|Monday, 21 May, 2001, 15:33 GMT 16:33 UK
Astronomers reach the 'edge' of Universe
The SDF looks through uncluttered space
By BBC News Online science editor Dr David Whitehouse
Looking at a region of space where galaxies are scattered like coloured jewels, astronomers have obtained their most penetrating view of the cosmos yet.
The Subaru Deep Field (SDF) image, produced by the Japanese Subaru Telescope in Hawaii, goes almost to the edge of the observable Universe.
It was obtained by looking into space in a direction out towards our galaxy's north pole, a view that is relatively uncluttered with local stars and nearby galaxies.
The image shows large elliptical galaxies, faint galaxies of intense blue, and puzzling red objects thought to be youthful star systems gorged with dust.
Obtaining the SDF was one of the first tasks for the Subaru Telescope after it was commissioned in 1998. Its giant 8.2-metre (27-foot) mirror is probably the most accurate in the world. Astronomers have been delighted with the clarity of its images so far.
The Subaru Telescope is an exceptional piece of engineering
The SDF is a region about a degree in size, well away from the plane of our galaxy and very suitable for study from Hawaii as it passes almost overhead most nights.
Investigation of the SDF has revealed that the galaxies detected in the image, if spread throughout the cosmos, would account for more than 90% of all the galactic light in the Universe.
This is a different result from that obtained by the other well-known deep image of the Universe, the Hubble Deep Field (HDF).
The objects seen in the HDF, if typical of the Universe, would account for less than 90% of the Universe's light. The higher figure obtained by the SDF suggests that it is the deeper view of the cosmos.
The SDF shows some of the faintest galaxies ever observed, technically described as fainter than magnitude 24.5.
Some have delicate traces of structure, suggesting they are advanced spiral galaxies. This is puzzling since they are being seen soon after galaxies first formed. Some galaxies are clearly interacting with one another, and others are the bluest and the reddest objects ever detected in space.
Researchers used models of galaxy evolution to predict how many faint galaxies would be missed in other deep images of the sky. They discovered that the galaxies they detected in the SDF image accounted for more than 90% of the total near-infrared light from all the galaxies in the Universe along this line of sight.
It is now believed that Subaru is seeing almost to the edge of the observable Universe and that there is very little extra light to be detected from even fainter galaxies.
However, although the Subaru observations can account for almost all of the light emitted by galaxies in the Universe, measurements from satellites show that the total amount of background light scattered through the cosmos is three times larger.
It is becoming clear, researchers say, that there is a great deal of light in the Universe that cannot have come from normal galaxies and for which, as yet, we have no adequate explanation.
The scientists say that resolving this discrepancy is an important task for astronomy.