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Astronomers see birth cluster of galaxies in early universe

An international team of astronomers has discovered a large reservoir of hot gas in the cluster-in-formation around the Spiderweb Galaxy. Based partly on that hot gas, the astronomers predict that the cluster-in-formation will grow into one of the largest objects in the universe. A step closer to discovering when the first clusters formed, says Leiden astronomer George Miley. Their findings were published in Nature.

Clusters can contain thousands of galaxies. Between the galaxies is the so-called intracluster medium. That hot gas has more mass than the galaxies themselves. The presence of hot gas in clusters-in-formation has been predicted by cosmological simulations for more than a decade, but observations that could confirm the prediction were lacking.

Research led by Luca di Mascolo (University of Trieste, Italy) on the Spiderweb protocluster now clearly shows that there must be hot gas between the galaxies. The Spiderweb cluster is located more than 10 billion light years from Earth in the Universe that was only 3 billion years old at the time.

Shadow reveals mass and shape

The astronomers discovered the intracluster medium via the so-called thermal Sunayev-Zeldovich effect. This effect occurs when light from the cosmic background radiation - the residual radiation from the Big Bang - passes through the intracluster medium. When interacting with the fast-moving electrons in the hot gas, this light gains some energy and changes its colour, or wavelength, slightly. At certain wavelengths, this causes a cluster of galaxies to cast a shadow of sorts over the cosmic background radiation.

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By measuring these shadows using the telescopes of the ALMA observatory, astronomers were not only able to deduce the existence of the hot gas, but also to estimate its mass and map its shape.

The researchers found that the Spiderweb protocluster contains a huge reservoir of hot gas with a temperature of several tens of millions of degrees Celsius. The mass of the hot gas now detected is thousands of times larger than the cold gas previously discovered. The discovery strengthens the expectation that the Spiderweb protocluster will have grown into a heavy cluster of galaxies in about 10 billion years.

Forty years

Leiden emeritus professor George Miley co-authored the publication on the Spider Web Cluster. 'More than forty years ago, we first observed the iconic cluster with the Westerbork telescope. Later, Hubble showed us its spiderweb-like appearance. And now we have discovered the hot gas with ALMA.'

Miley added: 'And it's not finished yet. The LOFAR telescopes are currently scanning the sky in search of even more Spiderweb cluster-like objects. I expect that at some point we will discover when the first clusters formed.'

Scientific paper

Forming intracluster gas in a galaxy protocluster at a redshift of 2.16. Luca Di Mascolo et al.

This article appeared as a press release on the website of NOVA.

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