The Large-scale Distribution of Galaxies

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Are galaxies randomly distributed in the Universe or do they form well defined structures? Extensive galaxy surveys have shown that structures resembling sheets and filaments characterize the galaxy distribution. This distribution resembles a complicated spider's web several hundred megaparsecs in diameter (1 megaparsec=3.26 million light-years). We can study the formation of the structures in the galaxy distribution by using computer simulations, which follow the motion of a set of particles that interact with each other gravitationally. These particles represent the distribution of matter in a finite volume of the Universe.

Figure 1

Figure 1 shows four snapshots from a simulation carried out on the Cray parallel supercomputer at the Garching Computer Centre, which followed the motions of 17 million particles within a volume of 5 million cubic megaparsecs. Time runs from left to right and from top to bottom. In the cosmological model shown, at redshift z=3, 2, 1, and 0, the Universe is 1.6, 2.5, 4.6, and 13 billion years old, respectively. The present time corresponds to z=0. Each panel is a slice 170 megaparsecs on a side and 16 megaparsecs thick. Bright spots indicate regions where the matter density is several orders of magnitude larger than the mean. Filaments and underdense regions are found in the simulated matter distribution, much as in the real Universe. The Universe is mostly filled by material which does not emit light. This ``dark'' matter regulates the way in which structures grow from tiny seed perturbations in the early Universe. Simulations like the one shown in Figure 1 only follow the motion of this dark matter. There is no information about the position and luminosity of galaxies. So how can we compare these simulations with observations? We have included a set of phenomenological prescriptions for where and how galaxies form and evolve in these dark matter-only simulations. These physically-motivated ``recipes'' describe the cooling and condensation of gas at the centres of dense regions of dark matter, the transformation of this gas into stars, the effect of supernova explosions on the intergalactic medium, and the merging of close pairs of galaxies.

Figure 2

Figure 2 shows the evolution of a region 42 megaparsecs on a side taken from the simulation shown in Figure 1. In this smaller region, a cluster of galaxies forms. The dark matter is shown in grey scale. Galaxies are shown as circles colour-coded according to their star formation rate: the sequence red, yellow, green and blue represents an increasing rate of star formation. At early times (top left panel) there exist only a few galaxies forming stars at a very high rate. As time goes on, from z=3 to z=0, more galaxies form and, at the same time, some galaxies run out of gas, form fewer stars and become green, yellow and red. At the present time (z=0), red galaxies populate the central region of the cluster, whereas blue galaxies can be found in the outskirts of the system. This is very similar to what is observed in the real Universe. We have performed quantitative comparisons with observational data. In the research articles listed below, you can find analyses of galaxy properties at high and low redshift and a comparison of the properties of groups and clusters with results from galaxy surveys. The research is on-going and we plan to study the properties of galaxies in high-redshift clusters for comparison with observations from the Hubble Space Telescope and large ground-based telescopes.

A. Diaferio, G. Kauffmann

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Last modified: Tue May 18 14:59:32 MDT 1999 by Markus Rampp
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