INFO 24-1998: ISO sees the pattern in the cosmic wallpaper
24 July 1998
The European Infrared Space Telescope (ISO) finds a new population of galaxies, the first objects known to contribute to the background glow of the universe. The discovery will help to solve the mystery of galaxy formation.Two dozen distant galaxies that are undergoing a process of intense evolution --either merging to build larger ones or reaching their final shape - have been detected by a team of French astronomers using the European Space Agency's ISO space telescope. These are the first individual objects known to contribute their energy to the bulk of the Cosmic Infrared Background, a radiation that fills the entire universe like a wallpaper, emitted at the era when galaxies were formed. The new-found distant galaxies are indeed like the 'pattern' in this 'cosmic wallpaper'.
This discovery will for the first time enable scientists to test different theories of galaxy formation, and therefore to tackle a key problem of astronomy - the birth process of galaxies which has remained a mystery so far, mainly because current telescopes cannot reach that far back in time, about 12 000 million years ago.
This obstacle has been partially overcome by the French team headed by Jean-Loup Puget, at the Institut d'Astrophysique Spatiale in Paris, precisely because they searched for the primeval galaxies by focusing first on the study of the cosmic wallpaper, the 'Cosmic Infrared Background' (see note to editors below). This background glow that fills the whole universe is a by-product of the galaxy formation itself, a relic of the era when the first galaxies were being born. Its existence was predicted three decades ago and it was known to be detectable only at infrared wavelengths, as the dust enshrouding young galaxies causes them to be both opaque in visible light but bright at infrared wavelengths. However, this cosmic wallpaper turned out to be very dim: only two years ago Puget's team detected it after a careful analysis of data from NASA's COBE satellite.
Once the 'Cosmic Infrared Background' had been found, the next step was to disentangle it into the sources contributing to it, that is, into the young galaxies in evolution or the 'pattern' in the wallpaper.
The two dozen distant galaxies that Puget's team now report to have found are extremely faint. Their detection could only be attempted by using the spectrophotometer ISOPHOT on-board ESA's ISO, the best and most sensitive infrared space telescope so far.
Commenting on this discovery, Puget said: "Astronomers didn't expect too many sources at first, but although we have confirmed our prediction of a large number of galaxies we know also that these objects are only the tip of the iceberg. Using our method and with more sensitive instruments, especially the FIRST satellite now planned by ESA as ISO's successor, many more new sources will come out."
When searching for the Cosmic Infrared Background as well as for the individual galaxies contributing to it, in both cases the most difficult task the team had to face was to subtract all the infrared emission coming from foreground sources, such as the dust in our own Galaxy and the Solar System.
"It was like trying to hear a bird singing behind the noise of heavy traffic," explains Puget. One of the most surprising implications of the finding of the Cosmic Infrared Background - recently confirmed by three other groups of astronomers examining the COBE data - is that many more stars than previously known did form in the early universe.
Dark Age of the Universe
To identify the faint sources contributing to the infrared background glow demanded development of a specific method. First, the French team ran computer models to infer the populations of galaxies required to produce the observed background, then they defined the characteristics of an infrared survey tailored to detect that predicted population. That's how the search with the ISO satellite was started. "The spectrophotometer ISOPHOT on board ISO is the only instrument nowadays capable of detecting faint sources at the wavelengths we needed, that is, beyond 100 microns," Puget points out. The area of the sky they surveyed, called the Marano Fields, is in the southern hemisphere; it is well studied at other wavelengths and especially clean of the 'noise' from foreground sources.
The detection of the new 24 far infrared sources proved the method successful. Using it, astronomers will probably be able to peer into the 'dark age' of the universe, a period more than 12 000 million years ago beyond the reach of the current telescopes, when the first galaxies were formed. This discovery shows that the exploration of this period needs to be done at infrared wavelengths.
But there's still much work to be done, such as finding many more sources contributing to the Cosmic Infrared Background and establishing the precise age of the new population of galaxies found by ISOPHOT.
"We need to find them by visible light if we can, and measure their distances and ages. What we do know is that these galaxies at the range now detected contribute less than 12 of the Cosmic Infrared Background, which means that the largest contribution comes from fainter sources we have not yet caught. We have very hard and long work ahead," indicates Puget.
