How to ‘un-distort’ a galaxy

How to ‘un-distort’ a galaxy

Read time: 3 minutes

A distant galaxy that looked like a near-perfect ring to ALMA turns out to be a chaotic collection of gas clumps instead. Some of these gas clumps are only 200 light years across. Never before astronomers have seen so much detail at such a large distance in the universe – more than 11 billion light years away.

Suppose you would make a selfie in a distorting mirror. If you put it as an avatar on your Facebook account, people might think that you actually have a very big head and short, stubby legs. But if they knew the exact shape of the mirror, they could 'reconstruct' how you really look like.

This is what astronomers have done. In 2014, with the 66 ALMA antennas in their widest configuration, they observed the extremely distant galaxy SDP.81, which is seen as it was just a few billion years after the big bang. The ALMA image of SDP.81 is distorted into a ring by the gravity of another galaxy in the foreground, which acts like a distorting lens. You can read more about these unique ALMA observations in an earlier news story.

In the earlier story, it says: "Astronomers will now try to find out what the galaxy really looks like – without the distorting effect of the gravitational lens." And that's just what they've done. By taking into account the gravity of the foreground galaxy, they could reconstruct the true shape of SDP.81, just like your distorted selfie can be reconstructed by someone who knows the properties of the distorting mirror.

The result: SDP.81 appears to be a shapeless collection of giant clumps of cold gas. Some of these gas clouds are falling into the core of the galaxy, where they will probably turn into huge numbers of new stars. Thus, thanks to the eagle-eyed vision of ALMA, astronomers can now witness a very early stage of star formation in the history of the universe.

And there's a bonus. The analysis of the ring that was observed by ALMA reveals that the foreground galaxy must harbour a very large black hole in its core. It probably weighs in at 200 or 300 million times the mass of the sun.


SDP.81 was discovered a few years ago by the European Herschel Space Observatory. It is at a distance of 11.4 billion light years in the constellation Hydra the Water Snake. (Note that the earlier news story put the galaxy at a distance of 12 billion light years; since then, the distance estimate has been improved.) Because of the large distance, we see the galaxy as it looked some 2.4 billion years after the big bang. The light of SDP.81 is bend into a ring by the gravity of a foreground galaxy, at a distance of 'only' 4 billion light years.


Astronomy is teamwork. It's hard to say who 'reconstructed' the 'true' image of SDP.81. Many dozens of astronomers, in eight different teams, have worked on the problem. This shows how excited astronomers were about the original ALMA observations. The eight teams all reached more or less similar results. Four of the eight scientific publications have been accepted for publication, or have already been published in scientific journals; four others will likely be published later this year.