Observations with Herschel have revealed unprecedented views of a ring in the centre of our Milky Way galaxy. The ribbon of gas and dust is more than 600 light years across and appears to be twisted, for reasons which have yet to be explained. The origin of the ring could provide insight into the history of the Milky Way.
Professor Bruce Swinyard of STFC's Rutherford Appleton Laboratory said “Herschel’s detectors are ideally suited to see through the dust lying between us and the centre of our Galaxy, and to find the relatively cold material, at only 15 degrees above absolute zero, which we have learned makes up the ring”. The new results are published in a recent issue of the Astrophysical Journal Letters.
Warmer gas and dust from the Centre of our Galaxy is shown in blue in the above image, while the colder material appears red. The ring, in yellow, is made of gas and dust at a temperature of just 15 degrees above absolute zero. The bright regions are denser, and include some of the most massive and active sites of star formation in our Galaxy.
“Hints of this feature were seen in previous images of the Galactic Centre made from the ground, but no-one realised what it was.”, explained Dr Mark Thompson of the University of Hertfordshire. “It was not until the launch of Herschel, with its unparalleled wavelength coverage, that we could measure the temperature of the dust clouds and determine its true nature.”
The central region of our Galaxy is dominated by a bar-like structure, which stirs up the material in the outer galaxy as it rotates over millions of years and is thought to be responsible for its spiral structure. The ring seen by Herschel lies right in the middle of this bar, encircling the region which harbours a super-massive black hole at the centre of our galaxy. Professor Glenn White of The Open University and STFC's Rutherford Appleton Laboratory said that “although bars have been seen in other galaxies, this ring of cold material revealed by Herschel, and the way it twists around the Galactic Centre, were completely unexpected, revealing several surprises.”
Firstly, the ring of gas is twisted, so from our vantage point we see two loops which appear to meet in the middle. These are seen in yellow in the image above, tilted slightly such that they run from top-left to bottom-right. Secondly, it seems to be slightly offset from the very centre of our Galaxy, where a super-massive black hole lurks. "This is what is so exciting about launching a new space telescope like Herschel," said Sergio Molinari of the Institute of Space Physics in Rome, Italy, lead author of the new paper. "We have a new and exciting mystery on our hands, right at the centre of our own galaxy."
The reason for the ring’s twist and offset are unknown, but understanding their origin may help explain the origin of the ring itself. Computer simulations indicate that bars and rings such as those we see in the centre of our Galaxy can be formed by gravitational interactions. It is possible that the structures in the heart of the Milky Way were caused by interactions with our largest neighbour, the Andromeda Galaxy.
“Like all good science experiments, Herschel is creating as many questions as it answers”, said Professor Matt Griffin, of the University of Cardiff, and Principle Investigator on one of Herschel’s detectors used in this study. “Unravelling the mystery of this ring could help us to explore the processes which have taken place deep in the heart of our Galaxy over billions of years.”
