as extended as the stellar emission and the molecular gas traced by CO(2-1), with a common center, and is brightest in the strongly-obscured nuclear part of the galaxy. Approximately one-third of the total dust emission emerges from the nucleus and the most prominent clump to the south, and (only) 60% from the central 3.5x1.5 kpc (0.5"-0.2"), implying that the starburst is very extended. The combined JWST and NOEMA morphology suggests GN20 experienced a recent interaction or merger, likely invigorating the starburst. The radial surface brightness profiles of the molecular gas and near-IR stellar emission are similar, while in contrast, the dust emission appears significantly more concentrated. Through self-consistent radiative transfer modeling of the integrated and resolved CO and dust emission, we derive an $M_{mol}=2.9^{+0.4}_{-0.3}\times10^{11}$ Msun with $α_{CO}=2.8^{+0.5}_{-0.3}$. We find that the extended dust implies a lower global dust optical depth than previously reported, but a high dust mass of $M_{dust}=5.7^{+0.8}_{-0.6}\times 10^{9}$ Msun and gas-to-dust ratio of ~50. Furthermore, we show the distinct apparent radial profiles of the gas and dust can be explained purely by radiative transfer effects and the observations are consistent with the gas and dust mass being similarly distributed throughout the starburst. The latter highlights the importance of accounting for radiative transfer effects when comparing molecular gas and dust distributions from different tracers.

Figures (11)

• Figure 1: IRAM/NOEMA long-baseline 1.1 mm imaging of GN20. The left and right columns show the cleaned images before and after self calibration. The panels show the (a) highest resolution A-configuration tracks only, at natural resolution ($0\hbox{$.\!\!^{\prime\prime}$}24\times0\hbox{$.\!\!^{\prime\prime}$}2$), before and (b) after self calibration, (c) A tracks, robust weighted to higher resolution ($0\hbox{$.\!\!^{\prime\prime}$}14\times0\hbox{$.\!\!^{\prime\prime}$}12$), before and (d) after self calibration, (e) A+C configuration tracks, robust weighted to the A configuration-only resolution ($0\hbox{$.\!\!^{\prime\prime}$}26\times0\hbox{$.\!\!^{\prime\prime}$}2$) before and (f) after self-calibration. Contours are overlaid on all images, starting at $5\sigma$ increasing in steps of $10\sigma$ up to $55\sigma$ and then steps of $20\sigma$.
• Figure 2: JWST/NIRCam and NOEMA 1.1 mm imaging ($4'\times4'$ cutouts) of GN20 at $z=4.055$. Dust contours start at $5\sigma$ and increase in steps of $10\sigma$ up to $55\sigma$ and then steps of $20\sigma$ up to $115\sigma$.
• Figure 3: Multi-wavelength cutouts of GN20. The top two rows show the NIRCam and MIRI imaging (Colina2023Crespo2024) covering the rest-frame 0.23 $\mu$m--3.56 $\mu$m range. The bottom row shows the $\mathrm{H}\alpha$ map Ubler2024, $\mathrm{Pa}\alpha$ map Bik2024, the dust continuum at 880 $\mu$m Hodge2012 and 1.1 mm (NOEMA; this work) and the JVLA/CO(2--1) Hodge2012. All maps are shown on a linear colorscale and NOEMA contours are overlaid on selected images, starting at $5\sigma$ and increasing in steps of $10\sigma$ up to $55\sigma$ and then steps of $20\sigma$ up to $115\sigma$. Beam sizes of the radio maps are shown in the bottom left corner.
• Figure 4: $3"\times3"$ false color image of GN20 at $z=4.055$ highlighting the distinct spatial distribution of the different components: rest-frame UV (NIRCam/F150W), rest-frame near-IR (MIRI F560W, yellow), rest-frame 220 $\mu$m dust continuum (NOEMA/1.1mm) and the molecular gas traced by CO(2--1) (VLA). The stellar distribution traced by the rest-frame near-IR is largely cospatial with the CO and the dust emission, though the emission from the latter appears more centrally concentrated. The (unattenuated) rest-frame UV light is only visible in the outskirts.
• Figure 5: Azimuthally averaged and integral-normalized radial profiles, measured from the peak of the NOEMA continuum emission in bins of 1 kpc.