ar X iv : a st ro - p h / 96 04 10 8 v 1 1 8 A pr 1 99 6 Void Hierarchy – a guiding principle to the study of faint structures in voids

We introduce Void Hierarchy as an important property of the Large–Scale Structure in the Universe and demonstrate how it can be used to interpret observations. Moreover the void hierarchy constraints any realistic galaxy and structure formation scenario. 1 Hierarchy of voids Voids were defined as low density regions or, alternatively, as regions completely devoid of a certain type of object. Mean void diameters listed in Table 1 demonstrate the dependence of the void size on the type of object used in the (second) void definition. Both definitions imply that voids are not completely empty. Thus, the question is meaningful whether the distribution of galaxies in voids is homogeneous or reveals any structure. For example, it was concluded that Blue Compact Galaxies (BCG) from the Second Byurakan Survey (SBS) or other peculiar galaxies occur isolated within voids (Pustil’nik et al. 1995). Such questions are very relevant concerning scenarios of large scale structure and galaxy formation, but they are not conclusively answered up to now. Table 1 Mean diameters of voids surrounded by different types of object type of object mean void diameter rich clusters (Abell/ACO–Catalogue) 100 h Mpc poor clusters (Zwicky–Catalogue) 37 h Mpc bright (M ≤ −20.3) elliptical galaxies 30 h Mpc galaxies brighter than M = −20.3 23 h Mpc galaxies brighter than M = −19.7 16 h Mpc galaxies brighter than M = −18.8 13 h Mpc Using the second void definition we have studied the properties of voids surrounded by galaxies from three different luminosity (absolute magnitude M) limited samples. Three void catalogues have been compiled. Comparisons of voids from different catalogues revealed that voids form a hierarchical system (cf. Lindner et al. 1995, A&A 301, 329) as it is visualized in Fig. 1a). In this hierarchical concept apparently isolated galaxies in voids may have faint close neighbors which are not detected because of selection effects as it is shown in Fig. 1b). 2 UlrichLindner, JaanEinasto, Maret Einasto, and Klaus J. Fricke 3000 15 14 13 12 11 10 9 49 < δ < 57 a) b) M < -19.7 -19.7 < M < -18.8 M > -18.8 BCG 15 000 14 13 12 11 10 9 Fig. 1. Wedge diagrams of a slice of the Universe 6000 km s deep and bordered by about 9 < α < 15 and 49◦ < δ < 57◦. a) The three circles indicate an example of hierarchically interlaced voids defined by galaxies of different luminosity limit. b) Additionally BCGs from SBS are shown (crosses). The circle indicates the distance to the nearest bright (M < −19.7) neigboring galaxy.