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A Glimpse into the Time Before Quasars Were Born

28. marraskuuta 1996

According to the widely accepted Big Bang theory, the first galaxies formed by gravitational accretion from slight irregularities in a primordial sea of matter, a process that required considerable time. Hence it would be expected that there would be a delay between the Big Bang and the appearance of the first galaxies. Or, looking back in time from the present, we would expect to find an epoch in the distant past when galaxies had not yet come into being.

The space density of quasars

An international group of astronomers 1 has now performed observations that seem to offer a glimpse into this very early period. They show that, when looking further and further into space and therefore successively farther back in time, the space density of quasars, after first increasing towards a maximum, then declines rapidly towards zero.

Quasars are thought to be the nuclei of active galaxies, that is galaxies in the process of formation or undergoing violent interactions with other galaxies. Such objects are extremely bright and they can be seen across the Universe. That is the reason why quasars, rather than the much fainter normal galaxies, have been used to study the distant Universe.

The possible existence of a rapid decline in the number of quasars as we look into the very distant Universe has been suspected for many years. Recent searches for distant quasars by means of optical telescopes observing their visible light have provided the strongest evidence. However, it has also been suggested that this decline could be merely due to obscuration caused by material in intervening galaxies - the distant Universe may be hidden from view.

Observations of radio sources

Radio waves are unaffected by dust, however, and many quasars are strong radio sources. Therefore, the group of astronomers from Germany, Great Britain, and the United States recently undertook a search for very distant quasars based on their radio emission.

This involved measuring accurate positions of hundreds of radio sources using two large radio telescopes, the Australia Telescope in New South Wales and the Very Large Array in New Mexico, and identifying them with visible counterparts using the European Southern Observatory's 3.6-metre telescope on La Silla, Chile.

The blue light from objects in the early Universe is known to be absorbed by atomic hydrogen in intervening gas clouds, and this provides a signature for very distant objects. Thus, the astronomers sought optical identifications of the radio sources which are visible only at the red end of the optical spectrum, and which are point-like in appearance (from this originates the term quasi-stellar objects, or quasars), in contrast to the fuzzy appearance of galaxies.

The most distant radio quasars

One of the identified objects had these characteristics, and indeed it was found to be the most distant radio quasar known. But no other radio quasars were found at greater distances, although they could easily have been detected by these observations if they were there.

Because all of the other radio sources were identified with more nearby quasars and galaxies, by a process of elimination there were simply none left that could be quasars at very large distances.

It was therefore possible to confirm that, beyond a distance corresponding to the time when the Universe was less than 10 percent of its present age (i.e. less than about 1-2 billion years old), the number of quasars decreases dramatically (see the figure) - a conclusion which is independent of any possible complications due to dust obscuration.

Looking into the Dark Ages

It thus appears that astronomers may now be seeing beyond the most distant quasars, and possibly galaxies, into the so-called Dark Ages when the first galaxies had not yet formed.

This provides further support for the Big Bang evolutionary cosmologies, according to which there should be a significant delay between the Big Bang and the appearance of the first galaxies. Detection of the pre-existing gas clouds out of which the galaxies formed at this early epoch will be one of the major challenges for astronomy in the years to come.

Publication of the results

These results will appear in a scientific article to be published in the December 5, 1996, issue of the journal Nature.

Lisähuomiot

[1] The group consists of Peter Shaver (European Southern Observatory, Garching, Germany), Jasper Wall (Royal Greenwich Observatory, Cambridge, UK), Ken Kellermann (National Radio Astronomy Observatory, Charlottesville, VA, USA), Carole Jackson (Institute of Astronomy, Cambridge, UK), and Mike Hawkins (Royal Observatory, Edinburgh, UK).

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