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RSAA News of the Month: August 2006

Cosmic stocktake reveals what's left of big bang

 

An international team of astronomers involving researchers at the Australian National University have announced the completion of a large survey of the nearby Universe. Amongst their findings is that 20% of the normal matter in the Universe has already been turned into stars.

One of the most important goals for cosmologists is to find out where all the normal matter that was produced in the Big Bang is today, 14 billion years later. The new survey reveals that about 20% is in stars, a further 0.1% lies in dust expelled from massive stars (and from which solid structures like the Earth and ourselves are made), and about 0.01% is in super-massive black holes.

According to the survey leader, Dr Simon Driver at the University of St Andrews, the remaining material is almost completely in gaseous form lying both within and between the galaxies, and forming a reservoir from which future generations of stars may develop.

The cosmic inventory

"I guess the simplest prognosis is that the Universe will be able to form stars for a further 70 billion years or so after which it will start to go dark," said Dr Driver. "However, the Universe is definitely tightening its belt with a steady decline in the rate at which new stars are forming."

The survey data was released at the General Assembly of the International Astronomical Union in Prague on August 18, 2006. This 21st century database, aptly named the Millennium Galaxy Catalogue, was constructed from over 100 nights of telescope time at the ANU's Siding Spring Observatory in NSW and observatories in the Canary Islands and Chile. The final Catalogue contains over ten thousand giant galaxies, each of these containing 10 million to 1000 billion stars.

A selection of galaxy images from the Millennium Galaxy Catalogue

"Basically, we were able to determine how much matter is in stars through what was, in effect, a cosmic stocktake," said Dr Alister Graham. "By careful separation of a galaxy's stars into its central bulge component and surrounding disc-like structure we have found that the Universe has been remarkably even-handed, placing half of the stars in galaxy discs and the other half in galaxy bulges."

Example of a galaxy image decomposed into its bulge and disc components

Measuring the concentration of stars in each galaxy's bulge enabled the researchers to additionally determine each galaxy's central super-massive black hole mass. "Some of these are up to one million billion times more massive than the Earth," said Dr Graham. Once we had these masses it was then a relatively simple task of summing them up to determine what percentage of the Universe's matter is locked away in black holes at the centres of galaxies."

"Future facilities will allow astronomers to probe well beyond the nearby Universe and back into the distant past", reports Dr Graham. "We will be able to observe how galaxies and their black holes have evolved into what we see around us today."

Telescopes such as the international Giant Magellan Telescope (GMT) will enable astronomers to directly measure black hole masses in galaxies ten times further away than can presently be measured. With a mirror diameter ten times bigger than the Hubble Space Telescope's mirror, the GMT will be capable of peering right back to the epoch of the Universe's very first stars. "With this new telescope, the future looks bright, as soon will the past," said Dr Graham with a smile.

Other members of the research team include Paul Allen and Ewan Cameron of The Australian National University, Jochen Liske of the European Southern Observatory and Roberto De Propris of the Cerro Tololo Inter-American Observatory.

The Millennium Galaxy Catalogue consists of imaging data from the Isaac Newton Telescope at the Spanish Observatorio del Roque de Los Muchachos of the Instituto de Astrofisica de Canarias, and spectroscopic data from the Anglo Australian Telescope, the Australian National University's 2.3 m telescope at Siding Spring Observatory; the ESO New Technology Telescope, the Telescopio Nazionale Galileo, and the Gemini Telescope.

Financial support for this project was jointly provided through grants from the Australian Research Council and the United Kingdom's Particle Physics and Astrophysics Research Council.

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