Stars cheek by jowl in the early Universe - RAS PN 09/4

The background image was taken by Dr Michael Hilker of the University of Bonn using the 2.5-metre Du Pont telescope, part of the Las Campanas Observatory in Chile. The two boxes show close-ups of two UCD galaxies in the Hilker image. These images were made using the Hubble Space Telescope by a team led by Professor Michael Drinkwater of the University of Queensland.

Friday, February 13, 2009

In our Galaxy, we are used to the idea that even the nearest stars are light years away from the Sun. But a team of scientists led by Professor Pavel Kroupa of the University of Bonn think things were very different in the early Universe. In particular, Ultra Compact Dwarf galaxies (UCDs), a recently discovered class of object, may have had stars a hundred times closer together than in the solar neighbourhood, according to calculations made by team member and PhD student Joerg Dabringhausen and presented in a paper in Monthly Notices of the Royal Astronomical Society.

UCDs were discovered in 1999. Although they are still enormous by everyday standards, at about 60 light years across, they are less than 1/1000th the diameter of our own Galaxy, the Milky Way. (In more familiar units, a light year is about 10 million million km). Astronomers believe that UCDs were created when more normal galaxies collided in the early Universe. But oddly, UCDs clearly have more mass than the light from the stars they contain would imply.

Up to now, exotic dark matter has been suggested to explain this ‘missing mass’, but this is not thought to gather in sufficient quantities within a UCD. In their paper Mr Dabringhausen, Professor Kroupa and their colleague Dr Holger Baumgardt present a different explanation.

The astronomers think that at one time, each UCD had an incredibly high density of stars, with perhaps 1 million in each cubic light year of space, compared with the 1 that we see in the region of space around the Sun. These stars would have been close enough to merge from time to time, creating many much more massive stars in their place. These more massive stars consume hydrogen (their nuclear fuel) much more rapidly, before ending their lives in violent supernova explosions. All that then remains is either a superdense neutron star or sometimes a black hole.

So in today’s UCDs, a good part of their mass is made up of these dark remnants, largely invisible to Earth-based telescopes but fossils of a more dramatic past.

Mr Dabringhausen comments, “Billions of years ago, UCDs must have been extraordinary. To have such a vast number of stars packed closely together is quite unlike anything we see today. An observer on a (hypothetical) planet inside a UCD would have seen a night sky as bright as day on Earth.”

Cosmologists "see" the Cosmic Dawn

(1) The Universe 590 billion years after the Big Bang
Credit: Alvaro Orsi, Institute for Computational Cosmology, Durham University.


(2) The Universe 1 billion years after the Big Bang
Credit: Alvaro Orsi, Institute for Computational Cosmology, Durham University.


(3) The Universe 1.9 billion years after the Big Bang
Credit: Alvaro Orsi, Institute for Computational Cosmology, Durham University.


(4) The Universe 13.6 billion years after the Big Bang
The Universe today.
Credit: Alvaro Orsi, Institute for Computational Cosmology, Durham University.

Tuesday, February 10, 2009

Scientists have used a computer simulation to predict what the very early Universe would have appeared like 500 million years after the Big Bang.

The images, produced by scientists at Durham University's Institute for Computational Cosmology, show the "Cosmic Dawn" - the formation of the first big galaxies in the Universe.

The Cosmic Dawn began as galaxies began to form out of the debris of massive stars which died explosively shortly after the beginning of the Universe. The Durham calculation predicts where these galaxies appear and how they evolve to the present day, over 13 billion years later.

The researchers hope their findings, which highlight star forming galaxies, will improve their understanding of dark matter - a mysterious substance believed to make up 80 per cent of the mass in the Universe.

Gravity produced by dark matter is an essential ingredient in galaxy formation and by studying its effects the scientists eventually hope to learn more about what the substance is.

The research is published in the Monthly Notices of the Royal Astronomical Society and was funded by the Science and Technology Facilities Council (STFC) and the European Commission.

The work combined a massive simulation showing how structures grow in dark matter with a model showing how normal matter, such as gas, behaves to predict how galaxies grow.

Gas feels the pull of gravity from dark matter and is heated up before cooling by releasing radiation and turning into stars.

The simulation images show which galaxies are forming stars most vigorously at a given time. Although the galaxies are biggest at the present day, the rate at which they are making new stars has dropped greatly compared with the rate in the early Universe.

The calculations of the Durham team, supported by scientists at the Universidad Catolica in Santiago, Chile, can be tested against new observations reaching back to early stages in the history of the Universe almost one billion years after the Big Bang.

Lead author, Alvaro Orsi, a research postgraduate in Durham University's Institute for Computational Cosmology (ICC), said: "We are effectively looking back in time and by doing so we hope to learn how galaxies like our own were made and to understand more about dark matter.

"The presence of dark matter is the key to building galaxies - without dark matter we wouldn't be here today"

Co-author Dr Carlton Baugh, a Royal Society Research Fellow, in the ICC, at Durham University, said: "Our research predicts which galaxies are growing through the formation of stars at different times in the history of the Universe and how these relate to the dark matter.

"We give the computer what we think is the recipe for galaxy formation and we see what is produced which is then tested against observations of real galaxies"

Professor Keith Mason, Chief Executive of the Science and Technology Facilities Council, said: "Computational cosmology plays an important part in our understanding of the Universe. Not only do these simulations allow us to look back in time to the early Universe but they complement the work and observations of our astronomers."