5. Summary and conclusions
Our present study did not find the voids occupied by a homogenous population of dwarf galaxies. We found a few galaxies in the very well defined nearby voids, but the number of void galaxies is not significant at the density level of field galaxies. We could interpret this result in the sense that a void population, if any, should have the density at least a factor 4.5 lower than the density of walls and filaments. Another possibility is that we start to see the brightest peaks of such population and we are still not faint enough to really sample these objects. Alternatively, there is no void population, and the galaxies we found in voids were only fluctuations from the normal distribution. This would explain why some voids were found to be empty and other to contain a few galaxies. On the other hand we should also remember that we were probing the voids only with emission-line galaxies, and from these, we are mainly sensitive in the high ionization objects. We are complete in the galaxies with large equivalent widths and we start to miss objects that have fainter EW. These objects are mainly low ionization and are better detected by H surveys, like the surveys based on objective prism IIIaF plates (UCM survey, Zamorano et al. (1994)). Therefore the voids were still not probed by low ionization emission-line objects. Nevertheless the UCM galaxies seem also to follow the same structures as the normal galaxies (Gallego 1995), but the UCM sample do not contain galaxies as faint as we detect. We should also keep in mind that the IIIaJ selected emission-line galaxies contribute only 7 from the total number of galaxies (Salzer 1989). It was suggested that the small HII galaxies are the best candidates to fill the voids, but it could still be that a population of old red dwarf galaxies would fill the voids. These galaxies do not have a strong spectroscopic signature like an emission spectrum and they are gas poor and therefore are not detected by HI surveys. If they were fainter than =-15, they would be not properly sampled by any survey that search them on direct images.
Coming back to our ELGs we suggest that the few galaxies that we found in voids have also a tendency for clustering. This result is far from being secure, but some of our void galaxies seem to associate with faint late-type ZCAT galaxies. For example our Arch of 7 ELGs in Void 2 is followed by four faint ZCAT galaxies. The Arch seem to divide that bigger void in 3 smaller voids. And not at the end, 75 from our isolated galaxies have their nearest neighbours among themself. These results are close to some results of N-body simulations (Dubinski et al. 1993, van de Weygaert & van Kampten 1993) or with the observational results of an HI survey in the Bootes void (Szomoru 1996). It was thus suggested that smaller scale voids disappear within larger voids but the frozen-in remnants of small walls would produce smaller substructures inside the larger voids. Lindner et al. (1996) also suggest a hierarchical distribution of galaxies and voids, in the sense that superclusters and clusters delimit bigger voids, bright galaxies some smaller voids and faint galaxies delimit very small voids, inside the bigger ones.
Overall the main characteristics of the ELG spatial distributions are:
1. The ELGs are better than normal galaxies for tracing the luminous matter at further distance;
2. The ELGs have a small tendency to be more evenly distributed than the ZCAT galaxies, with some galaxies lying in some voids or at the rim of the voids.
3. A filamentary structure (the Arch), populated only by faint ELGs, has been found to cross a big void in front of the Great Wall.
4. The void galaxies are intrinsically faint and they do not have special spectroscopic properties in comparison with the other ELGs in the field.
© European Southern Observatory (ESO) 1997
Online publication: April 28, 1998