A bulge is a very important component of a disk galaxy. In the context of structure formation in a cold dark matter (CDM) cosmology, bulges may form by hierarchical merging of disk galaxies, a process thought to lead to elliptical galaxies if the disks have approximately equal mass. Bulges formed in this way should, then, resemble elliptical galaxies, especially for early-type spirals. The bulges of later-type spirals, however, can be very different from the expectations of a merger-built bulge (also known as a "classical" bulge). In many cases, the bulge appears to be made of material associated with the disk.
KK04 reviewed the concept of "pseudobulges," referring to galaxy bulges that may have formed by slow secular movement of disk gas to the central regions (also known as disk-like bulges; Athanassoula 2005). The main driving agent for movement of the gas is thought to be bars, which are widespread among spiral galaxies and which exert gravity torques that can move material by redistributing the angular momentum. Inside the corotation resonance, where the bar pattern speed equals the disk rotation rate, gas may be driven into the center to provide the raw material for building up a pseudobulge. KK04 review the evidence for such processes and argue that pseudobulges are a strong indication that secular evolution is an important process in disk-shaped galaxies.
Figure 26 shows the morphologies of both classical bulges and pseudobulges. The four classical bulge galaxies shown in the lower row, M31, NGC 2841, M81, and M104, have bright smooth centers and no evidence for spiral structure or star formation. Classical bulges also tend to have rounder shapes than disks, and can have significant bulge-to-total luminosity ratios as illustrated by M104. Classical bulges are also more supported by random motions than by rotation. Many references to classical bulge studies are given by KK04. Formation mechanisms of such bulges are discussed in detail by Athanassoula (2005).
Figure 26. Examples of pseudobulges and classical bulges in spiral galaxies (left to right): Row 1: NGC 3177, 4030, 5377, and 1353 (all HST wide V-band filter F606W; KK04); Row 2: NGC 6782 (I-band, F814W), 3081 (wide B, F450W), 128 (Ks), and 1381 (Ks); Row 3: NGC 224 (M31), 2841, 3031 (M81), and 4594 (M104) (all B-band). The images of NGC 128 and 1381 are from Bureau et al. (2006).
The two upper rows of Figure 26 are all pseudobulges as recognized by KK04. The first row shows HST wide V-band (filter F606W) images of the inner 1-1.3 kpc of four galaxies, NGC 3177, 4030, 5377, and 1353, in the type range Sa-Sbc. The areas shown account for much of the rise in surface brightness above the inward extrapolation of the outer disk light in these galaxies, and would be considered bulges just on this basis. The HST images show, however, considerable spiral structure, dust, small rings, and likely star formation in these regions, characteristics not expected for a classical bulge. KK04 argue that instead these are pseudobulges that are highly flattened, have a projected shape similar to the outer disk light, have approximately exponential brightness profiles (Sersic index n 1-2), and have a high ratio of ordered rotation to random motions. KK04 argue that a low Sersic index compared to n = 4 appears to be the hallmark of these pseudobulges, and a signature of secular evolution.
The second row in Figure 26 shows other kinds of pseudobulges discussed by KK04. NGC 6782 and 3081 (two left frames) have secondary bars, and KK04 considered that such features indicate the presence of a pseudobulge because bars are always disk features. In each case, the secondary bar lies inside a nuclear ring.
The other two galaxies in the second row of Figure 26, NGC 128 and 1381, are examples of boxy or box-peanut bulges. These features have been linked to the vertical heating of bars, and if this is what they actually are, then KK04 argue that boxy and box-peanut bulges are also examples of pseudobulges. However, boxy and box/peanut bulges would not necessarily be the result of slow movement of gas by bar torques, and subsequent star formation in the central regions, but instead would be related to the orbital structure of the bar itself (Athanassoula 2005).
Recent studies have shown that pseudobulges are the dominant type of central component in disk galaxies. Although originally thought to be important only for late-type galaxies, Laurikainen et al. (2007) showed that pseudobulges are found throughout the Hubble sequence, including among S0-S0/a galaxies, based on sophisticated two-dimensional photometric decompositions. Such galaxies frequently have nuclear bars, nuclear disks, or nuclear rings. Laurikainen et al. also found that bulge-to-total (B / T) flux ratios are much less than indicated by earlier studies, especially for early Hubble types, and that the Sersic index averages 2 across all types. The lack of gas in S0 and S0/a galaxies complicates the interpretation of their pseudobulges in terms of bar-driven gas flow and subsequent star formation. Instead, Laurikainen et al. link the pseudobulges in early-type galaxies to the evolution of bars. Laurikainen et al. (2010) also showed that S0s can have pseudobulges if they are stripped spirals, without invoking any bar-induced evolution.