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Galaxies of stars shrouded in dust found in the early universe

March 31, 2003

A team of astronomers based in the UK and the US has for the first time measured the redshifts of a significant sample of puzzling "submillimetre galaxies", discovered by some members of the team in 1997. Dr Ian Smail of the University of Durham will tell the UK/Ireland National Astronomy Meeting that these are remote galaxies with high redshifts, and are likely to contain huge numbers of young stars heavily enshrouded by dust. Because of the time it takes light to travel, they are seen how they were when the universe was only one fifth its present age.

Until now the nature of submillimetre galaxies has remained an enigma. Astronomers detect them at the rate of one a night with the Submillimetre Common User Bolometer Array (SCUBA) on the 15-m James Clerk Maxwell Telescope (JCMT) located on the 4,000-metre-high volcano Mauna Kea on Hawaii. To date, more than 100 have been identified. They appear very bright at submillimetre wavelengths but their extreme faintness in the optical and near-infrared parts of the spectrum means that very little has been found out about them. One possibility was that these are galaxies of relatively modest luminosity at similar distances to the optically-bright galaxies that dominate pictures of the extragalactic sky, such as the Hubble Deep Field. Alternatively, they could lie at far greater distances and be intrinsically much more luminous. It has even been suggested that some of the objects might not be galaxies at all but very cold, very faint structures within our own galaxy.

To measure the redshifts of submillimetre galaxies, astronomers needed to obtain spectra of their visible light but until recently they had been deterred by the extreme faintness of these objects and the difficulty of pinning down their exact positions. But now the team of Ian Smail, Rob Ivison (UK Astronomy Technology Centre, Edinburgh), Scott Chapman and Andrew Blain (both of the California Institute of Technolgoy) has measured redshifts for a large sample of submillimetre galaxies by using the LRIS-B spectrograph on the Keck-I 10-m telescope on Mauna Kea. They focused on the extreme blue end of the visible spectrum and identified strong emission lines in the spectra of many submillimetre galaxies. This made it possible to secure accurate redshifts for a statistically significant sample of submillimetre galaxies for the first time and increased nearly tenfold the number of submillimetre galaxies with known redshifts.

On the basis of this new, large sample of data the team have concluded that a typical submillimetre galaxy lies at a high redshift, with a look-back time equivalent to 80% of the age of the universe. That puts them at a much earlier epoch in the history of the universe than optically-bright galaxies seen in deep images of the sky, and their high luminosities suggest that they contain vast numbers of young stars concealed by dust. The total number of stars formed in this population of submillimetre galaxies is comparable to or greater than the numbers of stars in optically-bright galaxies at these epochs. Many of the old stars we see in the universe around us today were probably formed in such galaxies long ago.

Royal Astronomical Society (RAS)