‘Hole in the ALMA sky’ is produced by hot cluster gas
New discoveries

‘Hole in the ALMA sky’ is produced by hot cluster gas

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Do you see the large ‘hole’ in this ALMA image? It looks as if someone cut a circular hole in a blue veil, with a large cluster of galaxies right behind the hole. But appearances can be deceiving. In fact, the ‘blue veil’ originates behind the galaxy cluster. It’s the so-called cosmic background radiation – the faint remnant from the energy of the Big Bang. This background radiation is reaching us from 13.8 billion light-years. The galaxy cluster is much closer: less than 5 billion light-years. So, what’s going on?

The effect that causes the ‘hole’ was first predicted almost 50 years ago by two Russian astronomers: Rashid Sunyaev and Yakov Zel’dovich. That’s why it’s called the Sunyaev-Zel’dovich effect. Because those names are so hard to pronounce, everyone just calls it the SZ effect. Here’s how it works.

The cosmic background radiation is observed at millimeter and submillimeter wavelengths – precisely the wavelengths that ALMA can see. The photons of the background radiation (the individual particles of light) have very little energy. To observe them, ALMA uses ‘receivers’ that are sensitive to those low energy levels. So far, so good.

But something funny happens when the radiation passes through a cluster of galaxies. In between the galaxies in the cluster is a lot of very hot gas. The photons of the cosmic background radiation interact with charged particles in the hot gas. As a result, they get an energy boost, as if they are kicked in the butt. So, when they leave the cluster again, on their way to Earth, they have a much higher energy then when they entered.

Since ALMA is only observing at low energies, it cannot see these ‘kicked’ photons anymore. That’s why there appears a hole in the ALMA observations. In the image, the ALMA data (the ‘blue veil’) are combined with a Hubble Space Telescope photo of the cluster. The cluster neatly coincides with the ‘hole’, just as you would expect.

Astronomers are excited about these new observations. It’s the first time that the SZ effect has been observed by ALMA. By studying the effect, it’s possible to learn more about the distribution and the properties of the hot gas in the cluster, even though ALMA cannot observe this gas directly.


The galaxy cluster in the image is called RX J1347.5-1145. It consists of many hundreds of individual galaxies. The cluster is located at 4.8 billion light-years, in the constellation Virgo the Virgin. The Sunyav-Zel’dovich effect of this cluster had already been measured by other telescopes, but the ALMA observations are much more sensitive and show more detail.


The ALMA measurements of the SZ-effect in this cluster were carried out by a very large team of Japanese astronomers, led by Tetsu Kitayama. To achieve a large field of view, Tetsu and his colleagues didn’t use the full array of 66 ALMA antennas, but the smaller Morita Array, which is part of ALMA. The Morita array (also known as the ALMA Compact Array) consists of just 12 antennas, each with a diameter of 7 meters (instead of 12 meters for the other 54 antennas). The results have been published in October 2016 in the Publications of the Astronomical Society of Japan.