Properties of Dwarf Spheroidals

Low surface brightness dwarf galaxies (at least the gas-poor variety) represent a fundamentally distinct family of objects, rather than just a low luminosity extension of the Hubble sequence; they are not “dwarf ellipticals” (Wirth and Gallagher, 1984; Kormendy, 1985; see Kormendy and Djorgovski, 1989 for a brief discussion and further references). Kormendy (1985) has demonstrated that dwarfs obey simple scaling laws and correlations which are quite different from those of normal ellipticals, pointing towards different formation histories. We extended this line of inquiry to multivariate correlations, using the sample of Local Group dwarf spheroidals (dSph), and samples of low surface brightness dwarfs (LSBD) in the Virgo and Fornax clusters. Whereas normal ellipticals form a statistically two-dimensional family in the parameter space of observables (Djorgovski, this volume, and references therein), dwarfs form nearly one-dimensional families; i.e., all of their properties seem to be well correlated (Djorgovski and de Carvalho, 1990). The only exception are velocity dispersion (only 5 available) , which do not correlate with any other observed properties. Furthermore, the corresponding correlations for dSph’s and ellipticals often have the opposite signs. Some examples are shown in figures 1 and 2. These results are only preliminary: selection effects are notoriously difficult