We have space in this paper to review only a few features of pseudobulges. Kormendy (1993) and Kormendy & Kennicutt (2004) discuss others. In this section, we list these other features to provide a preliminary prescription for identifying pseudobulges.
Any prescription must recognize that we expect a continuum from classical, merger-built bulges through objects with some E-like and some disk-like characteristics to pseudobulges built completely by secular processes. Uncertainties are inevitable when we deal with transition objects. Keeping these in mind, a list of pseudobulge characteristics includes:
The candidate pseudobulge is seen to be a disk in images: it shows spiral structure or its apparent flattening is similar to that of the outer disk.
It is or it contains a nuclear bar (in face-on galaxies). Bars are disk phenomena; they are fundamentally different from triaxial ellipticals.
It is box-shaped (in edge-on galaxies). Box-shaped bulges are intimately related to bars; they are believed to be - or to be made by - edge-on bars that heated themselves in the axial direction.
It has n 1 to 2 in a Sersic (1968) function, I(r) e-K[(r / re)1/n-1], fit to the brightness profile. Here n = 1 for an exponential, n = 4 for an r1/4 law, and K(n) is chosen so that radius re contains half of the light in the Sersic component. Nearly exponential profiles prove to be a characteristic of many pseudobulges (e. g., Andredakis & Sanders 1994; Andredakis, Peletier, & Balcells 1995; Courteau, de Jong, & Broeils 1996; Carollo et al. 2002; Balcells et al. 2003; MacArthur, Courteau, & Holtzman 2003; Kormendy & Kennicutt 2004).
It is more rotation-dominated than are classical bulges in the Vmax / - diagram; e. g., Vmax / is larger than the value on the oblate line.
It is a low- outlier in the Faber-Jackson (1976) correlation between (pseudo)bulge luminosity and velocity dispersion.
It is dominated by Population I material (young stars, gas, and dust), but there is no sign of a merger in progress.
If any of these characteristics are extreme or very well developed, it seems safe to identify the central component as a pseudobulge. The more of 1 - 7 apply, the safer the classification becomes.
Small bulge-to-total luminosity ratios B / T do not guarantee that a galaxy contains a pseudobulge, but if B / T 1/3 to 1/2, it seems safe to conclude that the galaxy contains a classical bulge.
Based on these criteria, galaxies with classical bulges include M 31, M 81, NGC 2841, NGC 3115, and NGC 4594. Galaxies with prototypical pseudobulges include NGC 3885 (Figure 5), NGC 3945 (Figures 2, 6, 9), NGC 4314 (Figure 3), NGC 4321 (Figure 4), NGC 4371 (Figures 6, 7, 8), and NGC 4736 (Figures 3, 6). The classification of the bulge of our Galaxy is ambiguous; the observation that it is box shaped strongly favors a pseudobulge, but stellar population data are most easily understood if the bulge is classical.