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
= 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:
The observed velocity
dispersion in the GW is
500 km s-1
(Ramella et al. 1992). The TF data can place a 90% confidence
upper limit on mean infall velocity of
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.
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).
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.