Dust in a distant galaxy is transformed into a purple swirl in a stunning JWST photograph

A beautiful image of the spiral galaxy NGC 628, produced using data from the James Webb Space Telescope, may provide insights into how dust behaves in space

Space 19 July 2022

A mid-infrared image of the centre of the NGC 628 galaxy, based on data taken by the James Webb Space Telescope on 17 July

Gabriel Brammer / Janice Lee et al. and the PHANGS-JWST collaboration

An extraordinary image of the centre of the spiral galaxy NGC 628 taken by the James Webb Space Telescope (JWST) has drawn comparisons to a Doctor Who vortex, but it could reveal important clues about how dust behaves in galaxies.

The image is a composite of three sets of data at different wavelengths taken by JWST’s mid-infrared instrument team. Gabriel Brammer at the University of Copenhagen in Denmark, who isn’t affiliated with the team, downloaded the data and translated each of the infrared wavelengths to red, green and blue before combining them to produce one image.

NGC 628 has been imaged with visible light by other telescopes, including Hubble, and looks similar to our own Milky Way if viewed from above the galactic plane. But JWST’s ability to observe infrared light in high resolution reveals a hidden structure. “If our eyes could see in these mid-infrared wavelengths, the night sky would look a lot more like this picture, which I think would be spectacular, maybe a little terrifying,” says Brammer.

The distinctive purple appearance of Brammer’s image is due to the unique chemical composition of NGC 628’s dust clouds, which are primarily made up of large molecules called polycyclic aromatic hydrocarbons, says Michael Merrifield at the University of Nottingham, UK.

These molecules only emit specific wavelengths of light, so when Brammer mapped the three wavelengths to red, green and blue, there was very little green. The remaining red and blue emissions, when combined, made a pinky-purple colour.

The wavelengths were only captured by JWST on Sunday at 12pm BST, so astronomers haven’t had a chance to pore through the findings yet. However, some scientists have already noted on Brammer’s social media posts of the image that the empty centre is different from what visual images of the galaxy show, and could indicate interesting physical processes.

Merrifield says the data could also tell us about how dust gets made and redistributed in galaxies. However, one problem with the information, he adds, is that it is too zoomed in and high-resolution to draw conclusions about how whole galaxies form and evolve. “It’s kind of too much information, in the sense that we’re seeing the weather rather than the climate.”

The actual process of making the image was “staggeringly simple”, says Brammer, and represents a less talked-about advantage of JWST over Hubble. “As great a resource as the [James Webb] telescope is, we’ve also made similar improvements in the technology for processing the data and distributing the data such that anybody, from astronomers to non-experts, can explore the data like this,” he says.

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