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In physical cosmology, galaxy filaments, also called supercluster complexes or great walls, are, so far, the largest known cosmic structures in the universe. They are massive, thread-like structures with a typical length of 50 to 80 megaparsecs h-1 that form the boundaries between large voids in the universe.[3] Filaments consist of gravitationally-bound galaxies; parts where a large number of galaxies are very close to each other are called superclusters.

So I've always wondered: how are these structures formed?

One would (naively) expect that gravitationally bound structures would form as either spheres or discs.

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My PhD thesis was on the numerical simulation of the formation of clusters of galaxies, but it's not easy to give a concise answer. A useful first approximation is the so-called Zel'dovich Pancake, where on very large scales corresponding to the size of clusters of galaxies or larger, such that pressure is negligible and the gas dynamics is (to a reasonable approximation) entirely gravitational, an ellipsoidal overdensity will collapse most rapidly along its shortest axis, to form a 'pancake'. This will then collapse further to form a filament which then itself fragments to clumps that form galaxies. So the distribution of galaxies will follow the pancake/filamentary structure of the initial collapse.

However, in reality the density field is a composite of random Gaussian fluctuations at many scales and not the simple overdensity considered in the Zel'dovich Pancake, so collapse is complex and is observed at a variety of stages on different scales. Also, as an overdense region collapses the gas shock-heats to millions of Kelvin and pressure ceases to be negligible (as an aside, strong thermal X-ray emission from this gas is observed by X-ray telescopes, and as it is in hydrostatic equilibrium in the cluster gravitational potential it can be used to derive the distribution of mass). The coupled gravitational/hydrodynamic collapse is highly non-linear, and needs to be followed numerically rather than analytically - my thesis was on N-body (i.e. gravity) + Smoothed-Particle Hydrodynamics (gas dynamics) simulations of this process.

Regarding star formation, this is another extremely complex process and there are both similarities (fundamentally they are both processes governed by a balance of gravitational and gas-dynamic forces) and profound differences. However, star formation tends to be triggered - i.e supernovae in one generation of stars triggers the collapse of molecular clouds and the formation of a new generation - and so is not directly analogous.

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Wow - so cool! I'll definitely have to read more into this –  InquilineKea Aug 18 '11 at 13:52
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