Gravity bends light of distant galaxy into a ring
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Have you ever looked at yourself in a distorting mirror? It's fun! Depending on the shape of the mirror, you will appear to be very thin, or extremely short and fat. You can also watch the reflection of your face in a spoon. It looks weird!
Every curved mirror creates a distorted view. But the same is true for a strongly curved lens. You may have seen photos taken with a wide-angle lens, or with a so-called fisheye lens. Straight lines appear to be curved in the photo, and everything is a bit distorted.
Last year, using ALMA, astronomers have made a strange 'photo' of a very distant galaxy which is also strongly distorted. Not because they used a curved mirror or a funny camera lens, but because the distant galaxy was observed through a 'cosmic lens'.
The galaxy is really far away: some 12 billion light-years. But between the distant galaxy and the Earth is another galaxy, at a distance of only some 4 billion light-years. The image of the distant galaxy is distorted by the gravity of the nearer one. Astronomers call this effect 'gravitational lensing', for obvious reasons.
In the case of the galaxy SDP.81, the gravitational lensing effect is extremely strong. The light of the distant galaxy is bent all the way into a ring. The ring has been imaged by ALMA in extremely high detail. This was possible because the ALMA antennas were very far away from each other during the observations – up to 15 kilometres. In such a 'long baseline' configuration, ALMA achieves its sharpest possible vision.
When looking at ALMA's image of SDP.81, it's good to realize that the distant galaxy is not ring-shaped at all. Astronomers will now try to find out what the galaxy really looks like – without the distorting effect of the gravitational lens.
You might think that gravitational lenses are a nuisance to astronomers because of the distortions they create. But in fact, they're a boon: because of the gravitational lens, the galaxy looks brighter, and it can be studied in much more detail.
The distant galaxy SDP.81 was discovered a few years ago by the European Herschel Space Observatory. It is also known as HATLAS 090311.6+003906 – the numbers refer to its position in the sky, in the constellation Hydra the Water Snake. The galaxy contains a lot of cold dust; ALMA detected the millimetre and submillimetre radiation of this dust. SDP.81 is so far away that its light took 12 billion years to reach us. As a result, we see the galaxy as it was 12 billion years ago, when the Universe was in its infancy.
The ALMA observations of SDP.81 were carried out by a large international team of astronomers, led by Catherine Vlahakis. Catherine is ALMA's Deputy Program Scientist. The new results will be published in Astrophysical Journal Letters.