Mar. 25, 1999 ATLANTA -- At a meeting of the American Physical Society today (March 24), astronomers announced they have turned back the hands of time and taken a look at the earliest structures that formed in the universe. "The very first objects to form were low-mass clusters of metal-free stars that condensed in the cores of dark-matter halos," said Mike Norman, a professor of astronomy at the University of Illinois and a researcher at the university's National Center for Supercomputing Applications. Norman and his colleagues used numerical cosmology to examine the earliest gravitationally bound astrophysical objects.
"Numerical cosmology usually involves simulating the large structures in the universe -- the formation of galaxies and clusters of galaxies," Norman said. "But you can use the same numerical and physical approaches to study the very first structures that formed from the Big Bang."
Theorists had proposed a number of likely candidates for the first cosmological objects -- from Jupiter-size "clumpuscules" to brown dwarfs to massive black holes.
"What we found in our simulation, however, were tiny star clusters, each containing about 1,000 to 10,000 solar masses," Norman said. "Although these were low-mass clusters, each star was massive - typically containing 100 solar masses. Because the stars were so massive, they were blown to bits long ago." A star's lifetime is determined by its mass. The greater the mass, the more rapidly a star will consume its nuclear fuel. As a star ages, it produces heavier elements through nuclear fusion. Massive stars typically end their days as supernovae that spew shock waves and heavy elements into space, triggering another generation of star formation.
The first star clusters likely formed between 50- and 100-million years after the Big Bang, Norman said. "But because they disappeared so long ago, we are basically studying a ghost that physics tells us once existed, but which we can't see anymore."
Today, the oldest visible objects are globular star clusters -- densely packed systems containing around one million solar masses. "It is probable that the earliest clusters, through a process of continued star formation, metal enrichment and mergers, eventually aggregated into these globular clusters," Norman said.
The initial conversion of gas into stars was highly inefficient and produced a very small number of stars, Norman said. "Probably less than 1 percent of the gas in these primordial clouds actually cooled and collapsed to sufficiently high densities to form stars, so there was plenty of fuel left over to make more stars. But these early star clusters were the 'spark plugs' that started the whole thing off."
The simulation used by Norman and his colleagues includes relevant dark matter dynamics, chemical and radiative processes, nonlinear hydrodynamics, and nonequilibrium physics to determine the collapse and possible fragmentation of gravitationally and thermally unstable primordial gas clouds. Density perturbations within the gas clouds initially created compact objects called halos, which then condensed into the first stars.
A major technical accomplishment -- called adaptive mesh refinement -- made the high-resolution simulation possible. The technique employs an algorithm that can collapse the resolution from cosmological scales down to the scale of an individual star.
"Our three-dimensional adaptive mesh refinement code utilizes an adaptive hierarchy of resolution grids to achieve an extremely high spatial dynamic range," Norman said. "A smart algorithm places subgrids around regions of high interest, and these subgrids then use even finer grid cells to increase the local resolution. So, we have a numerical grid that can zoom in automatically and adaptively to as fine a level of detail as is required by the solution."
The adaptive mesh refinement code was developed by Greg Bryan at the Massachusetts Institute of Technology. Tom Abel, at the Max Planck Institute for Astrophysics in Garching, Germany, developed the nonequilibrium chemical model of the primordial gas.
Funding for the project came from NASA and the National Science Foundation.
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