Relics of the Universe’s
first Galaxies are drifting through
Stuart Clark joins the hunt to find them.
* * * * * * * *
WHEN IT COMES TO GALAXIES, ASTRONOMERS HAVE A LOT IN COMMON WITH VICTORIAN FATHERS. THEY HAVE A PRETTY GOOD IDEA OF WHAT IS INVOLVED IN THEIR CONCEPTION AND THEY KNOW WHAT THE END PRODUCT LOOKS LIKE . BUT THE ACTUAL BIRTH ITSELF IS A TOTAL MYSTERY.
Using optical telescopes, astronomers can peer back through space to a time around a billion years after the big bang. Even at this early stage there are galaxies, not as fully grown as the ones today, but recognisable as galaxies nonetheless.
To look back further in time means studying microwaves rather than light, and that’s when we lose sight of galaxies altogether. Instead, we see a large universal bath of radiation that carries the imprint of the way the universe looked just 300,000 years after the big bang. At that point in cosmic history, there were no galaxies — just rippling undulations in the density of the gas that filled space.
The conclusion is inescapable: galaxies formed somewhere between 300,000 years and 1 billion years after the universe began. However, precisely when and how is still anyone’s guess. “It is a very important piece in the puzzle and at the moment it is missing,” says Jayaram Chengalur of the Indian National Centre for Radio Astrophysics in Pune.
We can start to fill this gaping hole in our understanding by looking at the effect galaxies had on the universe. Their formation caused one of the most profound watersheds in cosmic history.
Astronomers call it the era of reionisation, and it was a catastrophic event in which almost every hydrogen atom in the universe was ripped apart. It happened when the first objects began to appear inside galaxies. As they formed, huge amounts of high-energy radiation were released, and this flooded into space, stripping electrons from their nuclei. The question is, what were these first galactic objects?
One school of thought holds that they were stars, blazing their glory into space. Another thinks they were black holes, sucking glowing gas into oblivion. To find out, astronomers are gearing up to look for the effect these celestial objects have on their surroundings . That means building a new generation of telescopes specifically designed to detect radio waves emitted by hydrogen gas in the early universe before it was reionised.
For the first 300,000 years of its existence the universe was so hot that there were no atoms, only free electrons and nuclei . It was less than a thousandth of its current size and it had a temperature of thousands of kelvin rather than the chilly 3K above absolute zero of the cosmos today. Only when the observable universe had expanded beyond 300,000 light years across did the temperature fall low enough for nuclei and electrons to combine to form neutral atoms, most of which were hydrogen. As they combined, the particles allowed a burst of radiation to flood the universe — and we now know it as the cosmic microwave background.
After this, a period known as the dark ages set in, so called because there was nothing around emitting light at that time. Hydrogen atoms continued to linger during the first stages of galaxy formation, and only when the first objects formed and began radiating did reionisation begin.
Eventually it swept through the entire universe, with only one in every 10,000 atoms escaping destruction. There is a way to detect this crucial period of cosmic history, or at least to detect the disappearance of hydrogen atoms as they ionise.
Hydrogen atoms consist of a lone electron orbiting a proton . Both possess a quantum mechanical property called spin, on and the spins of the electron and the proton can be in the same or opposite directions. When an electron flips its spin from the same to the opposite direction, which it can do spontaneously, it releases radiation — radio waves, in fact — with a distinctive wavelength of 21 centimetres.
SLICE AT A TIME
As this happened in the early universe, by the time these fossil radio waves reach us, their wavelength will have been stretched by their passage across the expanding universe.
The further radiation travels, the longer the wavelength becomes. By scanning a range of wavelengths from it to several metres, you can focus on hydrogen at different times in the universe’s early history. In effect, you can construct a slice- by-slice scan of the era of reionisation . In fact, 21-cm radiation will tell cosmologists far more about the universe than the cosmic microwave background ever did, filling in many of the details from shortly after the big bang right up to the present day.
The main feature astronomers will be looking for is a dwindling signal. As hydrogen was ripped apart during reionisation, it lost the ability to emit radiation. So astronomers expect to see the 21-cm radiation — so-called even though the wavelengths are much longer when they reach Earth — fade as reionisation takes hold. By charting the speed of this disappearance and its distribution, astronomers hope to determine the nature of the first objects and the way galaxies form.
No one knows for sure what these first objects were like . “I think they were probably stars,” says Saleem Zaroubi of the University of Groningen in the Netherlands, “but I wouldn’t put a lot of money on it.” If they were stars, though, they were not like stars as we think of them today. Made almost exclusively from hydrogen and helium, they must have been monstrously large, with each one between 100 and 1000 times the mass of the sun. These stellar behemoths would have pumped out ultraviolet light, ionising the surrounding hydrogen to form a giant bubble.
On the other hand, the first objects to appear in the early galaxies might have been their central black holes. Around 300,000 years after the big bang, the density of matter in the universe varied from place to place. Some clumps of matter were so big that they may have automatically collapsed into black holes thousands of times more massive than the sun. Other clumps might have reached this condition after accreting more matter.
Either way, the resulting galaxy would resemble a scaled-down version of a quasar, a type of powerful active galaxy that dominated the universe 4 to 5 billion years later.
There is a way to tell whether galaxies were first characterised by stars or black holes. Astronomers believe mini quasars would have been powered by hydrogen gas falling into the black hole. As the gas spirals to its doom it is violently heated, reaching millions of kelvin. At such extreme temperatures, the black hole would emit much higher-energy radiation than stars, shining X-rays and gamma rays across the universe.
These can travel further than light from the first stars, so an ionised bubble created by a mini quasar will be larger than that created by a star. It will differ in another respect, too: the further the X-rays and gamma rays travel, the weaker they become. Hence the bubble edges from mini quasars are not as well defined as those from stars.
Deciding between the two options —monstrous stars or mini quasars with black holes — is where the new radio telescopes come in. The image slices they produce of the dark ages should allow astronomers to see bubbles of ionised hydrogen showing up as dark spots that form and spread as the dark ages ebb way. In principle, we should be able to see which came first: stars or black holes.
It is more than just a matter of which came first.
The speed at which reionisation happened is important too for both star and galaxy formation . As it ionises, hydrogen gas heats to 10,000 kelvin, making it better able to resist gravitational collapse. So stars would form less efficiently, and small galaxies would fail to take shape altogether. And with fewer stars, there will be less ionising radiation produced and reionisation will slow down.
So the pattern of ionisation will give an idea of whether galaxies were forming simultaneously all over the universe, or in fits and starts in individual pockets. At the moment it is anyone’s guess.
How easy it will be to untangle all of this is still a moot point. “I think a lot of the interpretation is going to be dependent on which models are used to analyse the data,” says Chengalur.
Eventually, NASA’s James Webb Space Telescope will come to the rescue. Planned for launch in 2013 and sensitive to infrared radiation, it will search for the stretched light from the first galactic objects. These objects should be sitting in the bubbles of ionised hydrogen revealed by the 21-cm radio telescopes, especially by the proposed Square Kilometre Array.
This light will tell us immediately whether the early galactic objects were stars or quasars. The ambiguity will be over and we’ll have everything we need to finally witness the process of galaxy formation.
That won’t happen soon, though. It will be no easy task to get the James Webb telescope and the Square Kilometre Array working together In the meantime, the search for reionisation will proceed with smaller radio telescopes. It’s a monumental undertaking, with the first tentative results expected around 2010.
“Twenty-one-centimetre astronomy opens a new window on the universe . Whenever you do that, you expect surprises,” says Zaroubi.
The bottom line is that we simply don’t know what to expect, but whatever we get, it will tell us something new about the universe. “We are on a fishing trip,” says Ue-Li Pen at the University of Toronto in Canada, “but it’s a well motivated fishing trip.”
Stuart Clark is a science writer based in
Hertfordshire, UK. www.stuartclark.com)
19 May 2007. (Pgs. 44-46)
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