2.2. Kinematic (Sub)Structure in Bulges and Elliptical Galaxies
Recent work on ellipticals has been dominated by continued attempts to measure the degree of velocity anisotropy and by discoveries of signatures of accretion. The latter include kinematically decoupled (misaligned) gaseous and stellar components. Reviews are given in Kormendy and Djorgovski (1989) and in de Zeeuw and Franx (1991). This emerging richness in kinematic structure is shared by galaxy bulges.
Kinematic evidence for accretion in disk galaxies includes the following. Gas that counterrotates with respect to the stars has been detected in NGC 4546 (Galletta 1987) and in NGC 2768, NGC 4379, and IC 4889 (Bertola, Buson, and Zeilinger 1992). We also have one spectacular case of an edge-on galaxy containing two stellar disks that counterrotate (NGC 4550: Rubin, Graham, and Kenney 1992; Rix et al. 1992). Bulges are less thoroughly studied than ellipticals; the early discovery of kinematically decoupled components in so many objects argues that bulge formation histories are as rich and complicated as those of elliptical galaxies.
Ellipticals are triaxial; are bulges triaxial, too? It is fashionable to suspect that they are. But this is not an argument by analogy. In ellipticals, triaxiality follows from the unimportance of rotation, which implies that velocity dispersion anisotropy is needed to account for the flattening. Once we realized that z is smaller than the other two components, we had no reason to expect that r = , either. But bulges rotate rapidly enough to be isotropic. If they are triaxial, this is not because they are like nonrotating ellipticals but rather because they are like bars.
What do observations tell us? It is too early to tell. In one galaxy, NGC 4845, kinematic evidence for noncircular streaming motions has been found and interpreted as a sign of triaxiality (Bertola, Rubin, and Zeilinger 1989; Gerhard, Vietri, and Kent 1989). From slit spectra at five position angles, Bertola et al. (1989) conclude that the rotation velocity at 7" - 10" radius is smaller along the major axis than elsewhere in the bulge. A slight twist between bulge and disk isophotes is also seen. In the absence of complications, these observations imply that the bulge is triaxial with principal axes a : b : c 1 : 0.75 : 0.5. The above papers agree reasonably well on the implied ranges of b/a and c/a. However, there are significant uncertainties. NGC 4845 is almost edge-on (inclination i 75°). This means that the major axis (PA = 78°) and neighboring (PA = 44° and 98°) slits are separated by only 2.8" at 8" radius. How sure can we be that the velocities really differ when the deprojection corrections are factors of 2.9 and 1.8 for the two neighboring slits? Also, the galaxy is dusty; the brightness distribution clearly shows dust at radii of ~ 5". Do we really see to the same depth along the line of sight at all three slit positions? Clearly this is a difficult object. Further such work is needed.
Bertola et al. (1989) note that NGC 4845 has a peanut-shaped bulge. Such bulges rotate particularly rapidly (section 3); they may be related to or even formed by bars (section 6). Peanut-shaped bulges are particularly likely to be triaxial.