Mystery of Galaxy Formation May be Solved
A team of astronomers at Durham University believe they have solved the long-standing mystery of how galaxies formed. Using new theoretical calculations, they have concluded that observations with the largest telescopes in space and on the ground have uncovered the long sought-after population of primeval galaxies - galaxies seen in the throes of birth.These young galaxies are so distant that their light has taken over 90 percent of the age of the Universe to reach us. The Durham astronomers have combined these observations with others of less distant galaxies to come up with a complete description of how these young collections of thousands of millions of stars evolved into mature galaxies like our own Milky Way.
They picture a scenario in which embryo galaxies start to form from the matter in the early universe which is full of exotic elementary particles dubbed 'cold dark matter'. These embryo galaxies often collide and merge, changing in appearance from spiral to elliptical.
The work has been done by Professor Carlos Frenk with colleagues Dr Carlton Baugh and Dr Shaun Cole of the University of Durham, and Dr Cedric Lacey of the Theoretical Astrophysics Centre, Copenhagen, Denmark. Dr Baugh will present these new results on Thursday 10th April at the UK's National Astronomy Meeting at the University of Southampton.
New data from the refurbished Hubble Space Telescope, the giant Keck Observatory in Hawaii, and other large telescopes are now providing unprecedented quantitative information about the universe's population of galaxies - numbers, luminosities, colours, shape, structure and star formation rates - over a large span of cosmic time. These data are beginning to sketch out a picture of galaxy formation and evolution from the time when the universe was only 1 billion years old or about 10 percent of its current age.
On their own, the observations paint only a patchy picture. Professor Frenk explains, 'It is rather like having a snapshot of different individuals at selected stages of development - say a baby, an infant, a young teenager and an old man - and then trying to piece together a picture of genetic evolution from this information. Clearly, this is only possible if one has an understanding of the basic biological processes at work. The situation is quite similar in cosmology. We have snapshots of galaxies at various evolutionary stages and we try to make a theory of galaxy formation and evolution by interpreting these data in the light of what we know about the physical processes at work.'
The major breakthrough that has unravelled the enigma of galaxy formation is the discovery last year of so-called primeval or proto- galaxies. These are embryo galaxies caught at a very early stage of their life cycle, when they are forming stars in profusion for the first time. This discovery was made by an Anglo-American team of researchers using some of the best and largest telescopes in the world.
They first identified candidate protogalaxies by measuring the colours of faint images using the British William Herschel Telescope in the Canary Islands. The tell-tale signs are the blue colour of young stars, together with the a missing chunk in the spectrum of their light, absorbed by intervening gas clouds during its long journey from the early universe to terrestrial telescopes. Some of these candidate proto-galaxies were then reobserved with very long exposures at the 10-m Keck Telescope in Hawaii, the largest telescope in the world. The detailed data clearly revealed that these are the long-sought after primeval galaxies. The same technique was subsequently applied to data from the Hubble Deep Field, the longest exposure ever made of a small patch of sky. Many more primeval galaxies were found in this way, covering a range of look-back times.
The team of UK astronomers at Durham University, together with their collaborator in Denmark, were amongst the first to interpret the significance of this discovery. Armed with a theory of the evolution of the early universe, they compared their predictions with the Keck and Hubble data. The new data confirmed a prediction they had made two years earlier regarding the way in which the population of stars builds up in the expanding universe.
This theory is based on the assumption that the universe is full of exotic elementary particles (cold dark matter) which provide the cosmic gravity. Under the influence of this gravity, clumps of matter - the embryo galaxies - grow bigger over time and are able to pull gas into themselves. This gas is initially very hot, but it cools down inside the 'dark matter halos' and subsequently turns into young stars. The embryo clumps have the properties inferred from the ripples in the cosmic background radiation discovered in 1992 by NASA's COBE satellite.
Computer calculations by the Durham team followed in detail the growth of embryo galaxies. They were able to calculate the rate at which gas would cool and the rate at which new stars would form. This is a difficult calculation because underlying these processes is the highly chaotic evolution of the dark matter background, which sets the stage for galaxy formation. The original embryo galaxies evolve in this chaotic background, often running into one another and merging together into larger and larger galaxies. In the process, they transmute their appearance from spiral galaxies like the Milky Way, to elliptical galaxies like the central galaxy of the Virgo cluster, and then perhaps back to rejuvenated spiral galaxies.
The observational data, equivalent to snapshots of galaxies at different evolutionary stages, fit in exactly with these theoretical calculations. According to the calculations, the observations have now captured the formation of 95 percent of the stars that have ever come into being in our universe. The remaining 5 percent still remain to be discovered, hidden away in the 'cosmic dark ages', when the universe was less than a billion years old. But it seems that the history of the universe over the past 11 billion years or so, and the way in which large galaxies like our own Milky Way came into existence, is no longer a mystery.