INFO 15-1999: ISO sees the golden age of galaxy formation
4 November 1999The Milky Way is a fairly quiet galaxy now, but some thousands of millions years ago it was quite a different story. ESA's infrared space observatory, ISO, has taken pictures of the 'golden age' of galaxy formation, the epoch when galaxies were taking the shape we see now, and has unveiled more than a thousand very active young galaxies in which non-stop star-formation machines are at work. The results, being presented at a workshop at Ringberg Castle in Germany (8-12 November 1999), show that the ancestors of today's galaxies were much more active than hitherto thought, with many more stars being born.
The new results come from detailed analysis of the ISO 'deep surveys', observing programmes aimed at detecting the faintest and farthest objects ever seen at infrared wavelengths. In astronomy, looking far into space also means looking back in time; thus by observing distant galaxies, ISO was actually seeing how galaxies in today's universe, such as our own and its closest neighbours, looked about ten thousand million years ago. The goal, astronomers said, was to address the many unanswered questions about galaxy formation and evolution. For instance, did all galaxies form at about the same time or is there a continuous galaxy-making process going on in the Universe?
The first results from the ISO deep surveys, presented about a year ago, provided some early clues. ISO astronomers searched for signs of star formation in the distant Universe, since the birth of many stars means that a galaxy is undergoing a phase of intense evolution. What they found was that galaxies with intense star formation, and thus very actively evolving, were much more common in the past than they are now. ISO also suggested that previous estimates about past star formation were wrong: many more stars than previously thought were made in the ancestors of today's galaxies.
Earlier calculations had been based on surveys with optical telescopes, whose view is obscured by the dust created by stars during their life cycle. However, the dust becomes bright when observed with infrared telescopes, and it was precisely the obscuring dust that ISO used as a tracer of star formation. ISO demonstrated that young galaxies formed three to four times more stars than suggested by optical surveys. This is what ISO astronomers call 'the dusty ISO revolution'.
The new results, to be presented at a workshop (entitled 'ISO Surveys of a Dusty Universe') at Ringberg Castle organised by the Max Planck Institute of Astronomy, are the definitive analyses of the ISO surveys. About sixty astronomers, mostly from Germany, France, the UK, Italy and Finland, have compared the data from almost all the surveys, performed with two instruments and focused on six different regions of the sky, in the Southern and Northern hemispheres. These regions include the well-known Hubble Deep Fields (North and South), now seen with ISO at long infrared wavelengths.
"We can fully confirm our estimates of last year about star formation, and we can also be much more specific about the galaxies we have observed", Italian astronomer Dario Fadda said. "We have detected more than a thousand distant young galaxies, seen for the first time in the infrared. Many of them had been observed with optical telescopes, but the fact that they are bright in the infrared means that they are dusty, and hence that a lot of new stars are being born there. We are seeing galaxies evolving very quickly."
Fadda, from the Service d'Astrophysique at the Commissariat à l'Energie Atomique, CEA, Saclay (France), has worked with the data obtained by ISOCAM, the infrared camera on board ISO. The team still don't know the precise distance at which each galaxy lies, but using a sample of one hundred galaxies with measured distances, they believe they are seeing the epoch when the Universe was about two thirds its present age.
Galaxies behind the curtain
The results from the other instrument used for the deep surveys, ISO's photometer ISOPHOT, show the same scenario: a younger Universe in which galaxies are evolving very actively, with a lot of star formation. ISOPHOT focused on study of the Cosmic Infrared Background, a faint infrared glow that fills the entire Universe. This Infrared Background was emitted by all galaxies in the past, and in particular by the dust heated in star formation events.
The team - called the FIRBACK (Far InfraRed Background) consortium - working with ISOPHOT last year discovered the first objects that contribute to this infrared background: over two hundred distant galaxies undergoing a process of intense evolution.
Now they've found that there are many more galaxies 'pouring' energy into the infrared glow. These are either too faint or too far away to be seen as individual sources by ISOPHOT: astronomers detect these galaxies as brighter 'bumps' in the infrared background, as if hidden behind a curtain. The technical term is 'fluctuations' in the infrared background. Experts highlight this result as one of the most exciting to be presented at the Ringberg workshop.
"Studying these infrared background fluctuations in addition to the initial and important step of merely detecting this emission will bring unprecedented new insights into galaxy formation and evolution. This is similar to the spectacular breakthrough expected from the detailed studies of the Cosmic Microwave Background fluctuations which will be measured by ESA's Planck Mission." "The ISO results not only test the models of galaxy evolution, but also give new observational constraints on this process," said Hervi Dole, from the Institut d'Astrophysique Spatiale (France), a member of the FIRBACK team. "Now we can be sure that there was much more star formation than previously thought, and that the maximum for that star formation happened when the Universe was between three and one thousand million years old - assuming that the present Universe is fifteen thousand million years old. Afterwards, and until the present day, the star-forming rate decreased quickly".
The latest results from the ISOCAM and ISOPHOT deep surveys will be published soon in the scientific journal Astronomy and Astrophysics.
Footnote about ISO
The European Space Agency's infrared space observatory, ISO, operated from November 1995 to May 1998, almost a year longer than expected. An unprecedented observatory for infrared astronomy, able to examine cool and hidden places in the Universe, ISO made nearly 30 000 scientific observations.
For more information please contact:
ESA Public Relations Division:
Martin F. Kessler (ISO Project Scientist):
Tel: +34 91 813 1254
Journalists wishing to contact participants at the meeting 'ISO Surveys of a Dusty Universe' should contact the organiser of the workshop:
Dietrich Lemke, MPIA, Heidelberg, Germany
(also Principal Investigator for ISOPHOT)
Tel: + 49 (0) 6221 528 259
or, from 8-12 November, at:
MPG Tagungsstaette Schloss Ringberg
Tel.: + 49 (0) 8022 2790
Fax: + 49 (0) 8022 279259
Other science contacts:
Results from ISO's Camera (ISOCAM):
Dario Fadda, CEA Saclay, France
David Elbaz, CEA Saclay, France
Results from ISO's Photometer (ISOPHOT):
Hervi Dole, IAS, Orsay, France
Guilaine Lagache, IAS, Orsay, France
Jean-Loup Puget, IAS, Orsay, France