Merging Galaxy Groups and Clusters in the Shapley Supercluster

One of the nation's most sensitive radio telescopes, ASKAP (owned and operated by Australia's national science agency, CSIRO), has recently been used to observe radio emissions in perhaps the largest known gravitationally-bound structure in the Universe, once again proving the importance of the technologies that will be used by the SKA over the next decade or so. Reviewing the data, astronomers found that the radio emissions are acting as a bridge between a cluster and a group of galaxies in the Shapley supercluster.

Before explaining further, you might want to know the difference between galaxy groups and galaxy clusters. The explanation is quite simple. An aggregation of tens of galaxies is referred to as a group, while clusters can contain thousands of galaxies.

Indeed, our own home the Milky Way galaxy is itself part of a larger group, known as the Local Group, that consists of around 80 other galaxies. The Local Group is dominated by the Milky Way and the Andromeda galaxy, each having its own subsystems of orbiting dwarf galaxies.

Relatively close-by to us (at around 54-million light years distant) is the Virgo cluster, our closest galaxy cluster and one consisting of perhaps up to 2,000 member galaxies. Both groups and clusters of galaxies are held together by gravity, with galaxy clusters the largest known structures in the Universe that are bound in this way. Clusters may be part of superclusters, but the gravitational attraction of structures within massive superclusters is not enough to counter the expansion of the Universe trying to rip them apart.

Now we get to the Shapley supercluster. This enormous structure was mapped by Harlow Shapley in the 1930s (it wasn’t named until late in the 20^th century) through an exhaustive survey of the southern skies conducted using photographic plates. In all Shapley and his collaborators reported the discovery of 76,000 bright galaxies. The Shapley supercluster itself appears as an oval cloud of more than 8,000 galaxies about 650-million light-years away in the constellation of Centaurus and has a mass of at least 10-million-billion suns.

As it happens, the Shapley supercluster probably is gravitationally bound, supercluster moniker or not. If so, it is the single largest bound structure in the Universe that we know of. Not that you would bet on not finding something even bigger.

Recently there has been a surge in interest in the supercluster mostly because of its dynamic properties and the influence it has on our own Local Group and the Milky Way. It appears all galaxies in our region of the Universe are moving towards the Shapley supercluster. In fact, it is suspected that the supercluster may be able to account for half of a sought-after region in space called Great Attractor.

It also helps that recently our radio telescopes have improved to the point where they are far more sensitive than ever and able to discriminate the finest details in structures millions of light-years away. There’s more interest in Shapley now than ever before because we can finally see it in high definition.

And so, astronomers pointed Australia’s ASKAP radio telescope, South Africa’s MeerKAT radio telescope, and India’s Giant Metrewave Radio Telescope towards the Shapley supercluster to see if they could find evidence of galaxy mergers in the central region of the supercluster. The radio emissions they detected were the tell-tale signs of a so-called minor merger, the merger of two groupings of galaxies of relatively low, and similar, masses.

“The emission was triggered by the collision of these separate groupings of galaxies,” said co-author of the article published in Astronomy & Astrophysics, Professor Andrew Hopkins from AAO Macquarie. “Despite its difficulty to detect, this unique emission will now allow astronomers to better study the regions between clusters of galaxies.”

The astronomers were also able to see evidence of ram pressure stripping in the space between galaxies. When galaxies move through clusters, they experience the gas in intergalactic space as a wind – and one that causes their own gas to be stripped away.

“Ram pressure stripping can have a profound impact on the evolution of galaxies, removing the cooler gas that helps with star formation. This case shows that ram pressure stripping can involve both warm gas and radio-emitting plasma, and highlights the role of cluster-cluster interaction in triggering it,” said Professor Hopkins.

The data delivered by the ASKAP and MeerKAT telescopes provide a taste of the wealth of information and discoveries that we can expect to see over the next decade. Construction of SKA, the world’s largest radio telescope, gets underway in both Australia and South Africa this year, and galaxy evolution is one of the science goals of the project.

CSIRO acknowledges the Wajarri Yamatji as the traditional owners of the Murchison Radio-astronomy Observatory site where ASKAP is located.