THE STORY OF GALAXY EVOLUTION - IN FULL COLOUR
Cambridge astronomers are solving long-standing puzzles about how galaxies evolve by studying colour images from the Hubble Space Telescope of very remote galaxies. They have found strong evidence that galaxies grow when smaller clumps of stars and gas merge. They have also discovered that the bar structures seen at the centres of some spiral galaxies formed relatively recently.
Professor Richard Ellis and Dr Roberto Abraham (Institute of Astronomy, Cambridge) and Dr Nial Tanvir (now University of Hertfordshire), together with Cambridge graduate students Jarle Brinchmann and Felipe Menanteau have developed techniques to study, for the first time, the information available from the "internal" colours of distant galaxies revealed by the Hubble Deep Field. They are using the colours of the structures in the galaxies to gather information about the type and distribution of stars within the galaxies. Since these galaxies are very distant, we see them as they were long in the past, typically half the present age of the universe. The results will be presented at the National Astronomy Meeting being held in Guernsey from the 10th to the 13th of August.
Background and new results:
Nearby galaxies come in three categories: spirals, characterised by their elegant swirling arms; ellipticals, which appear like smooth featureless collections of stars; and irregulars which have no symmetric structure. The Hubble Space Telescope is able to identify such galaxies to enormous distances [figure 1], corresponding to eras when the Universe was only half its current age. In earlier work, the team showed that many very distant (and hence young) galaxies do not resemble their present-day counterparts.
The team has now developed powerful techniques to study the colours of structures INSIDE the galaxies (structures such as nuclei, central "bulges", and spiral arms, all of which show strikingly different colours, as seen in figure 1) in order to probe the age and chemical makeup of these distant galaxies. The aim is to understand the physical processes that govern the evolution of galaxies as well as how the peculiar star systems detected in Hubble Space Telescope images have evolved into the well-defined galaxy forms we see today.
A popular theory claims that most elliptical galaxies formed from the collision of two spiral galaxies. In this scenario, the spiral arms are destroyed and the gaseous material is expelled or converted into stars. The Cambridge team has found striking evidence that distant ellipticals are varied in their internal colour properties [figure 2a], supporting the idea that merging has indeed taken place. This provides strong support for the theory that galaxies grow via the collisions of smaller structures.
Their work has also addressed the history of spiral galaxies, such as our own Milky Way. A long-standing puzzle is why some of these systems have bar-like features in their centres [figure 2b]. By counting the fraction of barred and non-barred galaxies existing at different eras in the past, the Cambridge team (together with Professor Mike Merrifield at the University of Nottingham) have provided the first evidence that bars may be a relatively recent phenomenon. Few bars are found in the most distant spirals seen more than 5 billion years ago. (The age of the universe is thought to be about 12 to 14 billion years.) This may be because bars develop as unstable features only when a spiral has grown to a certain size. This work has taken on added importance with the growing acceptance by astronomers that our own galaxy, The Milky Way, is also barred.
The future is bright --- the future is infrared
The next stage in understanding the evolution of galaxies will be directed towards studying the very faint peculiar galaxies discovered by the Hubble Space Telescope. A higher proportion of the galaxies in the deep Hubble pictures are of these amorphous types [labelled "peculiar" in figure 1] and astronomers have puzzled for several years over what has become of them at the present time. The make-up of these systems is difficult to study using the Hubble Space Telescope, because their peculiar appearance may arise partly from absorption by dust clouds in the galaxies. Fortunately a detailed understanding will soon become possible with the completion of two revolutionary new advances in ground-based telescope technology: the pair of Gemini telescopes, the first of which is now in operation in Hawaii, and one of its associated instruments called CIRPASS (the Cambridge Infrared PAnoramic Survey Spectrograph) now being built in Cambridge.
The Gemini 8-m telescope on Mauna Kea, Hawaii (an international facility involving the United Kingdom) has already demonstrated its remarkable ability to study distant galaxies at infrared wavelengths with a resolution comparable to that of the Hubble Space Telescope. Working in the infrared effectively allows astronomers to look through the dust clouds that block visible light, providing data on the age and other properties of these galaxies.
Gemini's unique capabilities (its enormous light-gathering power because of its giant 8-m mirror, combined with ultra high-fidelity images) will be complemented by the unique spectroscopic mapping capabilities of CIRPASS which will provide the next stage in the internal colour study of galaxies. CIRPASS has been designed to take maximum advantage of Gemini's capacity to resolve distant galaxies and thus allows the study of their individual components. The instrument will further break down these colours into hundreds of detailed spectra [figure 3] distributed across the visible portions of the galaxies. Each spectrum can be used to study the rotation and chemical make-up of galaxies on a point-by-point basis for galaxy samples to enormous distances --- including those which formed soon after the Big Bang.
CONTACTING THE PRESS ROOM and PRESS OFFICERS AT THE UK NATIONAL ASTRONOMY MEETING
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Images referenced in this release may be found on the Web at the following URL: http://www.ast.cam.ac.uk/~abraham/galaxies.html