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Spatial variations in aromatic hydrocarbon emission in a dust-rich galaxy


Dust grains absorb half of the radiation emitted by stars throughout the history of the universe, re-emitting this energy at infrared wavelengths1,2,3. Polycyclic aromatic hydrocarbons (PAHs) are large organic molecules that trace millimetre-size dust grains and regulate the cooling of interstellar gas within galaxies4,5. Observations of PAH features in very distant galaxies have been difficult owing to the limited sensitivity and wavelength coverage of previous infrared telescopes6,7. Here we present James Webb Space Telescope observations that detect the 3.3 μm PAH feature in a galaxy observed less than 1.5 billion years after the Big Bang. The high equivalent width of the PAH feature indicates that star formation, rather than black hole accretion, dominates infrared emission throughout the galaxy. The light from PAH molecules, hot dust and large dust grains and stars are spatially distinct from one another, leading to order-of-magnitude variations in PAH equivalent width and ratio of PAH to total infrared luminosity across the galaxy. The spatial variations we observe suggest either a physical offset between PAHs and large dust grains or wide variations in the local ultraviolet radiation field. Our observations demonstrate that differences in emission from PAH molecules and large dust grains are a complex result of localized processes within early galaxies.

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Fig. 1: PAH 3.3 μm emission from SPT0418-47.
Fig. 2: Mid- and far-IR continua and PAH emission from SPT0418-47.
Fig. 3: Large spatial variations in PAH emission within SPT0418-47.

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Data availability

All JWST data are available from the Mikulski Archive for Space Telescopes ( under programme no. 1355. The reduced JWST data products used in this work are available from the TEMPLATES collaboration public data repository, This paper makes use of ALMA data under project code nos. 2016.1.01374.S and 2016.1.01499.S, available from the ALMA science archive (

Code availability

JWST and ALMA data were reduced using the publicly available pipeline software for both observatories. Our reduction and analysis scripts for MIRI/MRS data are available from the TEMPLATES collaboration public data repository,


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JWST is operated by the Space Telescope Science Institute under the management of the Association of Universities for Research in Astronomy, Inc., under NASA contract no. NAS 5-03127. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan) and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

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Authors and Affiliations



J.S.S. led data analysis and drafted the main text. K.A.P. and D.L. contributed to data analysis. J.D.T.S. assisted in interpretation of data. J.R.R. and J.D.V. contributed to management of the TEMPLATES programme. All authors contributed to interpretation of the results and editing of the text, and are ordered alphabetically after K.A.P.

Corresponding author

Correspondence to Justin S. Spilker.

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Extended data figures and tables

Extended Data Fig. 1 Demonstration of residual MRS data artifact removal process.

Each column shows a 100-channel average from the MRS data cube corresponding to the wavelength ranges indicated at top. The top row shows the original pipeline-processed data. Horizontal stripe features are evident, a manifestation of the so-called ‘shower’ artifacts in MIRI data. The middle row shows the estimated background to be subtracted averaged over the same wavelength range. The bottom row shows the final background-subtracted image. The circles show the region of the cube that was masked during the background estimation due to the presence of real source emission from SPT0418-47. All images are on the same color scale. The 3.3 μm PAH feature is mostly contained within the wavelength range of the third column.

Extended Data Fig. 2 Investigations of possible variations in dust temperature across SPT0418-47.

Using additional ALMA continuum data at rest-frame 120 μm, we calculate the implied changes in Tdust under standard assumptions about the shape of the dust spectral energy distribution. The 120 μm and 160 μm images are shown on a linear min/max color scale, masking pixels detected at S/N<5 in either band, to demonstrate their qualitative similarity. This similarity consequently implies only small changes in Tdust across the source. We use the resolved Tdust map to estimate the implied correction to our ‘default’ assumption of a constant conversion between LIR and 160 μm flux density; the right panel shows that only ≈10% variations are implied, subdominant to other sources of uncertainty in our analysis.

Extended Data Fig. 3 Recovery of variations in LPAH/LIR.

Using mock data with constant LPAH/LIR inserted into signal-free portions of the MRS data cube, we test the extent to which the faintness of the PAH feature and noise properties of the MRS data influence our conclusion that SPT0418-47 shows large variations in LPAH/LIR. Points show individual pixels from several of the mock realizations, while the black dashed line and grey shaded region illustrate the median and 16–84th percentile range of the distribution of all mock simulations. Even in the faintest regions, LPAH/LIR is still recovered to within a factor of ≈2, improving to ±25% in brighter regions.

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Spilker, J.S., Phadke, K.A., Aravena, M. et al. Spatial variations in aromatic hydrocarbon emission in a dust-rich galaxy. Nature 618, 708–711 (2023).

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