ISO unveils the hidden rings of Andromeda
15 October 1998The Andromeda galaxy, one of the closest and best-known companions of our own galaxy, has been hiding from the astronomers' eyes one of its secrets: although it has always been considered as a typical spiral galaxy, it has now been shown to be a spectacular ringed galaxy. This is one of the observations made by the European Space Agency's ISO infrared telescope, whose results are being presented at a meeting in Paris 20-23 October, attended by about 400 astronomers from all over the world.
"Observations such as the ringed skeleton of Andromeda confirm that ISO is unveiling a totally new face of the Universe, with its unique ability to peer into regions never seen before. ISO is making the invisible visible," said Martin Kessler, ISO Project Scientist.
Andromeda, at only two million light-years away, is a very well known galaxy, also called M31. But when observed in the infrared, what optical telescopes interpret as a big galaxy typically spiral-shaped, turns out to be structured in multiple concentric rings.
Moreover, a large fraction of the dust and most of the gas of the galaxy - roughly the same size as our own, the Milky Way - gathers in just one ring, the brightest. Many new stars are being born in this ring, which has a radius of 10 kiloparsec (30 000 light years, about the same distance as between the Solar System and the centre of our own galaxy, the Milky Way).
Why is it that these rings are hidden in the usual views of Andromeda? "Because they are made mostly of cold dust, and this kind of material cannot be seen by conventional telescopes; only by observing in the infrared do they become bright," explained Martin Haas, from the Max-Planck Institut in Heidelberg (Germany). The cold dust is indeed very cold, at almost -260 °C. This is considerably colder than previous estimates, which poses a new problem to astronomers. Also, scientists will have to explain the deficit of dust and star formation inside the ring.
"ISO is seeing for the first time the very cold dust pattern of Andromeda," Haas says. However, if his hypothesis is correct, the rings might in the distant future - but soon in astronomical terms - become visible to optical telescopes. This, at least, would be the most likely consequence if the star formation taking place in the rings keeps going on and on. "Andromeda might be undergoing a transition phase", says Haas, who has published his findings, jointly with colleagues from his institute and from the University of Helsinki, in this month's issue of the journal Astronomy and Astrophysics.