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Astron. Astrophys. 318, 405-415 (1997)

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1. Introduction

Recent large field redshift surveys like the "Center for Astrophysics Survey" (CfA, Huchra et al. 1983) or the "Southern Sky Redshift Survey" (SSRS I, da Costa et al. 1988) revealed a picture of the universe, where galaxies are concentrated in clusters and sheetlike structures. These surround large spheroidal regions apparently almost empty of galaxies with velocity diameters up to 5000 km/s, the so called voids (for details see the review of Giovanelli & Haynes 1991). These surveys are based on previous galaxy catalogues limited either to B magnitudes brighter than 15.5 (e.g. CGCG, Zwicky et al. 1961-1969) or to diameters larger than [FORMULA] 0 (e.g. UGC, Nilson 1973) as well as in surface brightness. The majority of galaxies in these catalogues are luminous objects of relatively high surface brightness, while the portion of dwarf systems is restricted to a few percent.

Therefore one may ask whether the void structure is affected by an observational bias due to the limitations of the catalogues employed. They might be surmounted if the observations penetrate deep enough to reach less luminous galaxies. For the comprehension of the formation and evolution of galaxies, it is doubtless of fundamental importance to know the real nature of the voids. Are they depleted of galaxies or occupied by a population different from the one in their surroundings?

Some theoretical models like the galaxy formation scenario in biased cold dark-matter dominated cosmologies (Dekel & Silk 1986, Hoffman et al. 1992) predict a smoother distribution of less luminous galaxies compared to the giants. Other authors stated the importance of special kinds of dwarf galaxies as Irregulars (Binggeli et al. 1988) or low surface brightness (LSB) objects (Schombert & Bothun 1988) for a proper determination of the galaxy number density and the large scale structure of the universe. Recent specially designed redshift surveys of dwarf as well as LSB galaxies (Bothun et al. 1986, Eder et al. 1989, Salzer et al. 1990, Binggeli et al. 1990, Thuan et al. 1991a, b, Schombert et al. 1992, Pustil'nik et al. 1995) find similar spatial distributions of massive and less luminous galaxies, with notable differences however.

It was our intention to overcome the limitations of previous surveys in magnitude, diameter and surface brightness as well as to be independent of published galaxy catalogues and their restrictions. For that we obtained deep direct images, taken towards the central region of nearby well defined voids already known from literature. Estimating diameters and applying morphological criteria we tried to single out foreground galaxies and to get interesting candidates for follow-up spectroscopy. To increase the fields covered, the same was done using POSS plates. Only galaxies fainter than [FORMULA] were taken into account. For further details see Kuhn (1994) and Paper I.

As the further discussion will show, this object selection is an important as well as a critical procedure. On our deep plates and CCD frames one finds of course a multitude of faint predominantly background galaxies, far too many for spectroscopy. On the one hand, our selection criteria were rather successful, as the luminosity distribution of our sample demonstrates, but another question is whether these criteria are adequate for identifying void galaxies. We cannot exclude that we failed to find a considerable number of interesting objects.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998