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2.1. The Dynamical Importance of Rotation

The "zeropoint" of this subject is well known (for reviews see Illingworth 1981; Kormendy 1982a; Binney 1982; Davies 1989; Kormendy and Djorgovski 1989, and de Zeeuw and Franx 1991). Work on elliptical galaxies was revolutionized by the discovery (Bertola and Capaccioli 1975; Illingworth 1977) that most bright ellipticals do not rotate significantly. Therefore their dynamics are controlled by velocity anisotropy, which implies that they are triaxial (Binney 1976, 1978a, b). Many observational signatures of triaxiality have been found. In contrast, bulges rotate essentially as rapidly as models of oblate spheroids that are isotropic and hence flattened only by rotation (e.g., Illingworth and Schechter 1982; Kormendy and Illingworth 1982; Kormendy 1982b); isotropic models are an excellent fit to the data (Jarvis and Freeman 1985a, b; Kent 1989). However, Davies et al. (1983) point out that most bulges are less luminous than the nonrotating ellipticals; low-luminosity ellipticals also rotate. It is therefore not clear whether bulges and ellipticals differ. The Sombrero Galaxy contains one of the few well-studied bulges that is as luminous as a typical nonrotating elliptical; it is an isotropic rotator, but so are a few bright ellipticals. Even now, too few high-luminosity bulges have been measured. We do not know whether virtually all bulges rotate rapidly or whether bulges and ellipticals show the same dependence of rotation on luminosity.

One complication (Kormendy and Djorgovski 1989) is that ellipticals can grow disks by accretion. If they originally did not rotate, they still will not rotate after becoming "bulges". They add noise to any intrinsic correlation between bulge rotation and luminosity.

Therefore we know of only modest dynamical differences between bulges and elliptical galaxies. Bulges lie nearly in the fundamental plane correlations for ellipticals (e. g., Kormendy 1985; Bender, Burstein, and Faber 1992). But there are some photometric differences (Hamabe and Kormendy 1987). Therefore a more quantitative comparison of bulges and elliptical galaxies may be profitable.

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