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September 07, 2007
Fixing the Holes
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By including black holes for the first time in a large-scale cosmological simulation, physicists uncover their function in regulating the growth of galaxies
Once thought to be rare, exotic bodies, black holes have turned out to be fundamental focal points in the architecture of the cosmos. Among the sprawling, intricate arrangement of matter in every galaxy, they are the invisible centerpiece. Their tremendous mass, which can be many billion times the mass of the Sun, gives them gravitational pull to swallow huge quantities of interstellar gas. As this gas swirls outside the lip of the drain, just before falling in, it heats to extreme temperatures and radiates energy as light waves, producing one of the brightest bulbs in the universe -- a quasar. Once over the edge of this lip, however, nothing escapes, not even light.
Despite their ubiquitous presence, black holes and the quasars they spawn have been until now absent from large-scale simulations of the universe -- too small to be resolved within the big picture of cosmic structure. Thanks to TeraGrid resources, however, in particular PSC's Cray XT3, Carnegie Mellon University theoretical cosmologist Tiziana Di Matteo and her colleagues were able to include black holes in a simulation encompassing a sizeable fraction of the universe.
"Ours is the first cosmological simulation to incorporate black-hole physics," says Di Matteo. "It involves more calculations than any prior similar modeling of the cosmos. The galaxies we see today look the way they do because of black holes. This result offers the best picture to date of how the cosmos formed and evolved."
Their findings, forthcoming in the Astrophysical Journal, report on evolution of structure over the 14 billion years of the universe's existence. Far from being only destroyers, gobbling up any matter within reach, black holes in this new picture are also regulators: Their mass is related to the size of the galaxy they reside in as well as to its total star mass.
The goal is a more fundamental understanding of the evolution of the universe. "What kind of quasars formed in what kind of galaxies at what time?" asks Di Matteo. "What is the progenitor of the most massive black hole today?" There is also a more immediate benefit -- directing astronomers where to aim the Next Generation Space Telescope -- the successor to the Hubble Space Telescope -- to observe the formation of the first galaxies and black holes.
A New Cosmic Recipe
For this huge simulation, Di Matteo started with the software called GADGET-2 developed by Volker Springel of the Max Planck Institute for Astrophysics. To account for black holes, Di Matteo added code to "seed" black holes at the centers of forming galaxies. Next, she added an equation to describe how black holes "accrete" or swallow gas, adding to their mass and gravitational pull. Finally, she included calculations for "feedback" -- heating of surrounding gas in the galaxy by quasar radiation produced at the lip of the black hole.
The researchers first applied this approach on a small system of two colliding galaxies with black holes at their centers. The success of this simulation, which revealed new behavior when black holes were included, led to a 2005 paper in Nature. It also prompted Di Matteo to move to a much bigger scale. The idea, says Di Matteo, was to simulate a large portion of the universe "at the same resolution and with the same spatial scale as those idealized calculations."
To obtain this high resolution, the researchers scattered 230 million hydrodynamic particles over a 33 megaparsec cube, a huge volume encompassing a million galaxies, a representative chunk of the universe. To track these particles, the simulation used 2,000 XT3 processors -- the whole system -- over four weeks of run time. They followed the evolution of superclusters of galaxies -- the largest structures in the universe -- while simultaneously resolving the growth of black holes at the centers of galaxies. The XT3 was key.
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