Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eso..pres...24.&link_type=abstract
ESO Press Release, 08/2003
Astronomy and Astrophysics
Astronomy
Scientific paper
VLT Discovers a Group of Early Inhabitants and Find Signs of Many More [1]
Summary
Using the ESO Very Large Telescope (VLT) , two astronomers from Germany and the UK [2] have discovered some of the most distant galaxies ever seen . They are located about 12,600 million light-years away.
It has taken the light now recorded by the VLT about nine-tenths of the age of the Universe to traverse this huge distance. We therefore observe those galaxies as they were at a time when the Universe was very young, less than about 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 per cent the size of the full moon! This allowed understanding 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 the energetic photons needed to ionise the hydrogen in the Universe at that particularly epoch.
PR Photo 25a/03 : Colour-composite of the sky field with the distant galaxies. PR Photo 25b/03 : Close-Up images of some of the most distant galaxies known in the Universe. PR Photo 25c/03 : Spectra of these galaxies. From the Big Bang to the Cosmic Renaissance
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 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 had been stretched by the cosmic expansion towards longer wavelengths and was therefore no more able to ionise the hydrogen. On the contrary, it was trapped by the hydrogen atoms just formed. 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 knowledge is still rudimentary 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 that had moved together. The first generation of stars and, somewhat later, the first galaxies and quasars, produced intensive ultraviolet radiation. That radiation could not travel very far, however, as it would be immediately absorbed by the hydrogen atoms which were again ionised in this process.
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 lifted !
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 ". Finding the Most Distant Galaxies with the VLT
ESO PR Photo 25a/03
ESO PR Photo 25a/03
[Preview - JPEG: 476 x 400 pix - 320k] [Normal - JPEG: 952 x 800 pix - 824k]
Caption : PR Photo 25a/03 shows a sky region imaged with the multi-mode FORS2 instrument on the 8.2-m VLT YEPUN telescope, in which a number of 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. Technical information about the photo is available below.
ESO PR Photo 25b/03
ESO PR Photo 25b/03
[Preview - JPEG: 601 x 400 pix - 376k] [Normal - JPEG: 1202 x 800 pix - 1M]
Caption : PR Photo 25b/03 shows close-ups of some of the galaxies indicated in PR Photo 25a/03 . 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.
ESO PR Photo 25c/03
ESO PR Photo 25c/03
[Preview - JPEG: 605 x 400 pix - 176k] [Normal - JPEG: 1209 x 800 pix - 452k]
Caption : PR Photo 25c/03 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 [3]. 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.
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 [2]) galaxies. Various observational methods may be used - for instance, distant galaxies have been found by means of narrow-band imaging (e.g., ESO PR 12/03), by use of images that have been gravitationally enhanced by massive clusters, and also serendipitously.
Matthew Lehnert from the MPE 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 [2] 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,
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