Distant galaxies shed light on the reionisation of the universe, but raise further questions


Astronomers from the UK and Germany have discovered some of the most distant galaxies ever seen, about 12,600 million light years from Earth. The discovery was made with the Very Large Telescope, the European Southern Observatory's (ESO) facility in Chile, and is one of the first significant discoveries by British scientists since the UK became a member of ESO in July 2002.

It has taken the light from the galaxies about nine-tenths of the age of the Universe to cover the huge distance. We therefore observe those galaxies as they were at a time when the Universe was very young, less than 10% of its present age. At this time, the Universe was emerging from a long period known as the "Dark Ages", entering the luminous "Cosmic Renaissance" epoch.

Unlike previous studies which resulted in the discovery of a few, widely dispersed galaxies at this early epoch, the present study found at least six remote citizens within a small sky area, less than five percent of the size of the full moon. This find has enabled astronomers to understand the evolution of these galaxies and how they affect the state of the Universe in its youth.

In particular, the astronomers conclude on the basis of their unique data that there were considerably fewer luminous galaxies in the Universe at this early stage than 500 million years later.

There must, therefore, be many less luminous galaxies in the region of space that they studied, too faint to be detected in this study. It must be those still unidentified galaxies that emit the majority of energetic photons needed to ionise the hydrogen in the Universe at that particular epoch.

The lead astronomer from the UK, Malcolm Bremer of the University of Bristol explains,

"Our findings show that the combined ultra violet light from the discovered galaxies is insufficient to fully ionise the surrounding gas. 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 unseen galaxies that emit the majority of the energetic photons necessary to ionise the hydrogen in theUniverse."

Malcolm continues, "The next step will be to use the VLT to find more and fainter galaxies at the higher redshifts. With a larger sample of such distant objects, we can obtain insight into their nature and the variation of their density in the sky."

The observations were made using the FORS2 multi-mode instrument on the 8.2m VLT Kueyen telescope at the ESO Paranal Observatory in Chile.

Professor Richard Wade from Particle Physics and Astronomy Research Council, who fund the UK subscription to the European Southern Observatory said:

" In joining the European Southern Observatory UK astronomers have been granted access to world leading facilities, such as the VLT. These exciting results, of which I am sure there will be many more to come, illustrate how UK astronomers are contributing to cutting edge discoveries."

Malcolm Bremer and other astronomers at the University of Bristol have shown through other work that the light from galaxies such as these results from stars as opposed to massive black holes. Black holes were the other potential source of ionising radiation but their research confirms that it is starlight that reionises the Universe. Further details about this aspect of the research is to be published at a later date.

Contacts

Gill Ormrod - PPARC Press Office
Tel: 01793 442012. Email: gill.ormrod@pparc.ac.uk
Malcolm Bremer
Department of Physics, University of Bristol,

H. H. Wills Physics Laboratory,
Tyndall Avenue, Bristol, BS8 1TL, U.K.
Tel: 0117 9288764
email: M.Bremer@bristol.ac.uk

Matthew Lehnert
Max-Planck-Institut fuer extraterrestrische Physik,
Postfach 1312, 85741 Garching
email: mlehnert@mpe.mpg.de

Notes to Editors

1. This is a coordinated ESO/PPARC release

2. The results described in this press release appear in the research journal Astrophysical Journal ("Luminous Lyman Break Galaxies at >5 and the Source of Reionisation" by Matthew.D.Lehnert and Malcolm.Bremer). August 20th, Vol 593, No2, Part 1. This paper can be viewed at:- http://www.journals.uchicago.edu/ApJ/journal/contents/ApJ/v593n2.html

3.This work was carried out by Malcolm Bremer (University of Bristol, UK) and Matthew Lehnert

(Max-Planck-Institut fuer extraterrestrische Physik, Garching, Germany).

4. Images 

Images can also be found on the ESO web site:-  http://www.eso.org/outreach/press-rel/pr-2003/pr-24-03.html

All images to be credited to ESO.

Image 1 - Colour-composite of the sky field with the distant galaxies

This image shows a sky region imaged with the multi-mode FORS2 instrument on the 8.2-m VLT YEPUN telescope, in which a number galaxies in the redshift range from 4.8 to 5.8 were discovered during the present study. They are accordingly located at a distance of about 12,600 million light-years from the Earth. The photo is a composite image where the blue, green and red colours correspond to the R- (central wavelength at 650 nm), the I- (about 780 nm), and the z-band filter (910 nm), respectively. The size of the sky field corresponds to about 1,000 million light-years at the distance of these galaxies. North is up and East is left.

Image 2 - Close-up image of some of the most distant galaxies known in the Universe

This image shows close-ups of some of the galaxies indicated in Image 1.  Each of the high-redshift galaxies is highlighted by a white circle. The redshift of the object is indicated at the lower left corner of each image. All objects appear green in this image because they are not seen in the blue image and most of their light is emitted in the middle band that is rendered as green in the above three-colour image. Each image measures 30 arcsec on a side; this corresponds to approx. 500,000 light-years at the distance of these objects.

Image 3 - Spectra of these distant galaxies.

Image 3 shows the spectra of ten objects in the studied field, all with confirmed redshifts. Each galaxy spectrum displays a sharp peak in colour showing the signature of its hydrogen gas - this is the redshifted Lyman-alpha emission line. The bright Lyman-alpha emission line is indicated in each of the spectra along with the redshift. The strong "break" seen in each spectrum over the region of Lyman-alpha emission is the reason why these objects appear green in the PR Photo 25b/03; there is simply no light visible on the short-wavelength (blue) side of the Lyman-emission alpha emission line.

The VLT telescopes

 

To cast some light on the state of the Universe at the end of the Dark Ages, it is necessary to discover and study extremely distant (i.e. high-redshift) galaxies. Various observational methods may be used - for instance, distant galaxies have been found by means of narrow-band imaging, by use of images that have been gravitationally enhanced by massive clusters, and also by serendipitous discoveries.

Matthew Lehnert from the MPE Institute in Garching, Germany, and Malcolm Bremer from the University of Bristol, UK, used a special technique that takes advantage of the change of the observed colours of a distant galaxy that is caused by absorption in the intervening

intergalactic medium. Galaxies at redshifts of 4.8 to 5.8 can be found by looking for galaxies which appear comparatively bright in red optical light and which are faint or undetected in the green light. Such "breaks" in the light distribution of individual galaxies provide strong evidence that the galaxy might be located at high redshift and that its light started on its long journey towards us, only some 1000 million years after the Big Bang.

For this, they first used the FORS2 multi-mode instrument on the 8.2-m VLT YEPUN telescope to take extremely "deep" pictures through three optical filters (green, red and very-red) of a small area of sky (40 square arcmin, or approx. 5 percent the size of the full moon). These images revealed about 20 galaxies with large breaks between the green and red filters, suggesting that they were located at high redshift. Spectra of these galaxies were then obtained with the same instrument, in order to measure their true redshifts.

The spectra indicated that six galaxies are located at distances corresponding to redshifts between 4.8 and 5.8; other galaxies were closer. Surprisingly, and to the delight of the astronomers, one emission line was seen in another faint galaxy that was observed by chance (it happened to be located in one of the FORS2 slitlets) that may possibly be located even further away, at a redshift of 6.6. If this would be confirmed by subsequent more detailed observations, that galaxy would be a contender for the gold medal as the most distant one known!

The spectra revealed that these galaxies are actively forming stars and are probably no older than 100 million years, perhaps even younger. However, their numbers and observed brightness suggest that luminous galaxies at these redshifts are fewer and less luminous than similarly selected galaxies nearer to us.

The measured redshifts of the galaxies in the "Bremer Deep Field" are z = 4.8-5.8, with one unexpected (and surprising) redshift of 6.6. In astronomy, the redshift denotes the fraction by which the lines in the spectrum of an object are shifted towards longer wavelengths. The observed redshift of a remote galaxy provides an estimate of its distance. The distances indicated in the present text are based on an age of the Universe of 13,700 million years. At the indicated redshift, the Lyman-alpha line of atomic hydrogen (rest wavelength 121.6 nm) is observed at 680 to 920 nm, i.e. in the red spectral region.

5. Background Information

Nowadays, the Universe is pervaded by energetic ultraviolet radiation, produced by quasars and hot stars. The short-wavelength photons "knocks" electrons off from the hydrogen atoms that make up the diffuse intergalactic medium and the latter is therefore almost completely ionised. There was, however, an early epoch in the history of the Universe when this was not so.

The Universe emanated from a hot and extremely dense initial state, the so-called Big Bang. Astronomers now believe that it took place about 13,700 million years ago.

During the first few minutes, enormous quantities of protons, neutrons and electrons were produced. The Universe was so hot that protons and electrons were floating freely: the Universe was fully ionised.

After some 100,000 years, the Universe had cooled down to a few thousand degrees and the nuclei and electrons now combined to form atoms. Cosmologists refer to this moment as the "recombination epoch". The microwave background radiation we now observe from all directions depicts the state of great uniformity in the Universe at that distant epoch.

However, this was also the time when the Universe plunged into darkness. On one side, the relic radiation from the primordial fireball was fading. On the other side, no stars nor quasars had yet

been formed which could illuminate the vast space. This sombre era is therefore quite reasonably dubbed the "Dark Ages". Observations have not yet been able to penetrate into this remote age - our rudimentary knowledge is still restricted and is all based on theoretical calculations.

A few hundred million years later, or at least so astronomers believe, some very first massive objects had formed out of the huge clouds of gas and had moved together. The first generation of stars and, somewhat later, the first galaxies and quasars, produced intensive ultraviolet radiation. That radiation did not travel very far, however, as it would be immediately absorbed by the hydrogen atoms which were again ionised.

The intergalactic gas thus again became ionised in steadily growing spheres around the ionising sources. At some moment, these spheres had become so big that they overlapped completely: the fog over the Universe had vanished !

This was the end of the Dark Ages and, with a term again taken over from human history, is sometimes referred as the "Cosmic Renaissance". Describing the most significant feature of this period, astronomers also call it the "epoch of reionisation".

6.The UK joined ESO in July 2002. In joining ESO, UK astronomers have gained access to the four 8.2-metre telescopes that comprise the Very Large Telescope (VLT) and to the several 1.8-metre telescopes that make up the Very Large Telescope Interferometer (VLTI); These facilities are located in the northern part of the Atacama desert in Chile. They also have access to two 4-metre class telescopes and several smaller ones at the ESO La Silla observatory further south in Chile.

The UK will benefit from increased involvement in the design, construction and scientific discoveries of the Atacama Large Millimetre Array (ALMA), a network of 64 twelve-metre radio telescopes also to be sited in Chile, and will have the opportunity to play a defining role in ESO's 100-metre Overwhelmingly Large (OWL) optical infrared telescope.

The 4-metre Visible and Infrared Survey Telescope (VISTA) forms part of the UK entry contribution to ESO. It is a specialised wide-angle facility equipped with a powerful camera of novel design and efficient detectors that will enable it to obtain deep images of large sky areas in short time. These survey observations will be made in several wavebands in the near-infrared region of the electromagnetic spectrum, with the capability of adding a visible region camera later. VISTA will be installed at the ESO Paranal Observatory (Chile).

7. The Particle Physics and Astronomy Research Council (PPARC) is the UK's strategic science investment agency. It funds research, education and public understanding in four broad areas of science - particle physics, astronomy, cosmology and space science.

PPARC is government funded and provides research grants and studentships to scientists in British universities, gives researchers access to world-class facilities and funds the UK membership of international bodies such as the European Organisation for Nuclear Research, CERN, the European Space Agency and the European Southern Observatory. It also contributes money for the UK telescopes overseas on La Palma, Hawaii, Australia and in Chile, the UK Astronomy Technology Centre at the Royal Observatory, Edinburgh and the MERLIN/VLBI National Facility.

Issued by the Particle Physics and Astronomy Research Council on 21 August 2003.

Id 1404: This page was written by Gill Ormrod, and last updated 21 August 2003 at 09:59

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