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LARGE-SCALE STRUCTURES IN THREE DIMENSIONS

A.P. Fairall and W.R. Paverd Departments of Astronomy and Computer Science, University of Cape To wn, Rondebosch, 7700 South Africa.

Abstract Three-dimensional views are used to explore the character of the large-scale distri­ bution of southern galaxies. They show that within great walls of galaxies, such as the Sculptor great wall, there exists a cellular texture with voids smaller in size than 1000 km s-1 (whereas the voids that separate great walls are of order 5000 km s-1) in redshift space. The view from the position of our own galaxy reveals that the concentration of galaxies forming the supergalactic equator is spread in depth from the nearest galaxies out to approximately cz= 10000 km s-1, with many small voids embedded. It is therefore suggested that our galaxy is itself within a (Centaurus) great wall structure.

1 Introduction

Our contribution at the Rencontres de Moriond consisted of the presentation of a sequence of images, showing galaxies as colour-coded dots against a black background. The audience was equipped with ChromoDepth(tm) spectacles, that translate colour into stereoscopic depth, thereby enabling the audience to view the structures in three dimensions (3D). A colour-coded video was also shown. Regrettably, there is no way that the 3D images can be successfully reproduced here with this article. The text and diagrams following give brief descriptions of the images, and convey the verbal information that accompanied the presentation. 68

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Figure 1: A projection of the distribution of galaxies in the Sculptor (top), Fornax (bottom) and Grus (vertical centre) Walls. The Hydra, Centaurus and Fornax clusters are bottom left, and serve to mark the position of our galaxy. At the conference, this view was presented in 3D.

2 Chromostereoscopic Images Using ChromoDepth(tm) spectacles, pure red dots may be perceived as being closer than 2 metres, pure blue dots as distant as 40 metres. Only single images need be projected; the spectacles create the stereoscopic pairs. This is a great advantage in the present application, where the eye may otherwise be confused by large numbers of dots. The colour coded images have been created, using a Silicon Graphics Indigo 2 workstation. The 35mm slides were made by photographing the monitor screen with a pin-registered camera using Fuji Velvia film. Suitable software was developed that converts a selected range of depth into colour. Such colours, when produced on the CRT display, are no longer monochromatic, since the display works by combination of three primary colours; similarly, the photographic emulsion. Nevertheless, the results produced are acceptable and serve for the present purpose.

3 The Database

The database of galaxies used is the first author's Southern Redshift Catalogue (Hereafter SRC [3]), an ongoing compilation of published redshifts that, inter alia, attempts to identify discrepant velocities and exclude them from plots. It is important to realise that the catalogue does not form a statistically-controlled sample, and suitable caution has to be applied. However, the first author [4] has frequently made the point that, nevertheless, such catalogued data can still be successfully used in the mapping and delineating of large-scale structures and voids. Suffice to say that all such features identified in SRC in the early 1980s [9] have all stood the test of time. Three general views of the database were shown in 3D form. The data are bounded by the equatorial plane, since only southern declinations are covered by SRC. The wedge of obscuration of the Milky Way is also apparent. Wall-like features can be discerned :amidst the data, but are better seen in isolated views. 69

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...... '• ...... · · ...... Figure 2: A schematic representation, suggesting that great walls are formed by collective diminution of void size.

4 Great walls

As presented at the conference, the isolated views show only the galaxies in the Sculptor, Fornax and Grus Walls (and the Hydra and Centaurus clusters). An indication is provided in Figure 1. At the workstation, a mouse is used to set the viewpoint. When displayed in 3D, these views reveal internal structures within the great walls. A great wall, such as the Sculptor Wall (also sometimes referred to as the Great Southern Wall), has a cross section of 5000 X 1000 km s-1. The 3D projections confirm thefinding reported previously [5], that there exists a form of cellular structure within the wall. Galaxies in the wall are seen to surround internal voids of diameter <1000 km s-1. These voids are then almost an order of magnitude smaller than the larger voids that separate the great wall structures; for example, the Sculptor Void between the Sculptor and Fornax Walls, has a diameter of approximately 5000 km s-1. Figure 2 summarizes the finding. It suggests that there is a fa.bric of small voids within a great wall. Moreover, the wall might delineate a region of retarded void growth - that way the voids would be smaller and the galaxies that surround those voids would be closer. As yet, this findingis largely qualitative. It remains for a quantitative tool, analogous to the "Voidsearch" algorithm [7] that examined the larger voids, to analyse the distribution within walls.

5 Our View from within a Great Wall The images also include "all-sky" panels, six of which can cover the hemispherical screen of a planetarium. Three-dimensional views have been created on the dome of the Planetarium of the South African Museum. Like the single-slide images, they serve as a research tool, aside from obvious education potential. The software permits selection of minimum and maximum distances for the range of the colour coding. At the conference, a sequence of the panel covering RA 10 to 14 hours (and Deel 0 to -90deg) was displayed. The sequence increased in depth: 0 to 1250, 0 to 2500, 0 to 5000, 0 to 10000 and 0 to 20000 km s-1. 70

This panel includes the nearby galaxies NGC 5128 (Centaurus A) and M83, and the Hydra I and Centaurus clusters. The image confirms a continuous distribution of galaxies between our galaxy and Centaurus, such that what is commonly referred to as the Local (or Virgo) Supercluster is not an independent entity to the Centaurus supercluster. The panel also shows the most conspicuous segment of the supergalactic equator [2]. The 3D views emphasize that the general concentration of galaxies to the supergalactic plane is continuous in depth, out to approximately cz=lOOOO km s-1. Furthermore, there are numerous voids intermingled with this concentration. Many of the voids appear large because they are relatively close, but measurements show them to be smaller than 1000 km s-1 in diameter - much as those describe above in great wall structures. It is therefore suggested that the well-known concentration towards the supergalactic equa­ tor may represent a great wall structure seen from within [6]. Our galaxy and the Local Supercluster are a part of it. The Hydra I cluster and associated structures are not a part of this great wall. Although in the past, the existence of a "Hydra-Centaurus" supercluster was often advocated, there are only weak connections [l] between the two, while new structures revealed by Kraan-Korteweg et al [8] suggest that Hydra may form a parallel wall-like structure to our own "Centaurus Great Wall"

References

[l] da Costa, L.N., Nunes, M.A., Pellegrini, P.S., Willmer, C., Chincarini, G. & Cowan, J.J., Astron. J. 91, 6(1986). [2] de Vaucouleurs, G. de Vaucouleurs, A., Reference Atlas of Bright Galaxies, Texas, 1964. & [3] Fairall, A.P., Southern Redshift Catalogue, Dept Astronomy, University of Cape Town, (available over Internet from [email protected]) [4] Fairall, A.P., Palumbo, G.C.C., Vettolani, G., Kauffmann, G., Jones, A. & Baiesi­ Pillastrini, G., Mon. Not. R. astr. Soc. 247, 21P(1990). [5] Fairall, A.P., Paverd, W.R., Ashley, R.P., in 4th DAEC Meeting on "Unveiling Large-Scale Structures behind the Milky Way", eds. C. Balkowski and R.C. Kraan-Kortweg, ASP Conf. Ser., 67, 21 (1994). [6] Fairall, A.P. & Paverd, W.R., in 35th Herstmonceux Conference "Wide-field Spectroscopy and the Distant Universe" (World Scientific), in press. [7] Kauffmann, G. & Fairall, A.P., 1991, Mo n. Not. R. astr. Soc. 248, 313(1991). [8] Kraan-Korteweg, R.C., Fairall, A.P. & Balkowski, C., 1995, Astr. Astrophys. in press. [9] Winkler, H., Mon. N. Astr. Soc. Sthn. Africa, 42, 74 (1983)