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3.7.2. Are Walls Really Thin?

A large I-band TF sample of 175 spiral galaxies located in the Great Wall (GW) structure has been obtained and analyzed by Dell'Antonio et al. (1996a,b). For this larger sample, the scatter in the TF relation is sigma = 0.30 mag. This scatter coupled with the sample size is sufficient to determine the physical thickness of the wall which is necessary to confirm that it is a 2D structure and not a 1D filament that is spread out in redshift space by peculiar velocities whose origin would be unknown. Using this data set, Dell'Antonio et al. examine four main issues: 1) the amount of infall onto the GW, 2) the large-scale peculiar velocity of the GW itself, and 3) the thickness of the GW and 4) the amount of shear along the entire length of the structure. These issues are all highly relevant to the stability and longevity of the GW.

The results of this study can be summarized as follows:

bullet The observed velocity dispersion in the GW is approx 500 km s-1 (Ramella et al. 1992). The TF data can place a 90% confidence upper limit on mean infall velocity of approx 500 km s-1. This produces an upper limit on GW wall thickness of 11 h-1 Mpc which strongly confirms that it is a cold, 2D structure in physical space as well as redshift space.

bullet Over the angular extent of the wall, the shear velocity is constrained to be -70 ± 210 km s-1. The lack of significant shear indicates the GW to be a quiet, rather unevolving structure. Furthermore, both the low infall and shear velocities (each consistent with zero) indicates that the GW is not subject to any significant perturbation from clusters that are embedded within it. As the angular extent of this structure corresponds to a linear size of a few 100 Mpc, the low degree of shear can place a limit on the power-spectrum (see Chapter 5) of large-scale fluctuations (e.g., Feldman and Watkins 1995).

bullet A larger data set is required to more confidently determine if the GW is at rest with respect to the CMB. The current data do not suggest that GW has significant large scale motion but the possibility that the GW participates in the same bulk flow pattern as the LG can only be ruled out with 70% confidence.

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