Mapping the Milky Way

A new map of the centre of our own galaxy, The Milky Way, is the biggest, most detailed, and most sensitive yet made. The map shows giant streamers and huge clouds of interstellar gas where stars are being born 26,000 light years from Earth - shedding new light on the exotic structures in this unusual region of our own galaxy.

The new map of gas and dust clouds in the Galactic centre, with the position of Sagittarius A* marked.

An international team of astronomers, led by groups from Cambridge University in the UK and the Joint Astronomy Centre (JAC) in Hawaii, has analysed the cold interstellar dust (particles similar to soot and very fine sand) in the centre of the Milky Way to map the clouds of molecular gas out of which new stars are formed.

Douglas Pierce-Price of Cambridge University said, "This breathtaking survey is bigger and more sensitive than any previously made. It is the first detailed map to show essentially all the interstellar gas in the galactic centre."

Cambridge Team Leader Dr John Richer added, "These observations were the result of many months of careful planning. But luck also played its part - during the observations we experienced the driest, clearest weather that I have seen in more than ten years of observing at the telescopes in Hawaii."

The unprecedented detail in the map reveals not only dense clouds of gas, but also a wide network of wispy filaments linking them. The region is also peppered with gas bubbles and shells. Although the origin of these structures is not fully understood, astronomers believe they must have been shaped by intense winds from stars, twisted magnetic field lines, and the explosions of supernovae.

The team used 'SCUBA', a state of the art camera built by the UK Astronomy Technology Centre (Royal Observatory, Edinburgh), together with the 15-metre 'James Clerk Maxwell Telescope' (JCMT) in Hawaii.

Wayne Holland, who led the group in Hawaii, said, "SCUBA is a wonderful instrument, and the most sensitive camera of its type. Its unmatched mapping speed has revolutionised this field of astronomy." Holland added, "Over a two year period we spent 15 nights at the telescope making these observations. SCUBA's successor, SCUBA-2, could do the same job in less than half an hour, so mapping the entire galactic plane becomes a real possibility."

The physical conditions in the central nucleus of our Galaxy are dramatically different from those nearer the Earth. The gas and dust clouds in what is known as the `Central Molecular Zone' are much denser and more turbulent. At the very centre is a mysterious object called Sagittarius A*, thought to be a supermassive black hole, which has a mass about 2.6 million times that of our Sun.

If the nucleus of our own galaxy is so complex, then what of other much more active galaxies such as the 'starburst' galaxy M82? Group member Jane Greaves said, "In M82 we have already used SCUBA to detect giant regions of star formation surrounded by complex magnetic fields. We now need to fully unravel its mysteries using even more sensitive instruments."

As other galaxies are much further away, they cannot yet be mapped with as much detail as this survey. However, the new Atacama Large Millimetre Array (ALMA) telescope consisting of 64, 12-meter radio telescopes to be built in the Atacama Desert, Chile, will enable UK astronomers to see at least 100 times more clearly to the nuclei of galaxies at the extremes of the Universe. ALMA will start construction in 2002.

Notes for editors

The centre of the Milky Way is about 26,000 light years from Earth, in the constellation of Sagittarius. Dust between the galactic centre and Earth blocks visible light, so the region must be observed using light of other wavelengths. The survey used sub-millimetre wavelengths to detect the cold dust - particles similar to soot and very fine sand - mixed in with the molecular gas.

SCUBA

The Submillimetre Common-User Bolometer Array (SCUBA) was built by the UK Astronomy Technology Centre (ATC) at the Royal Observatory Edinburgh, and is currently the world's most powerful "submillimetre-wave" camera. It has been in operation on the JCMT for about 4 years, and has revolutionised our knowledge of many areas of astronomy. The instrument contains highly sensitive detectors called bolometers, which are cooled to 0.06 degrees above absolute zero (-273 degrees Centigrade) to make them super-sensitive to the incoming sub-millimetre waves.

Submillimetre astronomy is still relatively new with only a tiny fraction of the sky having been so far been studied at these wavelengths. Although SCUBA is highly sensitive it has a limited field-of-view, which makes mapping extended areas like the Galactic Plane very time-consuming. Currently in a design phase is a next generation camera (called "SCUBA-2") which will be able to map a thousand times faster than the current SCUBA. SCUBA-2 will be able to carry out wide-field surveys of the submillimetre sky, and will be very complementary to facilities such as the ALMA telescope, which will study detail on very small size-scales.

ALMA

ALMA (the Atacama Large Millimetre Array) will be the most powerful telescope array ever built which operates at millimetre wavelengths: it builds on the technical and scientific success of existing millimetre wave telescopes, such as the JCMT, but will have 100 times better angular resolution and greatly improved sensitivity. It is being designed and built by a multinational consortium, including the UK, ten other European countries, the US, Canada, Chile and potentially Japan; it will be the largest international collaboration in astronomy ever undertaken. ALMA will be built on a plateau in the Andes, in the Atacama Desert of northern Chile, at an altitude of 5000m. It will be complete by 2010, when it will allow astronomers from the UK and around the world to make the first detailed images of dust and gas throughout the Universe, including images of the birth of stars and galaxies.