DM's Thesis - News articles


These are copies of articles published in news sources about my Ph.D. work.

Astronomy observations change view of early universe

University of Sydney News - Volume 22 No. 35 - 23 October 1990

Astronomers at the Anglo-Australian Telescope in New South Wales - including Dr Richard Hunstead, Senior Lecturer, and PhD student Mr David Mar, both of this University's Department of Astrophysics - have used a new instrument developed at University College London to probe hydrogen gas clouds near the edge of the observable universe. These clouds are believed to be in pristine condition since the time of the 'Big Bang' and may hold vital clues about the way galaxies formed, which remains one of the most intriguing problems in modern astronomy.

The clouds are much colder than previously thought - between five and ten thousand degrees rather than the 30 thousand degrees predicted by the favourite theories. The consequences of this difference are far-reaching and have already generated intense speculation as to the true nature of the clouds. They may be much denser than originally supposed and resemble long filaments or sheets. Alternatively, the low temperature may be an indication that the gas is expanding and cooling very efficiently. In either case it is clear that the new observations will change profoundly our view of the early universe.

The new instrument in operation at the Anglo-Australian Telescope at Siding Spring Observatory in NSW is an echelle spectrograph enabling astronomers to study spectra with both high resolution for detail and wide coverage across the spectrum. In conjunction with the spectrography, the team used the Image Photon Counting System, a very sensitive light detector. This combination of instruments represents a powerful tool for exploring conditions in distant hydrogen clouds.

The team looked at the spectra of quasars. Quasars are the most remote objects in the known universe at distances so great that, in some cases, the light now being received from them left the quasar twelve billion years ago. On its long journey through the universe, the quasar light passes through distant galaxies and clouds of intergalactic hydrogen gas. Each time a cloud is intersected, some of the light is absorbed, leaving a characteristic imprint - or absorption line - in the spectrum of the quasar. It is by measuring these absorption lines that astronomers can glean precious information on conditions prevailing in the universe's infancy. Were it not for the enormous energy output from quasars, these distant regions would remain hidden, even from the most powerful telescopes.

Astronomers refer to the part of each quasar spectrum that is particularly rich in absorption lines as the 'Lyman alpha forest.' The actual shapes of the lines give important clues to physical conditions in the clouds, such as the temperature. Until now, even the best available instruments gave only a blurred view of the lines that was not good enough to make a proper temperature measurement. In this study, the four astronomers recorded 79 Lyman alpha lines with a resolution twenty times better than most previous observations. Their startling discovery was that the lines are much narrower than expected from the popular theoretical predictions.

Results of the research appeared in the 15 October issue of the Monthly Notices of the Royal Astronomical Society.

Figure

Figure caption: A diagram of data received (above), illustrating how light is absorbed by Lyman alpha clouds. The team has now analysed the shapes of the absorption lines.

Far-away clouds yield their secret

The Australian - 14 November 1990, page 26

By DIANA THORP

A group of astronomers funded by the British and Australian Governments have broken new ground in the study of hydrogen clouds.

Based at the Ang1o—Australian Telescope in NSW, and using a new instrument that was developed at the University College, London, the group has been probing the hydrogen gas clouds near the edge of the observable universe.

These clouds are believed to have been left in pristine condition since the the time of the "Big Bang" and may hold vital clues about the way galaxies formed.

The team discovered that the clouds are much colder than previously thought - between 5000 and 10,000 degrees rather than the previously assumed 30,000 degrees.

This is significant because no one has ever measured the temperature of these clouds before, according to Mr David Mar, one of the team members and a member of the Department of Astrophysics at the University of Sydney.

"These clouds were previously thought to be very hot, large objects, but now we think that if the temperatures are less, then to have the same density that we observe they have to be very small," Mr Mar said.

The new instrument, an echelle spectrograph, enables astronomers to study spectra with both high resolution for detail and wide coverage across the spectrum.

In conjunction with this spectograph, the team used the Image Photon Counting System, a sensitive light detector.

The combination of these instruments results in a powerful tool for exploring the conditions in the distant hydrogen clouds.

They also allow the study of quasars, the most remote objects in the known universe, at distances so great that, in some cases, the light now being received from them left the quasar 12 billion years ago.

By the time the light reaches Earth, much of it has been absorbed by distant galaxies and clouds of intergalactic hydrogen gas. Each time a cloud is intersected, a characteristic imprint, or absorption line, is left in the spectrum of the quasar, and by measuring these absorption lines astronomers are able to establish conditions in the universe's infancy.


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