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What Are We Actually Seeing In JWST's First Deep Field Image?

The first image released by Hubble's successor is a beauty, but what exactly are we looking at?


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockJul 12 2022, 11:55 UTC
JWST First Deep Field. Image credit: NASA, ESA, CSA, and STScI
JWST's first Deep Field starring galaxy cluster SMACS 0723. Image credit: NASA, ESA, CSA, and STScI

Yesterday, President Biden and Vice President Harris presented the first full-color image from JWST, the deepest and highest resolution infrared image of the universe. The image is awe-inspiring but a short and slightly confusing press briefing did little to explain what we are actually seeing in this historic image.

The subject is SMACS 0723, a massive galaxy cluster that warps space-time around it. It acts as a lens, a gravitational lens to be precise, magnifying distant galaxies from the very early universe.


The picture is aesthetically pleasing in itself but knowing what you're looking at and how it was made gives it the gravitas it deserves. So let’s start with the technical side. Comparisons to the Hubble Deep Fields – the previous deepest images of the universe – are obvious (see the gorgeous side-by-side below) but JWST is seeing the universe in infrared, not visible light, and is much faster at doing it. It took JWST 12.5 hours to take the new image while it took Hubble 10 days to take its first Deep Field image.

Comparison between Hubble and JWST view of the same field. Image Credit: NASA, ESA, CSA, STScI
Comparison between Hubble and JWST view of the same field. Image Credit: NASA, ESA, CSA, STScI

Now let’s consider its size. If you were to put a grain of sand on your fingertip and hold it out at arm's length, everything you see in this image would be contained in there. Hundreds of galaxies, some with hundreds of billions of stars, as well as stars that are within our own galaxy. Those are the easiest to spot. All the bright lights with six spikes are stars in the Milky Way; the spikes are caused by the physical structure of the telescope.

At the very core of the image is SMACS 0723's central galaxy, the whitish-yellow blob. But do not be fooled by its shapelessness. This object is the massive core of a massive cluster. The mass of these objects is notoriously difficult to estimate, but one approach puts it around 839 trillion times the mass of our Sun or about 700 times the mass of the Andromeda galaxy. And it's not even one of the most massive ones, as the cluster goes.

Around the bright central galaxy, there is a diffuse light halo that is from dust and stars that the telescope, despite its sharpness, can’t resolve. The cluster is located 4.6 billion light-years away, but some of the light from this image comes from further afield. As mentioned earlier, the gravitational lens magnifies the light of distant background galaxies that are much, much older. 


Also in this image, there are arcs and twisted objects, some of which appear to be repeating, and you are not wrong. Gravitational lensing can create multiple images of the same object, often with a time delay. A curious effect that has allowed researchers in the past to see a supernova explode in one image and then predict it would appear later in another.

Some early analysis has been done on these objects leading to determining the chemical composition of the furthest galaxy yet. Its light comes from 13.1 billion years ago. In the coming months, researchers will be able to study even more galaxies in even more detail. 

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