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6. Spatial distribution and clustering properties

The well-known morphology-density relation for the classical Hubble types (Dressler 1980) holds also for dwarf galaxies (Binggeli et al. 1987; Ferguson and Sandage 1988; Binggeli et al. 1990). It may, in fact, even be stronger for dwarf galaxies: Gas-rich dwarfs appear to be the most weakly clustered, i.e., the most uniformly distributed species of galaxies (Salzer 1989), while early-type dwarfs have been said to be the most strongly clustered of all galaxies (Vader and Sandage 1991). The morphological segregation is plausibly stronger for low-mass galaxies, as these systems will also be more vulnerable to environmental influences than high-mass galaxies. Bright, nucleated dwarf ellipticals are infallible tracers of high-density places; they occur only in clusters of galaxies or, if outside, as close companions to massive galaxies. This is generally true for early-type dwarfs (Einasto et al. 1974; Binggeli 1989; Binggeli et al. 1990). However, it is important to note that there seem to exist some fairly isolated, faint, non-nucleated dE's. The prime example is the recently discovered Tucana system, which does not show any young, or even intermediate-age population (da Costa 1994). Another case of a possible isolated faint dE has been reported by Binggeli et al. (1990). This would suggest that faint dwarfs can exhaust their gas also without external pressure (Sect. 7.6). It is difficult to give an upper limit to the space density of isolated dE's, first, because of their low visibility, and second, because, once detected, their distances are in general not accessible (at least at present). However, recent deep searches for low-surface brightness objects suggest that there cannot be too many of them (Eder et al. 1989; Phillipps et al. 1994). The best guide we have is the Local Group, where the strong clustering of dE's around M31 and the Milky Way has recently been reinforced by the null result of an extended search for new local dwarfs (Irwin 1994; but see Ibata et al. 1994).