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.
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: