4.5. Nuclear Disks

"Nuclei" are central star clusters that are dynamically distinct from the bulge (see Kormendy and Djorgovski 1989 for a review). Some nuclei are disks. A particularly clear example is NGC 4594 (the Sombrero Galaxy): the nuclear isophotes are very flattened (Burkhead 1986, 1991; Kormendy 1988b), and even the observed spectrum, which is a composite of the bulge and nucleus, implies a velocity dispersion 181 ± 6 km s^-1 at r 3.7" that is smaller than the velocity dispersion 240 ± 4 km s^-1 of the bulge (Kormendy 1988b; Jarvis and Dubath 1988). Another possible example is the nucleus of M 31: after subtraction of the superposed bulge spectrum, this is colder at r 2" than the bulge (Kormendy 1988a, but contrast Dressler and Richstone 1988). Good spatial resolution is required to see a kinematic signature. More commonly, we recognize nuclear disks only through their effects on isophote shapes. Disky distortions in the central few arcsec of many ellipticals suggest that nuclear disks are common (Nieto et al. 1991; Scorza 1993). They are another example of high-density disk material near galaxy centers.

4.6. Triaxial Disklike "Bulges" in Barred Galaxies

In the discussion of the V_max / - diagram, I noted that bulges of barred galaxies tend to be more dominated by rotation than bulges of unbarred galaxies. That is, they are more disk-like. Also, it has been known for a long time that many of them are triaxial. Examples include the SB0 galaxies NGC 1291 and NGC 1543 (de Vaucouleurs 1975; Jarvis et al. 1988) and NGC 2950 (Kormendy 1981). In all of these, the isophotes clearly show nuclear bars that have position angles different from the main bar and from the outer disk. Other examples are discussed in Kormendy (1979) and in Buta (1986a, b, 1990). Sometimes the inner bar is a nucleus distinct from the bulge, sometimes the whole "bulge" in an SB galaxy is triaxial. Large V_max / values imply that these exceptionally triaxial "bulges" are dynamically like bars (Kormendy 1983; Kent and Glaudell 1989; Kent 1990) and not like triaxial giant ellipticals.