Infrared Space Observatory Sees Stars Forming
A team of astronomers using the orbiting Infrared Space Observatory (ISO) have detected evidence to show that stars formed at a prodigious rate in some of the remotest known galaxies in the universe. ISO was used to make infrared images of galaxies in the so-called Hubble Deep Field, which are up to 10 billion light years away. The radiation from these galaxies has taken so long to reach Earth, they are being seen now as they were shortly after they formed.
The details are being announced by Professor Michael Rowan- Robinson of Imperial College, London, at a Specialist Meeting of the Royal Astronomical Society on 'The Hubble Deep Field' on Friday 8 November 1996. (The meeting is taking place at the Scientific Societies Lecture Theatre , 23 Savile Row, London W1, from 10.30am-3.30pm.) The Imperial College astronomers suggested making the ISO observations after predicting that these galaxies would be emitting large amounts of infrared radiation, a characteristic sign that star formation is occurring.
Professor Rowan Robinson says, 'We believe we are seeing the main episode of star formation in some of these galaxies, during which most of the heavy elements in a galaxy, the stuff we are made of, is being manufactured'. Stars are being formed at an estimated rate of 10 to 1000 solar masses per year, much faster than the current star formation rate in our own Galaxy, which is just one solar mass per year. At such rates, the entire mass of stars in a typical galaxy could be made in about 1 billion years.
The Hubble Space Telescope obtained the 'Deep Field' image in December 1995, when it scanned a tiny area of sky (5 square arcminutes - about the size of Venus) 150 times to capture the faintest galaxies ever seen through four different light filters in the ultraviolet, visible and near infrared wavelength bands (0.3- 0.9 microns). The images were released to researchers in January and groups around the world have been working to try to unravel its secrets.
Redshifts of some of the brighter galaxies in the field have been measured with the Keck 10-metre telescope on Mauna Kea, Hawaii. However for the fainter galaxies in the field, astronomers have to use the colours of the galaxies to deduce their redshift and hence their distance. As a distant galaxy's spectrum is redshifted due to the expansion of the universe, its colours change in a predictable way. Analyses of this kind have been carried out by several groups, including the group at Imperial College led by Prof. Michael Rowan-Robinson. These studies have found two remarkable things about the Hubble Deep Field galaxies. Firstly they appear to be at very high redshifts, ranging up to 4, and with 50 % of them having redshift > 2. This means that over half the galaxies are seen no more than 1-3 billion years after the Big Bang and so are very close to the moment of their birth.
The second remarkable aspect of the Hubble Deep Field galaxies is that most of the fainter galaxies have the very blue coloration, which is a characteristic of galaxies undergoing strong star- formation. It seems that we are seeing the galaxies going through their first major episode of star formation, when most of the elements of which we are made (carbon, nitrogen, oxygen, iron etc) were formed inside stars. If this picture is correct the IC group predicted that the galaxies should be strong emitters of infrared radiation, as this is a characteristic of strong bursts of star formation. In the local universe most of the light from newly forming stars is absorbed by dust grains and re-emitted in the infrared. Their prediction was that the galaxies will be putting out ten times as much energy at far infrared wavelengths than in the optical and ultraviolet.
This is where ISO, the European Space Agency's Infrared Space Observatory, launched in November last year and already producing impressive results about the infrared sky from wavelengths 3 to 200 microns, comes in. As soon as they began to see the implications of the Hubble Deep Field, the IC group formed a consortium with other European collaborators and put in a bid to the Project Scientist of ISO, Martin Kessler, to carry out a survey of the Hubble Deep Field with ISO at its most sensitive wavelengths, 6.7 and 15 microns. The bid was successful and the observations were carried out in July this year, the data arriving at IC during August.
The maps at both 6.7 and 15 microns show several clear sources of infrared radiation, which can be associated with optical galaxies in the Hubble field. Of the seven best cases, two are probably just infrared radiation from red giant stars in the galaxies. For the other five, Prof. Rowan-Robinson and his collaborators interpret the infrared radiation as emission from a dust shrouded 'starburst', a tremendous burst of star formation.
Illustrations availableFig 1: False colour image of the ISO 15 micron map of the Hubble Deep Field area. The white spots are the detected infrared sources.
Fig 2: Optical image of the Hubble Deep Field from the Hubble Space Telescope (HST).
Fig 3: Mosaic of the area around the Hubble Deep Field, showing 12 'flanking' fields mapped by HST to lower depth and the positions of the sources detected in the ISO survey.
These images are available via the WWW at the following location:
Hard copies of the images may be obtained on request from Tom Miller at the Imperial College Press Office
Coinvestigators on the ISO Survey of the Hubble Deep FieldProf. L. Danese, SISSA Trieste; Prof. A. Franceschini, Padua Observatory; Dr E. Egami, Max-Planck-Institut fuer Extraterrestriche Physik, Munich; Dr M. Kontizas, University of Athens; Prof A. Lawrence, Institute for Astronomy, Edinburgh; Dr R. McMahon, Institute of Astronomy, Cambridge; Dr B. Mobasher, IC London; Dr H. Noorgaard-Nielsen, Danish Space Research Institute, Copenhagen; Dr S. Oliver, IC London; Prof M. Rowan-Robinson (Principal Investigator), IC London; Dr I. Perez- Fournon, Instituto de Astrofisica de Canarias, Tenerife; Dr J. I. Gonzalez-Serrano, Instituto de Fisica de Cantabria, Santander.
ISO Data Analaysis Team at Imperial College:
Dr N. Eaton, Dr A. Efstathiou, Dr P. Goldschmidt, Dr R. Mann, Dr S. Oliver (Project Scientist), Dr S. Serjeant, Dr T.Sumner (Project Manager).
ContactProf. Michael Rowan-Robinson, Astrophysics Group, Blackett Laboratory, Imperial College ( 0171-594-7530)
Issued jointly by
Tom Miller, Press Office
Imperial College of Science, Technology and Medicine, London
Phone: (0)171-594-6701; Fax: (0)171-594 6700
Dr Jacqueline Mitton, RAS Public Relations Officer