Hot cluster wind robs galaxies of stellar building material 

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Suppose you carry a tray with a lot of Lego blocks. More than enough to build a large number of houses. You want to play with your friends, at the center of the city square. But it’s very, very windy there. By the time you reach the center of the square, the wind has blown away most of your Lego. There’s nothing left to build with anymore. 

That’s the situation some galaxies are in, according to new observations by ALMA. Galaxies do not build LEGO houses, of course. Instead, they carry large amounts of cold gas from which new stars can be ‘build’. But as soon as a galaxy joins its ‘friends’ in a large galaxy cluster, the wind in the cluster starts to blow away most of the cold gas. By the time the galaxy ends up in the central regions of the cluster, it has lost most of its cold gas. As a result, the formation of new stars comes to a halt. 

Astronomers have used ALMA to look for cold gas in the galaxies that are part of an extremely remote cluster. Because it’s so far away, we see the cluster as it was when the Universe was still rather young – just 4.4 billion years. Back then, the cluster was still 

forming: some galaxies had already settled at the cluster’s center, but new galaxies were still entering the outskirts of the cluster. 

The astronomers found that the galaxies in the cluster’s center are almost devoid of cold gas. Apparently, the gas in those galaxies has been blown away by the strong cluster wind – the flow of tenuous hot gas that blows throughout the cluster. In contrast, galaxies at the cluster’s edge still contain quite a lot of cold gas. That’s probably because they just start to enter the cluster. The hot wind has not yet been able to strip all the cold gas away. 

So for this remote cluster, the formation of new stars slowly came to a halt some 9 to 10 billion years ago. Or at least, the star formation activity became much lower than it had been before. The same may well be true for other clusters of galaxies. Maybe that’s the reason why star formation in the Universe has been dwindling down over the past 10 billion years or so. 

What? 

ALMA made observations of a cluster of galaxies known as XMMXCS J2215.9–1738. As the name implies, the cluster was first discovered by XMM-Newton, an orbiting X-ray observatory of the European Space Agency. The cluster is located at a distance of 9.4 billion light-years, in the constellation Aquarius, the Water Bearer. Because of this huge distance, the light from the cluster took 9.4 billion years to reach Earth. That means astronomers observe the cluster as it was 9.4 billion years ago, when the Universe was just 4.4 billion 

years old. Earlier observations by the Japanese Subaru Telescope at Mauna Kea (Hawaii) had already indicated that many new stars are being formed in the cluster’s member galaxies. The new ALMA observations have now revealed that star formation (from cold gas and dust) is mainly taking place in galaxies at the edge of the cluster, not at its center. 

Who? 

The ALMA observations of the remote galaxy cluster were carried out by a large team of Japanese astronomers, led by Masao Hayashi of the National Astronomical Observatory of Japan in Tokyo. The team published their new results in a paper in Astrophysical Journal Letters.

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