ESA Science News

05 Nov 99

ISO sees the golden age of galaxy formation

The ancestors of today's galaxies revealed

The 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 ŕ 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 Hervé 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.

USEFUL LINKS FOR THIS STORY

  • ISO science homepage
  • FIRBACK survey homepage

    IMAGE CAPTION:

    This is the FIRBACK Marano Field, in the Southern Hemisphere, as seen by the instrument ISOPHOT on board ISO. Several ISO 'deep surveys' were performed within the FIRBACK international programme. As a result, the FIRBACK team discovered the first objects known to contribute their energy to the infrared background of the Universe: more than 200 distant galaxies undergoing a process of intense evolution. Some of them are seen as white points in the image. The diffuse brighter 'bumps' in the image are also galaxies, but they are either too far away or too faint to be resolved by ISOPHOT; however, knowing that they are indeed there is already considered to be a great achievement by the scientists.
    Credits: ISO/ISOPHOT FIRBACK team.

    ROYAL ASTRONOMICAL SOCIETY

    Date: 22 August 1997

    INFRARED SPACE OBSERVATORY PROBES OLDEST GALAXIES

    A team of astronomers led by Professor Michael Rowan- Robinson from Imperial College, London, has used the European Space Agency's Infrared Space Observatory (ISO) to obtain the longest exposure ever made of the 6.7-micron- wavelength infrared radiation from a single patch of sky. The images reveal remote galaxies giving out 10 to 100 times more infrared radiation than visible light. These powerful infrared signals show that new stars are being created within these galaxies at a phenomenal rate. The total rate of star formation in the universe 7 billion years ago implied by these results is considerably higher than has been estimated previously from studies in visible light. Such a result confirms the suspicion that there was much more star formation activity then than now. It seems that ISO has captured young galaxies at the peak of their star-forming ability.

    A total of 13 hours of observations have been added together for this long exposure, the last observations being obtained just a few weeks ago in July. The very latest results from the observations are being presented by Dr Seb Oliver, of Imperial College, at the XXIIIrd General Assembly of the International Astronomical Union (IAU) in Kyoto, Japan, on 25 August.

    The new images show galaxies in their early stages of development when star formation was much more rapid than it is today. Their radiation has taken so long to travel to us, we see them as they were 7 billion years ago. Many of the newly forming stars are very massive, but they are hidden in dense shrouds of dust. The light from the stars heats the dust and the galaxies appear much brighter in infrared telescopes than they do in optical telescopes, such as the Hubble Space Telescope (HST). "Some of these galaxies are 10 to 100 times more luminous in the infrared than the optical," said Professor Rowan-Robinson. "On average perhaps 80% of the star formation is hidden from normal view and is only visible in the infrared."

    The Imperial College team members concentrated their observations on the area of sky known as the Hubble Deep Field because the Hubble Space Telescope looked at it for about 10 days in December 1995. The patch of sky is about the same as the apparent size of the planet Venus (5 square arcminutes). One of their objectives was to use ISO's unique sensitivity in the infrared to discover more about the distant galaxies in the Hubble Deep Field, which we now see as they were when they were so young that they were still in the process of being assembled. To achieve this, the team has been carrying out a survey of this tiny segment of sky at the infrared wavelengths to which ISO is most sensitive - 6.7 and 15 microns.

    Following a preliminary report at a meeting of the Royal Astronomical Society on 8 November 1996, the team repeated its observations of the Hubble Deep Field at 6.7 microns in order to obtain maps of even fainter objects and study more galaxies at this interesting wavelength. The new observations were made in July 1997 in the last few days that the field was visible to ISO.

    After collecting ISO observations equivalent to a total exposure lasting 13 hours, the Imperial College team and their collaborators have found many additional galaxies. There are far more galaxies than they expected taking into account the normal amount of infrared radiation emitted by ordinary stars and galaxies like our own Milky Way. The extra infrared emission is a sign of dramatic bursts of star formation taking place deep inside clouds of gas and dust. These findings indicate that galaxies emitting strongly in the infrared were either much more common or more luminous in the past than they are now.

    Matching the galaxies seen by ISO with the galaxies detected in visible light (for which distances are either known or have been estimated on the basis of their colours) allows the distance to each galaxy to be determined. This, in turn, makes it possible to estimate their luminosities and the rate at which they are forming stars.

    A particular surprise was the prodigious rate at which stars were being born in these galaxies - from 8 to 1000 solar masses per year - compared with about 1 solar mass per year in our Milky Way galaxy today. Many of the galaxies at redshifts between 0.5 and 1 (at the limit of ISO's sensitivity) are undergoing dramatic episodes of star formation known as 'starbursts'. The total rate of star formation in the universe at around a redshift of 1 implied by these results is considerably higher than has been estimated previously from studies at optical wavelengths. It confirms suspicions that there was much more star formation activity at these epochs than at the present time, but that much of that activity is hidden from optical telescopes by large quantities of dust.

    NOTES

    ESA's Infrared Space Observatory was launched in November 1995, and is scanning the infrared sky t wavelengths from 3 to 200 microns. ESA has recently announced that the satellite has been using up its supply of liquid helium coolant - which maintains the telescope's temperature close to absolute zero - much more slowly than anticipated. ESA has, therefore, extended its mission until April 1998.

    Complementary to these very deep observations of the Hubble Deep Field, the group at Imperial College is also leading a much larger project (the largest single project of the ISO mission) to study many more of these dusty, star forming objects at slightly more modest distances. The galaxies seen by this European Large Area ISO Survey (ELAIS), a project involving 19 European institutes and partly funded by the European Commission, will help paint the picture of how galaxies have changed between the epoch of the distant objects seen by ISO in the Hubble Deep Field and the present day. Preliminary results from this project will also be presented at the IAU meeting. So far over 1000 infrared galaxies have been found in this survey.

    Images are available on Web pages at:

    http://artemis.ph.ic.ack.uk/hdf

    and

    http://artemis.ph.ic.ac.uk/hdf/newimages.html.

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