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.

CONTACT

Dr Ian Smail, Institute for Computational Cosmology, Dept. of Physics,
University of Durham, South Road, Durham DH1 3LE
Phone: (+44) (0)191 334 3605
FAX: (+44) (0)191 334 3645
e-mail: ian.smail@durham.ac.uk

Note: Dr Smail is in Hawaii observing until 7 April, but can be contacted by
e-mail.