Astronomers from the University of Cape Town (UCT) and the South African Astronomical Observatory (SAAO) have played a leading role in a successful international cooperative programme to locate galaxies hidden on the far side of our own galaxy, the Milky Way. The project involved astronomers from Australia, the Netherlands and the US as well as South Africa, and made use of the 64 m diameter Parkes radio telescope in New South Wales, Australia.

The South Africa-based participants are UCT chair of astronomy Professor Renée C Kraan-Korteweg and SAAO astronomer Dr Anja Schröder, who have both been involved in the project since it started in 1997. They are two of the lead authors of the paper ‘The Parkes HI Zone of Avoidance Study’, published in the Astronomical Journal on February 9. The leader of the project is Professor Lister Staveley-Smith of Australia’s International Centre for Radio Astronomy Research, University of Western Australia node.

The Parkes instrument is equipped with an innovative receiver, which can scan the sky 13 times faster than previously. “Even so, the data gathering went on for various years,” points out Kraan-Korteweg. Altogether, the project detected 883 galaxies. “About half of which had never been seen before,” highlights Dr Schröder. Although they are relatively close in astronomical terms – about 250-million light years from earth – they were previously unknown because, being beyond the far side of the Milky Way from earth, they were hidden by our own galaxy. This hidden sector of space is called the Zone of Avoidance.

The importance of this research lies in the contribution it makes towards the understanding of a major and currently mysterious gravitational anomaly, known as the Great Attractor. This seems to be pulling hundreds of thousands of galaxies, including the Milky Way, towards it. In the case of our galaxy, this is at a speed of two-million kilometres an hour. The gravitational force exerted by the Great Attractor indicates that it has a mass equivalent to about one-million billion suns, resulting in it being described as a “mass overdensity”.

“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,” reports Kraan-Korteweg. “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.”

According to Kraan-Korteweg, these previously unknown galaxies are evidence that the Great Attractor is the result of a major supercluster near to the Milky Way. A supercluster is a large grouping of galaxies and clusters of galaxies and this one seems to cross the Milky Way diagonally. The team has detected a number of new astronomical structures that might help explain our galaxy’s movements. These include three galaxy concentrations (designated NW1, NW2 and NW3) and two new clusters (CW1 and CW2).

Kraan-Korteweg is an internationally recognised expert in researching the galaxy and mass distribution hidden behind the plane of our home galaxy. She has been working on these issues for 25 years now and has involved many of her students, particularly at UCT, in this research. Both she and Dr Schröder are busy with follow-up research, making use of data acquired from the SAAO’s Infrared Survey Facility, to analyse the movements of the detected galaxies to obtain improved estimates of the mass density in our local part of the Universe.