Galaxy Formation: Are We There Yet?

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Science  06 Jul 2001:
Vol. 293, Issue 5527, pp. 15-17
DOI: 10.1126/science.293.5527.15e

The formation of the large-scale structure of the universe as revealed by observations of luminous ancient galaxies depends on the nature of dark matter. After the Big Bang, it is thought that dark matter started to clump together into halos and that the halos eventually merged to form galaxies. Dark matter can be cold or hot, and these two types lead to different scenarios for galaxy evolution. Although there is a growing preference for cold dark matter because it is consistent with recent observations of the background radiation in the universe (produced before galaxies formed), cold dark matter confounds galaxy formation by removing angular momentum from a protogalaxy during the halo-merging process.

Thacker and Couch have developed a smoothed particle hydrodynamic simulation of galaxy formation that keeps the rotating gaseous disk of the protogalaxy from losing too much of its angular momentum. They tracked the evolution of tens of thousands of interacting dark matter, gas, and star particles during condensation (gravitational collapse). A gaseous high-density disk formed first, and then star formation began; later, stellar explosions (supernovae) heated the gas, which rose into the halo, but then returned to the disk, adding energy back to the system. It is this supernova energy feedback that allowed the disk to maintain its angular momentum long enough to form a simulated galaxy with properties similar to those of observed galaxies. Thus, the Big Bang and present-day galaxies can be connected through cold dark matter, and astrophysicists may be getting closer to a unified model of the evolution of the universe. — LR

Astrophys. J., in press (astro-ph/0106060).

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