SA astronomer leads new galaxy discovery

Read time 3min 40sec
The Vela supercluster had gone unnoticed due to its location behind the plane of the Milky Way.
The Vela supercluster had gone unnoticed due to its location behind the plane of the Milky Way.

Professor Ren'ee C Kraan-Korteweg, a South African astronomer from the University of Cape Town, has led a team of international astronomers to discover a previously unknown major concentration of galaxies in the constellation Vela, which they have dubbed the Vela supercluster.

Earlier this year, Kraan-Korteweg, together with another team, played a big part in making a ground-breaking discovery by peering through the stars and dust of the Milky Way with a radio telescope to discover hidden galaxies beyond.

Superclusters of galaxies are the largest and most massive known structures in the universe. They consist of clusters, embedded in wall-like structures of galaxies that span up to 200 million light-years across the sky. The most famous supercluster is the Shapley Supercluster, some 650 million light-years away. It is believed to be the largest of its kind in our cosmic neighbourhood.

In the latest discovery, Kraan-Korteweg led a team from SA, the Netherlands, Germany and Australia to discover another major supercluster, slightly further away in distance (800 million light-years), which covers even more sky than Shapley.

The Vela supercluster had gone unnoticed due to its location behind the plane of the Milky Way, where dust and stars obscure background galaxies, resulting in a broad band void of extragalactic sources. The team's results suggest the Vela supercluster might be as massive as Shapley - and, given the relative proximity in space of these major structures, this might actually pose a cosmological conundrum, the astronomers say.

The discovery was based on multi-object spectroscopic observations of thousands of partly obscured galaxies. Observations in 2012, with the refurbished spectrograph of the Southern African Large Telescope, confirmed eight new clusters reside within the Vela area. Subsequent spectroscopic observations with the Anglo-Australian Telescope in Australia provided thousands of galaxy redshifts and revealed the vast extent of this new structure.

Despite her decade-long quest of charting the galaxy distribution hidden by the Milky Way, Kraan-Korteweg says: "I could not believe such a major structure would pop up so prominently" when she and colleague Dr Michelle Cluver of the University of the Western Cape analysed the spectra almost as soon as the photons hit the spectrograph.

Cluver says: "As I looked at each new spectrum, it became obvious we were uncovering a massive network of galaxies, extending much further than we had ever expected."

Following the discovery of this substantial structure, the team assessed its cosmological impact. They say such huge galaxy concentrations attract matter through their immense gravity, and shape the pattern of cosmic flows on enormous scales.

"Despite its large distance from us, the Vela supercluster might have a significant influence on the motion of our local group of galaxies, which includes the Milky Way, and could help resolve some puzzles in the observed flows of galaxies around," says co-researcher Dr Maciej Bilicki.

But there is still much to do, the researchers say, adding that further follow-up observations are needed to determine the full extent, mass and influence of the Vela supercluster. So far, this region of the sky is sparsely sampled, while the part closest to the Milky Way has not been probed at all because dense star and dust layers block our view, they note.

They believe the central core of the supercluster can only be mapped with dedicated radio surveys that can penetrate this "Zone of Avoidance".

It is here that the South African Square Kilometre Array Pathfinder, MeerKAT, can pave the way, says Kraan-Korteweg. "We have proposed to use this powerful radio telescope in early-science mode when 32 of its total of 64 dishes are in place during 2017 for a systematic search of the fully hidden core of the supercluster."

Meanwhile, the supercluster's outer edges will be mapped with the innovative new instrument Taipan, which will start science operations in 2017. Taipan includes an innovative starbugs optical fibre positioner and a purpose-built spectrograph.

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