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3.2. Hierarchical Structures Uncovered

In the late 70's dedicated redshift surveys of galaxies were begun. These surveys have culminated in the discovery of large voids (regions where there are few or no galaxies), great linear chains of galaxies, great attractors which perturb the expansion motion of galaxies, and an overall complex distribution of galaxy clustering on a variety of size scales. Prior to redshift surveys, significant structure in the Universe was known just from galaxy positional data. Both Abell and Zwicky defined criteria for grouping positional points together into structures and generating catalogs of galaxy clusters. These catalogs remain a basic source for the study of LSS.

In characterizing LSS on the basis of 2D positional data, their can be much ambiguity due to the dependence of observed small scale structure on larger scale structures. Figure 3-1 shows the results of an N-body simulation involving the distribution of dark matter (see Chapter 4) which is assumed to dominate the mass. Notice that the distribution is quite filamentary and is essentially 1D. While the observed Universe also exhibits filamentary structure in the 2D positional data, the filaments are really 2D sheets of galaxies that are not adequately present in the N-body models. Nevertheless, these filaments or sheets can give rise to apparent structure if you are looking along the line of site of a particular filament. It is therefore important to develop good diagnostics to separate out gravitationally bound structures with the more weakly bound structures that are embedded in an overall large scale structure component such as a filament or wall.

Figure 3-1

Figure 3-1: Large scale structure in a Dark Matter supercomputer simulation. Image courtesy of the HPCC group at the Univeristy of Washington and George Lake. This simulation shows a void filled Universe with much filamentary structure. Clusters of galaxies appear to form at the intersections of voids.

Unfortunately, this is rather difficult to do. Numerous studies have shown that the velocity distribution of galaxies within a structure is a very poor predictor of whether or not that structure is bound. This is because small, dense structures can generate infall from the larger lower density region in which they are embedded. Thus the velocity distribution of the galaxies in any structure represents the convolution of those which originally formed there and those which have only recently arrived. But this issue may not even be relevant in a filamentary Universe as the structure there is mostly defined by the distribution of large low density regions (e.g., voids).

In general terms, X-ray emission is a reasonable signature of a bound structure. The X-rays from gas in the Intracluster Medium (ICM) which is heated by the gravitational potential of the structure. The origin of this gas is unclear but likely sources include gas which has been liberated from galactic potentials (presumably through tidal interactions) and left over gas that never was part of a galactic potential. Until recently, most X-ray satellites would measure only a flux which could then be transformed into an X-ray luminosity using the distance to the structure. X-ray luminosity, however, depends on both the X-ray temperature and the total gas mass. While there is a loose correlation between X-ray luminosity and the observed velocity distribution, this correlation in and of itself provides at best ambiguous evidence that the structure is gravitationally bound. X-ray temperature is a more direct way to measure the depth of the potential but until the recent ROSAT X-ray satellite, X-ray temperatures were difficult to measure. In a bound system which is in hydrodynamic equilibrium, there will be a natural and predictable correlation between X-ray luminosity and x-ray temperature. Mergers of two subsystems with different potential well depths (temperatures) or line-of-site projection of subsystems along a filament will tend to smear out this correlation making X-ray identification of bound systems on scale less than 0.5 Mpc difficult.

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