It has been known for some time that many bulges have a radial light profile that is not an elliptical-like r1/4 law (Andredakis & Sanders 1994; de Jong 1995; Courteau, de Jong, & Broeils 1996); instead, they are reasonably well described by an exponential light profile. Incidentally, the bulge of our own Milky Way also has an exponential light profile (Binney, Gerhard, & Spergel 1997). Recent high-resolution investigations using data from the Hubble Space Telecope (HST) have strengthened the evidence for exponential light profiles down to the smallest scales at the end of the spheroids luminosity sequence (Carollo et al. 1998, 2001; Carollo 1999). Several studies have used the generalized surface density profile I(r) exp[-(r / ro)(1/n)] introduced by Sérsic (1968) to model the bulge light (Andredakis, Peletier, & Balcells 1995; Graham 2001; MacArthur, Courteau, & Holtzman 2003). These studies report shape-parameter values for bulges of late-type spirals ranging between n = 0.1 and 2. Some of the Sérsic analyses attribute a significant meaning to the derived precise values of n (Graham 2001; Balcells et al. 2003). Tests based on simulated data, however, show a large dependence of the derived parameters on, for example, the input parameters; indeed, MacArthur et al. (2003) stress that, on average, the underlying surface density profile for the late-type bulges is adequately described by an exponential distribution. The same studies show the existence of a coupling between bulges and disks that is manifested by an almost-constant scale lengths ratio hbulge / hdisk 0.1 for late-type spirals, and a similar scaling relation even for earlier-type systems. This is interpreted to indicate a similar origin for bulges of all sizes in hosts of any Hubble type.
For more massive, early-type bulges, ground-based studies using the Sérsic law to describe their light distribution have found values of n close to, or even in excess of the elliptical-like de Vaucouleur's (1948) value of n = 4 (Graham 2001). However, the analysis of high-resolution HST images for a sample of early-type bulges provides values of the Sérsic shape index n not in excess of ~ 3 (Balcells et al. 2003). The difference in the estimates for n is due to the contribution of photometrically distinct central point sources, which at ground-based resolution are confused for bulge light (see Section 1.5). Balcells et al. interpret the n < 3 Sérsic shape indices in the massive bulges as an indication that even these systems, like the smaller exponential-type bulges, are not the outcome of violent relaxation during collisionless accretion of matter. Both in the ground-based and HST analyses, a trend remains between the bulge Sérsic shape parameter n and the bulge luminosity and half-light radius; the trend is in the direction of brighter, bigger bulges having larger n values (Graham et al. 2001; Balcells et al. 2003).
Detailed studies of the integrated stellar populations of bulges of all Hubble types have also been pushed forward by the availability of high-resolution multi-color images from the HST (Peletier et al. 1999; Carollo et al. 2001). The independent analyses agree on the basic result that (1) massive early-type bulges have very red colors, unambiguously indicating old ages ( 8 Gyr) for the average stellar populations of these systems, and (2) the smaller, later-type (almost) exponential bulges have on average significantly bluer colors.
Kinematically, the Sb pseudo-bulges studied by Kormendy (1993) represent the extreme case of a general behavior shown by bulges of any Hubble type and mass: these all appear to have kinematic properties that are closer to disklike structures rather than to elliptical galaxies. Indeed, based on the comparison between minor axis radial velocity dispersions of disks and bulges, Falcón-Barroso et al. (2003) report that even the early-type, massive bulges are actually thickened disks.
The following important considerations on bulges that emerge from the above analyses. (1) The earlier-type bulges form a continuum with the late-type bulges in terms of the shapes of the surface brightness profiles. The smallest bulges are exponential structures, and the largest appear to be intermediate cases between the exponential and the elliptical-galaxy ones. (2) Bulges of spirals are coupled to their host disks in a similar way along the Hubble sequence (with a possible weak trend toward marginally higher hbulge / hdisk ratios for early-type bulges). (3) There is a spread in average stellar metallicity and ages amongst bulges, but also a clear trend toward smaller bulges being less enriched and younger stellar structures than the more massive, earlier-type bulges. (4) Bulges of any size show some kinematic features that are typical of disks.