Proto-galaxies tip cold dark matter
Nov 28, 2007 5 comments
Astronomers have spotted some of the first galaxies ever to form. These so-called proto-galaxies existed over 11 billion years ago and their discovery supports the “cold dark matter” model of how the universe evolved after the Big Bang.
In all, 27 proto-galaxies were found by an international team using the Very Large Telescope in Chile. Data from the observation should help astrophysicists refine models of galaxy formation.
The first galaxies are believed to have formed about one billion years after the universe was created in the Big Bang – which itself happened nearly 14 billion years ago. Astrophysicists believe that around this time, cold dark matter – invisible stuff that accounts for 95% of the mass of the universe – began to gather in small clumps under its own gravitational attraction. These clumps joined together to create larger clumps, which in turn combined to make even larger clumps and so on.
The first stars
Astrophysicists believe that normal matter such as hydrogen simply tagged along with the dark matter because of its gravitational attraction. Eventually, these clumps of matter and dark matter became proto-galaxies – structures about 1000-times smaller than our Milky Way galaxy – which contained the first stars. These galaxies are then believed to join together to form the galaxies we know today.
The new observations confirm theoretical research proposing that galaxies like our own have formed by the amalgamation of small proto-galaxies
In principle, proto-galaxies should still be visible because the light from some of these very distant structures is only now reaching Earth. However, this light is very faint and astronomers have struggled to detect it – and instead they have had to infer the existence of proto-galaxies from their apparent ability to block radiation from even further away.
The astronomers discovered the proto-galaxies in a small patch of sky using the European Southern Observatory’s Very Large Telecsope (VLT) (arXiv 0711.1354v1). The observations were made between 2004 and 2006 for a total of 92 hours, which allowed the team to resolve extremely faint and distant objects.
Damped Lyman alpha system
The galaxies were identified by their distinct ultraviolet light – called Lyman alpha light - that is given off by hydrogen gas when it is ionized by radiation from a star. Proto-galaxies are expected to contain large amounts of gaseous hydrogen at relatively high densities, which gives the Lyman alpha light the spectral characteristics of a "damped Lyman alpha system" (DLAS). DLAS light was spotted coming from 27 distant objects and the shifts in wavelength of the DLAS light confirmed that the galaxies existed about 2 billion years after the Big Bang.
“The new observations confirm theoretical research proposing that galaxies like our own have formed by the amalgamation of small proto-galaxies early on in the history of the Universe”, said Andy Bunker of the Anglo-Australian Observatory in Sydney, who is one of the leaders of the study.
According to team member Cedric Lacey, who is an astrophysicist at Durham University in the UK, the VLT data will allow astronomers to work out how many stars each proto-galaxy contains. This and other parameters extracted from the observations will then be used to improve current models of galaxy formation.
Lacey also told physicsworld.com that there is “nothing special” about the part of the sky where the proto-galaxies were found. “It’s an average bit of the sky”, he said, “which means that we should see [proto-galaxies] in all directions”.
As is often the case with scientific discoveries, the proto-galaxies were found by mistake. The team were actually looking for evidence of Lyman alpha fluorescence, whereby radiation created shortly after the Big Bang interacts with hydrogen throughout the universe, causing it to glow in the ultraviolet. According to Lacey, the team also saw evidence of this fluorescence and have asked for more time on the VLT to study it in more detail.
About the author
Hamish Johnston is editor of physicsworld.com