Recent surveys with the HST that have focused on the late-type, allegedly bulgeless Scd-Sm disks, find that only ~ 30% of these systems have light profiles consistent with being single-exponential structures (Böker et al. 2003). The remaining disks are not well fitted by a single exponential; in particular, the surface brightness in the central few kiloparsecs exceeds the inward extrapolation of the outer exponential disk. The surface brightness profiles of many of these late-type disks are often equally well described either by the sum of two exponential components, or by a single Sérsic profile over the entire radial range with shape parameter n up to a value of 2.5. In the earlier-type systems, a second central exponential component in addition to the outer exponential disk is typically interpreted as a bulge component. Böker et al. (2003) suggest that the frequent detection of such central exponential "excesses" also in systems that, according to the classical classification scheme, should host no bulge component, together with the fact that a single Sérsic profile is often a good alternative to the sum of two exponentials, may indicate that in fact these excesses are not bulges, but rather denser regions of the disks themselves.
A key issue in the context of understanding the nature of the central regions of disk galaxies is one of definitions (see Carollo et al. 1999). Böker et al. distinguish between what they call "the modern theorist" view, assumed to be the correct one, which asserts that a bulge is a kinematically hot component with an extended three-dimensional structure, and the "observers" view, which, in photometric studies, relies on the assumption that disks are exponential structures and that bulges are identifiable as additional light (mass) contributions in the central regions. All the photometric analyses of the local (and distant; see, e.g., Shade et al. 1996) disk galaxy population that are aimed at studying bulge properties indeed assume a constant scale length exponential profile for the disk, and attribute to a bulge any central concentration of light in excess of the inward extrapolation of the outer, constant scale length disk (e.g., Andredakis & Sanders 1994; Andredakis et al. 1995; de Jong 1995; Courteau et al. 1996; Carollo et al. 1998; Balcells et al. 2003; MacArthur et al. 2003). Böker et al. (2003) mention a lack of theoretical support for the disks being exponential, and point out that the assumption that disks remain exponential all the way into the center may not be correct, i.e., that the operational definition of bulges adopted in photometric studies may lead to attributing to a bulge what actually belongs to the disk. It is certainly not an easy task to disentangle into distinct subcomponents the centers of galaxies, where all of these subcomponents are expected to reach their largest densities.