Void boundaries in the southern sky : aspects of large scale structure in southern galaxy catalogues
Master Thesis
1991
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University of Cape Town
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Abstract
The evidence for structure at the largest observable scales has developed considerably in the last two and a half decades since the discovery, in 1967, of significant clustering in galaxy surveys. As will be shown by the review -material and galaxy spectrum redshift survey maps presented in chapter 1, it has become generally accepted that the universe contains a vast, lacunary structure. Naturally, a number of issues arise when the accuracy of the survey maps that display such structure is questioned. For example, visual bias on the part of the observer (chapter 1) and errors induced by the inability of a survey to pick out non-luminous or too distant matter, or errors that exist as a result of observational techniques (see chapter 2) may affect the accuracy with which redshift maps represent real structure in the universe. Attempts at mathematically modelling these effects are also reviewed in chapter 2. Redshift distributions contain more than spatial information however, and potential inaccuracies are further exacerbated by the fact that there is a wide range of galaxy properties. This fact is the source of so-called segregation effects. However it is argued that the lacunae (more commonly called voids) which are evident in redshift surveys that make use of all available redshifts, agree with features evident in controlled surveys (i.e. surveys for which galaxies are selected subject to some sort of observational criterion). In addition, voids are able to "hide" in the cosmic microwave background. Thus even though the largest structures found in redshift surveys (superclusters) tend to be as large as the extents of the surveys, it is conceivable that one way, of fitting a homogeneous and isotropic Friedmann model to the real universe is by assuming that voids are in fact the structural units of the Friedmann model. The question of examining redshift data for possible interconnections between voids has prompted the discovery of a striking two-dimensional sheet of galaxies-the main topic of this thesis. In chapter 2 the southern catalogues from which the data has been drawn are discussed. In fact the use of all available redshifts allows a detailed examination of large volumes of the 3-d distribution to be made. Computer software called COELIS has been especially developed for the purpose of providing a number of data manipulation and graphics tools for studying the 3-d distribution and, in particular, flat (i.e. 2-d) sub-sets of the real data. A description of COELIS' functions is the subject of chapter 3 (the code is included in an appendix). In chapter 4, the discovery of a remarkably planar redshift feature is presented; some 6.5 h-¹ Mpc thick and approximately 20 x 20 h-² Mpc² wide, at the interface between the southern voids of Sculptor and Eridanus-termed the Wall. The planar nature of such a feature allows for 2-d statistical methods to test for significantly under-dense regions (in effect void interconnections). There is no evidence for clustering in the Wall (which is highly diffuse) and no evidence for significant under-densities. In fact the distribution of redshifts in the Wall appears to be quite random. A preliminary examination of other void interfaces is inconclusive; it reveals a mixture of apparent interconnections and walls'. When available galaxy-property information is used to derive galaxy-property gradients perpendicular to planar features it is possible to obtain a list of likely formation histories for the feature under study, and hence obtain some idea of the formation events that gave rise to the surrounding voids. This is the subject of chapter 5. The general implications of finding non-uniform distributions of properties across diffuse redshift features are discussed in terms of the postulate of uniform thermal histories. One advantage of studying low density features (such as the Wall) is that if non-uniform property distributions exist (as indicated by segregation effects) then it may be argued that the formation of the features predates galaxy formation, that is, proto-galaxies "know" about their eventual environment. This is the case for the Wall, for which segregation effects are presented in some detail. Thereafter a comparison with generally accepted models of large-structure formation is presented. The existence of the Wall appears to be consistent with two models: cosmic strings in a HDM model, or a class of explosion scenarios with a hybrid HDM/CDM model. Thus not only do segregation effects support the existence of the Wall as a real entity in the data but the list of indicated cosmogonies, for voids in the region of the Wall, is shortened considerably.
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Bibliography: p. 113-122.
Reference:
Maurellis, A. 1991. Void boundaries in the southern sky : aspects of large scale structure in southern galaxy catalogues. University of Cape Town.