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Thursday, September 16, 1999 Published at 14:27 GMT 15:27 UK


Merger mania in early Universe

Simulation of the early universe, stars, galaxies and dark matter

By BBC News Online Science Editor Dr David Whitehouse

A new simulation of the early Universe suggests that titanic collisions formed the galaxies we see today.

Scattered throughout a younger Universe were countless young galaxies, each much smaller than most of the galaxies that inhabit our Universe today. But if they were smaller, they were brighter, because they contained vast numbers of hot young stars.

These young galaxies did not stay still. Over hundreds of millions of years, they collided with each other and merged to form larger star systems. This triggered violent bursts of star formation. The galaxies crackled with new stars spreading through them like a grassfire.

This is the view of the first few billion years of our Universe as proposed by cosmologists, following a recent analysis of supercomputer data from the University of California.

Faint but bright

The new study sheds light on the nature of the most distant galaxies seen by telescopes. These so-called "high-redshift galaxies", the light from which has been strongly shifted toward longer (redder) wavelengths by the expansion of the Universe, have presented a challenge to cosmologists since they were first detected in the early 1990s.

[ image: A deep Hubble Space telescope image]
A deep Hubble Space telescope image
Although they appear faint and are difficult to study because they are so far away, they actually shine very brightly because of their young, hot stars.

"The mystery is that these very luminous galaxies are so far away that the light we see today left them when the universe was only 10 to 15 percent of its current age, and one wonders how so many bright galaxies had already formed so early in the evolution of the universe," said Tsafrir Kolatt, of the University of California.

A report in the Astrophysical Journal Letters called "Young galaxies: What turns them on?," describes what the authors call the "collisional starburst" scenario, in which collisions between small galaxies trigger intense but relatively short-lived bursts of star formation.

When two galaxies collide, clouds of gas get funnelled toward the centre of the larger galaxy, where it condenses to form new stars. To test the idea, the researchers used supercomputers to simulate the behaviour of matter over billions of years in a representative chunk of the Universe.

Collisons and mergers

The simulation provided an unprecedented level of detail, enabling them to analyse the interactions of relatively small clumps of matter and to detect collisions and mergers. Supercomputers were needed not only for the simulation itself but also for the analysis of the results.

[ image: A
A "high redshift" galaxy
"This project is right at the edge of what's possible with the biggest, fastest computers available," added another of the researchers, Professor Joel Primack.

The mysterious so-called "dark matter" had to be included in the simulation. This form of matter makes up at least 90% of the Universe although astronomers do not know what it is.

"For each individual collision in the simulation, we had to calculate how many stars would form, how long they would shine, and what colours would be observed in the light from them," Kolatt said.

The results showed that the observed properties of the high-redshift galaxies can indeed be explained by the collisional starburst scenario and provide a glimpse of what our universe was like 12 billion years ago.

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