New discoveries

Ring-like galaxy is actually a distant twin of the Milky Way

Read time: 3 minutes

Have you ever seen a galaxy that looks like a wedding ring?

This image, obtained by ALMA, shows a galaxy known as SPT 0418-47. It is extremely far away: the
galaxy’s light took 12.4 billion years to reach Earth. According to the astronomers who took the
image, it looks very much like our own Milky Way galaxy.

But wait a moment – our Milky Way is a flattened disk with a bright core and beautiful spiral arms,
right? So how can this ring-like galaxy be described as the Milky Way’s twin?

It’s because we don’t see the distant galaxy as it really is. Its light has been distorted and magnified
by the gravity of another galaxy that lies closer to us. Thanks to this ‘gravitational lens’, SPT 0418-47
looks weird, just like your face looks weird if it is reflected in the shiny surface of a spoon.

If you know how the image of the galaxy is distorted by the gravitational lens, you can calculate its
‘real’ appearance. It turns out that SPT 0418-47 is indeed very much like our own Milky Way: an
orderly, flattened disk with a bright, thick ‘bulge’ in the very center. The only thing that’s missing are
the spiral arms.

This is a surprising find. Because of the large distance, we see the galaxy as it was 12.4 billion years
ago, when the universe was still young, and the first galaxies were only just being born.

Astronomers always assumed that the earliest galaxies would be much more messy, without an
orderly form and structure. However, the ALMA observations of SPT 0418-47 reveal that at least
some newborn galaxies in the early universe already looked very mature.

You might think that the distortion by the gravitational lens made it more difficult to study the
distant galaxy. In a sense, that’s true of course: astronomers first had to calculate the galaxy’s ‘real’
shape. But without the magnification by the gravitational lens, they would never have been able to
observe the galaxy at all, because it is so extremely far away!

What?

SPT 0418-47 was first observed by the South Pole Telescope in Antarctica (hence the name). It is located in the inconspicuous constellation Horologium, in the southern sky. The galaxy’s light that is arriving at Earth right now was already emitted when the universe was just 1.4 billion years old – about ten percent of its present age. Somewhere between the distant galaxy and the Earth is another galaxy (not visible in the ALMA image). The gravity of this other galaxy is bending the light waves of SPT 0418-47, distorting its shape into an almost perfect ring.

Who?

The ALMA observations of SPT 0418-47 were carried out by a group of seven European astronomers,
led by Francesca Rizzo of the Max Planck Institute for Astrophysics in Germany. Francesca worked
together with colleagues from her own institute, and from the University of Groningen (The
Netherlands) and ASTRON, the Netherlands Institute for Radio Astronomy. The team published their
results in the August 12, 2020 edition of the scientific journal Nature.

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