Cosmologists spy on galaxies being born, and conclude that these massive stellar assemblages might have formed earlier than believed
star-gazers have hit pay dirt. The fog shrouding the mystery of galactic births in universe is gradually giving way to the onslaughts of astronomers' telescopes, which have recently caught sight of galaxies that were in the act of formation. Two groups of researchers, working independently in the us and uk , have found evidence of the existence of distant galaxies almost as old as the universe itself, suggesting that galaxies might have formed fairly early in the history of the universe.
"These distant galaxies could contain the key to the process of galaxy formation and show us what our own Milky Way galaxy may have looked like as it was forming," say scientists from the University of Durham, uk , who recently published their findings in Nature (Vol 383, No 6597). The Durham research team consisted of scientists from the Durham Cosmology Group, which has been active in the deep imaging (photographing distant gala xies) and galaxy count (counting number of objects in a galaxy) fields since 1976.
From clumps to dumps The authors of the Nature report have analysed what they claim to be the 'deepest astronomical pictures' of the faintest galaxies ever observed from ground- and space-based observatories. The ground-based pictures came from the British/Dutch/Spanish 4.2-metre (m) William Herschel Telescope in the Canary Islands and from the 3.8-m uk Infrared Telescope ( ukirt ) on Hawaii, whereas the space-based pictures were generated by the Hubble Space Telescope ( hst ). Light from these galaxies in the pictures has taken 10 billion years to reach the earth. This is close to the estimated age of the universe and hence, these deep pictures are representing galaxies as they appeared early in the history of the universe. In this sense, say the authors, they are like "time machines" that allow us to see deep into our past. They capture galaxies in the process of formation and enable us to test the cosmological theories of galaxy formation. Some of these theories suggest that galaxies formed early in the history of the universe, and others, that they formed much later and continue to form till date. The present findings lend credence to the former theory, say the authors.
Similar observations, based on images obtained from the hst , had been published a fortnight before ( Nature , Vol 383, No 6595) by a team of us scientists. Hubble's images revealed a grouping of 18 gigantic star clusters pictured as they were about 11 billion years ago (since light took that long to reach the earth), a period when it is commonly believed that galaxies started to form. According to the authors, these clusters constitute the "building blocks" of galaxies under construction. "We've never seen so many of these objects in a single exposure and so small," says S M Pascarelle from the department of physics and astronomy, Arizona State University, Arizona, one of the authors. "We are convinced that these objects are not peculiar, but a part of the general formation process of galaxies in early universe." These results, the authors say, add weight to a leading theory that galaxies grew by starting out as clumps of stars, which, through a complex series of encounters, consolidated into the larger assemblages that we see today.
Hubble revelations Astronomers can observe the birth of stars because star formation is an ongoing process. However, they have never directly observed the phenomenon of galactic formation, because it may have happened a long time ago, or because galaxy formation is not as spectacular as was once believed. "The observations from the hst offer some of the best direct visual evidence to date," says Pascarelle. "It is the first time anyone has seen that many star-forming objects in such a small space (separated by two million light years across). There are not nearly as many such luminous objects in the two million light years separating the Milky Way galaxy from the Andromeda Nebula, the nearest major galaxy," says Roger Windhorst, another member of the us research team.
hst shows a new level of detail for determining the true nature of these "pre-galactic blobs". These were seen in a two-day exposure by hst of a small region of sky in the northern part of Hercules constellation. Hubble resolved clumps as small as 2,000 light years across, each consisting of about a billion young stars. "We think that by repeated merging, they will grow big enough to become the bulges of nearby galaxies," says W C Keel, another member of the us team. "In fact, at least four of the objects in this field show double structure in their centers only a few thousand light years apart, as if we've caught them in the act of falling together," he points out.
According to Windhorst, though many of the objects found in the hst pictures appear isolated, they are close enough together in space to eventually merge. He sketches a scenario where two or more objects will pass through each other, drawing out hydrogen gas to form more stars later. Although the term collision is used, their individual stars do not collide. They may then evolve to form the numerous faint blue galaxies, as seen by Hubble and other telescopes. Later, the surrounding hydrogen gas settles into a disk to form a spiral galaxy.
If this construction plan is correct, our Milky Way galaxy contains all the pieces of the assembly process. The older, redder stars in the Milky Way's central bulge came from the merged clusters or "sub-galactic units". According to Pascarelle and his team, the spiral arm that our sun inhabits was made later after hydrogen settled into a disk. Some of the 140 globular star clusters which orbit the Milky Way may be the 'leftover' smaller building blocks which formed before the large units observed by these researchers, but were never pulled directly into larger assemblages.
Red and blue hold the clue
It is known for some time now that stars and galaxies are blue when young, and turn red with age. Faint and out of bounds for even the most powerful telescopes, it is very difficult to get detailed spectroscopic information about some of these galaxies. However, by taking pictures in blue, red and infrared light, the colours of the fainter galaxies can be obtained. Using this approach, astronomers have found that as they probe deeper into the universe, the galaxies turn more blue. Consequently, the observed galaxies -- faint blue in colour -- have attracted the attention of scientists trying to delve deeper into the mysteries of the genesis of galaxies.
Another important factor in the observation of early galaxies is the redshift. The redshift measures the speed at which stars and galaxies are moving away from us. Therefore, the higher the redshift, the farther away is the object being observed. The high redshift values of the objects observed by the American and British teams show that they were much farther removed from us than any any other object known so far. The accuracy of the measurement of redshift value is of crucial importance. The latest deep pictures, according to the Durham researchers, offer new clues to the redshift of faint blue galaxies and allow much more accurate measurements of the colours of these galaxies.
The biggest stumbling block in providing a satisfactory resolution through telescopic observations to the question of when galaxies formed, is the fact that about 90 per cent of the universe is filled with 'dark matter', invisible to any telescopes.
The dark matter is believed to be 'cold', meaning that its random motions are slower than the speed of light. In this scenario, galactic structures tend to form from the bottom up; that is, the smallest clumps of stars form first and merge together to form larger galaxies. In contrast, if the dark matter would have been 'hot' -- made up of particles that move nearly at the speed of light -- then only the largest structures could have condensed; smaller structures such as galaxies and star clusters would have formed later from fragments within these larger structures.
Says Windhorst, "Neither scenario can be completely correct because we now know that these sub-galactic objects already existed a very long time ago. They must have formed shortly after the Big Bang. But also, some structures on the scales of superclusters may have condensed out of the primeval soup as shortly as one million years after the Big Bang, leading to the seeds of largescale structure in the universe. Perhaps a hybrid model is necessary, but mostly leaning towards the cold dark matter theory. The idea that small clumps grow into bigger ones is very effective. It explains a lot of things, but it doesn't quite explain the existence of largescale structures early on."
Durham researchers, however, assert that the cold dark matter theory no longer holds good. "The theory suggests that galaxies form by collisions or mergers of previous generations of galaxies and that this process could continue right until the present day. With little spectroscopic information on the faint blue galaxies, it was difficult to get the 'redshift' information which could solve this problem. What little redshift information there was at relatively bright magnitudes indicated that the faint blue galaxies lay at relatively low redshifts and hence that galaxies had formed only recently, as predicted by the cold dark matter model.
But the new pictures show that the faint blue galaxies are likely to lie at much higher redshift than previously thought and, therefore, tie the epoch of galaxy formation to an earlier date in the history of the universe. Thus, the intriguing question about when galaxies form now seems to be answered: this was when the universe was only 20-30 per cent of its present age. The Durham team avers that this is much earlier than what is expected in the standard version of the cold dark matter model.
Notwithstanding the 'dark matter' -- cold or hot -- both the groups have provided convincing evidence that galaxies may have formed much earlier than thought so far, though the question of how they have formed still remains to be fully answered. It may take some more time and further investigations before these theoretical issues are finally resolved. But it seems the beginning of the end is clearly in sight.