SA astronomers help discover hidden galaxies behind Milky Way

11 Feb 2016 - 09:45

Professor Renée C. Kraan-Korteweg, Chair of Astronomy at UCT, and her close collaborator, Dr Anja Schröder, who is working with the SAAO, are among the lead authors of “The Parkes HI Zone of Avoidance Survey”. The paper was published in the Astronomical Journal on Tuesday, 9 February 2016. They are working with first author Professor Lister Staveley-Smith, from the University of Western Australia node of the International Centre for Radio Astronomy Research (ICRAR).

 Hundreds of hidden nearby galaxies have been discovered for the first time, shedding light, amongst others, on a mysterious gravitational anomaly dubbed the Great Attractor. Despite being just 250 million light years from Earth – very close in astronomical terms – the new galaxies had been hidden from view until now by our own Galaxy, the Milky Way. Thanks to the 64m Parkes Radio Telescope (Commonwealth Scientific and Industrial Research Organisation), which was equipped with an innovative receiver that can scan the sky 13 times faster than before, the scientists were able to survey this hidden part of the universe much more efficiently. “Even so, the data gathering went on for various years,” said Professor Kraan-Korteweg.


An annotated artist's impression showing radio waves travelling from the new galaxies, then passing through the Milky Way and arriving at the Parkes radio telescope on Earth (not to scale). Credit: ICRAR

The discovery may help to explain the Great Attractor region, which appears to be drawing the Milky Way and hundreds of thousands of other galaxies towards it with a gravitational force suggesting it contains a mass of the order of one million billion suns: our whole Milky Way is moving towards the Great Attractor at two million kilometres per hour. Professor Staveley-Smith said: “The Milky Way is very beautiful, of course, and it’s very interesting to study our own Galaxy, but it completely blocks out the view of the more distant galaxies behind it.” Scientists have been trying to get to the bottom of the mysterious Great Attractor since major deviations from universal expansion were first discovered in the 1970s and 1980s. We don’t actually understand what’s causing this gravitational acceleration on the Milky Way or where it’s coming from.


An artist’s impression of the galaxies found in the ‘Zone of Avoidance’ behind the Milky Way. This scene has been created using the actual positional data of the new galaxies and randomly populating the region with galaxies of different sizes, types and colours. Credit: ICRAR

The team found 883 galaxies, “About half of which had never been seen before,” said Dr Anja Schröder, who scrutinised all available multi-wavelength imaging data for possible counterparts.

Professor Kraan-Korteweg said that the newly identified galaxies provide evidence that the mass overdensity called the Great Attractor is due to the existence of a major nearby supercluster – a large collection of galaxies and clusters of galaxies – that crosses the Milky Way diagonally. The research identified several new structures that could help to explain the movement of the Milky Way, including three galaxy concentrations (named NW1, NW2 and NW3) and two new clusters (named CW1 and CW2).

Professor Kraan-Korteweg, (pictured above) who is internationally recognised as leader in unveiling the galaxy and mass distribution behind the Plane of the Milky Way, said astronomers have been trying to map the galaxy distribution hidden behind the Milky Way for decades. It has been a focus of her research over the last 25 years, and she has involved numerous students (notably at UCT) in this type of research. She said: “We’ve used a range of techniques, including telescopes at the SAAO, but only radio observations have really succeeded in allowing us to see through the thickest foreground layer of dust and stars in the inner Milky Way. An average galaxy contains 100 billion stars, so finding hundreds of new galaxies hidden behind the Milky Way points to a lot of mass we didn't know about until now.” Professor Kraan-Korteweg mentions that such systematic surveys with radio telescopes are in a sense precursors to the much deeper surveys for neutral gas planned with the Square Kilometre Array precursors, such as MeerKAT.

Both Professor Kraan-Korteweg and Dr Schröder have been involved in this project since its inception in 1997. This included regular observing runs over many years with the Parkes Radio Telescope in Australia, data reduction, the actual search for signatures of the galaxies in the data, and presenting preliminary results at international conferences. Both were in charge of  preparing major parts of the paper and are working on follow-up analysis, in particular using near-infrared data obtained with the SAAO IRSF (Infrared Survey Facility) using the motions of the detected galaxies to get better estimates of the overall mass density in the nearby universe.