It is not generally realized that many galaxies contain central components that look like bulges but that have disk-like dynamics. This section reviews the evidence. A remarkably large number of papers at this meeting address this subject; it is clearly an idea whose time has come. As in previous papers (Kormendy 1982a, b; Kormendy and Illingworth 1983), I suggest that high-density disks are formed from ordinary disk gas that has been concentrated toward the center and turned into stars.
The prototypical bulgelike disk is in the Sb galaxy NGC 4736
(Kormendy 1982a).
It is illustrated in the Hubble Atlas
(Sandage 1961). The
central brightness profile (Fig. 1) is an
r1/4 law that reaches the
high central brightness characteristic of a bulge
(Boroson 1981).
However, the r1/4-law component shows a nuclear bar and
spiral structure to within a few arcsec of the center
(Fig. 2). Bars are
disk phenomena. More importantly, it is not possible to make spiral
structure in a bulge. Thus the morphology already shows that the
r1/4-law profile belongs to the disk. This is shown more
quantitatively by the well-known
Vmax / 
- 
diagram
(Illingworth 1977;
Vmax = maximum rotation velocity;
=
mean velocity dispersion near the center;
=
ellipticity). Figure 3 shows that the "bulge" of
NGC 4736 has an
unusually large ratio of ordered to random velocities (the data are from
Pellet and Simien 1982).
It is well above the "oblate line" describing
oblate spheroids with isotropic velocity distributions. Disks observed
edge-on are near the oblate line;
0.8 and
V /
2. Seen more
nearly face-on, they project to positions above the oblate line.
Kormendy (1982a)
therefore concluded that most of the light near
the center is coming from a high-surface-brightness disk.
 |
Figure 1. Surface brightness profile of NGC 4736, from Boroson (1981). The "shelves" at 40" and 120" radius are characteristic of oval disks (Kormendy 1982a). |
 |
Figure 2. CFHT image of NGC 4736 (106" high). The radial brightness gradient has been removed: the image has been divided by a mask image with the brightness profile of the galaxy but exactly elliptical isophotes. The nuclear bar is elliptical and therefore also removed; it can be recognized by the spiral structure and dust morphology near the center. |
 |
Figure 3. Vmax /
|