3.4. Disk Galaxies
This review of galaxy photometry concentrates heavily on elliptical galaxies, because our physical understanding of them is currently making rapid progress. I will therefore have to give a very incomplete discussion of disk galaxies, to keep these lecture notes from growing too long. I will generally follow the review of Kormendy (1980).
The bulge components of disk galaxies are usually considered to be equivalent to elliptical galaxies. There are close similarities in morphology (Sandage 1961), brightness distributions (de Vaucouleurs 1959b) and stellar content (Faber 1977). However, there is growing evidence for physical differences between bulges and ellipticals, which suggest that bulges are intermediate in properties between ellipticals and disks.
These differences are as follows (see also Kormendy 1980).
(1) Bulges rotate more rapidly than most giant ellipticals (section 4). They appear to be rotationally flattened spheroids, while giant ellipticals are triaxial ellipsoids supported by anisotropic velocity dispersions. This dynamical difference, and the gravitational effect of the disk, may account for the photometric differences between bulges and ellipticals, listed below.
(2) The bulges of many edge-on galaxies have "box-shaped" outer isophotes which are not seen in elliptical galaxies. Examples include NGC 128 and 7332 (Sandage 1961) and NGC 4469, 4565 and 5746 (de Vaucouleurs 1974b). This distortion is associated with large amounts of rotation: unlike normal bulges, box-shaped bulges rotate rapidly even at large distances above the disk (section 4.2.7).
(3) The minor-axis brightness profiles of some bulges are more complicated than those of elliptical galaxies. For example, that of NGC 4565 (Fig. 5) is not well described by fitting functions for elliptical galaxies. Instead, it has two distinct segments, the outer being shallower than the inner. In contrast, major-axis profiles are well fitted by r1/4 laws (de Vaucouleurs 1959b; Kormendy 1977b; Burstein 1979b, c; Boroson 1981). These photometric peculiarities may be related to the fact that the minor axis is a special direction for bulges which is relatively unimportant for ellipticals - it is the rotation axis, and it is perpendicular to the disk potential.
(4) Some bulges are more diffuse than elliptical galaxies. That is, they have larger characteristic radii and fainter characteristic brightnesses than ellipticals of the same luminosity (Figure 25; Burstein 1979c). Boroson and Kormendy (1982) confirm this result with a larger sample and a better analysis of parameter coupling errors.
(5) Bulges appear on average to be flatter than ellipticals (see Kormendy 1980; Dressler and Sandage 1983). For example, Hamabe (1982) has studied the shapes of eight bulges of nearly edge-on galaxies. When corrected for disk light, their average shape is E5.0 ± 0.4 (dispersion / 81/2), flatter than the E3.5 - 3.8 shape of typical ellipticals. This is true despite the fact that the sample is probably biased toward objects which are abnormally similar to ellipticals: four of the bulges studied contribute more than 70% of the light of their galaxies. A high degree of flattening could be the result of rapid rotation or of the disk potential.
Figure 25. Comparison of the Be - log re relations for r1/4-law fits to the mean profiles of bulges and ellipticals (from Kormendy 1980). Bulges which contribute most of the light of their galaxies are consistent with the relation (straight line) for ellipticals. Bulges such as M31 and M81 have larger re and fainter Be than ellipticals of the same luminosity - lines of constant luminosity have slope 5 and are therefore steeper than the line shown for ellipticals (i.e., equation 6).
Many of the above results are tentative and in need of further study. However, the differences between bulges and giant ellipticals appear to be significant. It becomes interesting to ask whether any of the above photometric peculiarities are seen in small ellipticals, which are now known to rotate as rapidly as bulges (Davies et al. 1983, see section 4.2.6).