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3.2 (Non-) rotational support

The question of rotation in dwarf ellipticals has been addressed only recently. Obviously, the measurement of rotation requires spectroscopy at an even fainter level of surface brightness than measurement of a central sigma. The sparse data we have to date suggest that dwarf ellipticals are not supported by rotation. This excludes at least some ``dS0'' systems, which we have generally lumped together with the dE class. Data are published for two bright dE's in the Virgo cluster, VCC 351 and IC 794 (Bender and Nieto 1990); the three bright dE companions of M31: NGC 205 (Carter and Sadler 1990; Held et al. 1990; Bender et al. 1991), NGC 185 (Bender et al. 1991; Held et al. 1992), and NGC 147 (Bender et al. 1991); and the resolved Fornax system (Paltoglou and Freedman 1987; Mateo et al. 1991). A convenient measure of the dynamical importance of rotation is provided by the anisotropy parameter

Equation 5 (5)

where v is the rotational velocity, vsigma is the mean velocity dispersion, and epsilon is the ellipticity at the radius where v is measured (Binney 1978; Kormendy 1982). If barsigma geq 1, the object can be assumed to be flattened by rotation; if vsigma is significantly smaller than 1, the object must be supported by velocity anisotropy. The six dwarfs from above all have vsigma < 0.4, hence they are clearly not rotation-supported. As mentioned, the dS0's may not follow this trend, e.g., UGC 7436, a dS0(5),N system in the Virgo cluster is a rotating disk galaxy (Bender, private communication). Many dS0's exhibit disk-like morphologies (Binggeli and Cameron 1991, cf. Sect. 2), but their kinematic nature has yet to be explored.

Bender and Nieto (1990) have also measured a number of faint classical and compact E's in the Virgo cluster, which all seem to be rotation-supported, in accord with the trend found by Davies et al. (1983). An exception to this rule is the very compact galaxy M32 with vsigma approx 0.5 (Tonry 1984; Tonry 1987; Dressler and Richstone 1988), but M32 is likely tidally influenced by M31. The separation between rotating low-luminosity E's and anisotropic dE's apparently goes hand in hand with the photometric discontinuity between E's and dE's described above (Sect. 2.2). However, a much larger kinematic sample is needed to test for a real discontinuity in vsigma. Bender and Nieto (1990) argue that the specific angular momentum (J / M) of the dwarfs, unlike that of giant ellipticals, falls onto the J / M - M relation for rotating ellipticals. The dwarfs seem to compensate for their slow rotation with a larger effective radius. Bender and Nieto (1990) suggest that dE's gained their anisotropy from expansion due to winds after the onset of star formation, but admit that gas accretion and tidal interactions could be important as well. Shaya and Tully (1984) have suggested that the low angular momentum content of elliptical galaxies is due to tidal interaction with the collapsing protocluster. Such a mechanism may also apply to dE galaxies, although their higher specific angular momentum may be somewhat problematical.

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