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3.7. A Case Study: The Physical Structure of Voids and Walls

3.7.1. Are Voids Real Structures?

As emphasized earlier, the appearance of structure in redshift space may not be an accurate representation of their structure in physical space. This is particularly relevant to the apparently thin nature of the Great Wall as well as the apparent sharpness of the void boundaries. In general, these structures are of low galaxy density and are preferentially inhabited by spiral galaxies. If there are enough spiral galaxies available, the measures of relative distances using the TF relation can be used to compare the physical size of the structure with its apparent size in redshift space. For instance, the apparent voids that are seen in redshift space could be manifestations of smaller structures which have large outflow velocities away from the center. In this case, the physical size of the void would be significantly smaller than the size inferred from the redshift ratio of the near and far sides. However, if the voids are truly massless, some outflow is expected but that is generally small (see Regos and Geller 1991).

Figure 3-21

Figure 3-21: Structure of the large void seen in the first CFA Slice data in redshift space. Adapted from Bothun et al. (1992).

The physical structure of one of the larger voids detected in the CFA slice survey was measured in 1992 by Geller and collaborators (see Bothun et al. 1992b). Figure 3-21 shows the structure of this void in redshift space. In redshift space, its diameter is approx 5000 km s-1. Relative distances between spirals located on the near and far redshift edges of the void were measured using the I-band TF relation. The observed scatter in the TF relation for this sample is fairly small, sigma = 0.18 mag and this allows for fairly accurate measures of relative distance. Three principal results emerged from this study:

bullet 1. The mean redshift ratio of the far and near sides of the void is 2.16 ± 0.08 and the ratio determined from the relative distance measures is 2.22 ± 0.10. Thus the physical size of the void is the same as its redshift size. This means that the voids are physical structures and not artifacts in redshift space and that they likely formed at early times and have been expanding at the same rate as the rest of the Universe, thus preserving their character.

bullet 2. Consistent with the first result, no no large net outflow was detected at the void boundaries. The formal 1sigma upper limit on outflow is approx 5% of the void diameter.

bullet 3. The near side component of the void which is closest to the Coma cluster has an infall velocity of approx 900 ± 250 km s-1 at mean distance 26 h-1 Mpc from that structure. This is a substantially higher infall than that of the LG which is located about 15 Mpc away from the center of the Virgo Cluster. To first order, this indicated that Coma is a much more massive cluster than Virgo. Streaming velocities (towards virialized clusters) along the void wall of this amplitude are predicted by most models in which voids exist near clusters of galaxies (see Regos and Geller 1992). These streaming velocities give rise to the appearance in redshift space that the void wall smoothly connects to the virialized structure.

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