Looking back to the cosmic renaissance

5 October 2003

Six of the most distant galaxies ever seen have been discovered in a small area of the sky less than five per cent the size of the full Moon. Located approximately 12,600 million light-years away, we observe these galaxies as they were at a time when the universe was very young, less than about 10% of its current age. At this time the universe was emerging from a long period known as the “dark ages” and was entering the luminous “cosmic renaissance” epoch.

The discovery of these new galaxies, with redshift z between 4.8 and 5.8, by Matthew Lehnert from Germany and Malcolm Bremer from the UK, was made possible thanks to a deep-field observation with the Very Large Telescope (VLT) of the European Southern Observatory. Very distant galaxies (with a redshift z greater than five) have been discovered previously, but this is the first time that so many have been found together in a small area of the sky. The two astronomers conclude on the basis of their unique data that there were considerably fewer luminous galaxies in the universe at this early stage than there were 500 million years later. This suggests that the newly discovered sources are indeed some of the very first luminous galaxies after the Big Bang. Their spectra have revealed that at the time they emitted the light, these galaxies were actively forming stars and were probably no older than 100 million years, perhaps even younger.

Nowadays, the universe is pervaded by energetic ultraviolet radiation produced by quasars and hot stars. The short-wavelength photons liberate electrons from the hydrogen atoms that make up the diffuse intergalactic medium, and the latter is therefore almost completely ionized. There was, however, an early epoch in the history of the universe when this was not so. This epoch follows the recombination of the nuclei and electrons to form atoms, some 100,000 years after the Big Bang. The universe became transparent at that time and released the cosmic microwave background radiation we can still observe from all directions.

This was also the time when the universe plunged into darkness. On one side, the relic radiation from the primordial fireball had been stretched by the cosmic expansion towards longer wavelengths and trapped again by the newly formed hydrogen atoms. On the other side, no stars or quasars had yet been formed that could illuminate the vast space.

These “dark ages” were ended after a few hundred million years by the formation of the first generation of stars and, somewhat later, the first galaxies and quasars. Little by little their intense ultraviolet radiation re-ionized the intergalactic gas in steadily growing spheres around the ionizing sources. At some moment, these spheres had become so big that they overlapped, completely lifting the fog over the universe. This re-ionization epoch is sometimes referred to as the “cosmic renaissance”.

“Our findings show that the combined ultraviolet light from the discovered galaxies is insufficient to fully ionize the surrounding gas,” explained Malcolm Bremer. “This leads us to the conclusion that there must be many more smaller and less luminous galaxies in the region of space that we studied, too faint to be detected in this way. It must be these still unseen galaxies that emit the majority of the energetic photons necessary to ionize the hydrogen in the universe.”

“The next step will be to use the VLT to find more and fainter galaxies at even higher redshifts,” added Matthew Lehnert. “With a larger sample of such distant objects, we can obtain an insight into their nature and the variation of their density in the sky.”

Further reading

M D Lehnert and M Bremer 2003 ApJ 593 630.

Picture of the month

Mars completed almost half a rotation between these two observations, taken 11 hours apart with NASA’s Hubble Space Telescope (the left image was taken on 26 August 2003 at 23:00 UT and the right image on 27 August 2003 at 10:00 UT). These two sides of Mars were photographed as the red planet was making its closest approach to Earth in almost 60,000 years. (NASA, J Bell (Cornell University) and M Wolff (SSI).)


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