Classical bulges, which have R1/4 light profiles, are not strongly flattened, and rotate rather slowly, are believed to have been formed from violent mergers of protogalactic fragments in the early stages of galaxy formation, as described in Section 2.1. Galaxy disks with an embedded classical bulge are presumed to have built-up subsequently through the usual process of dissipative gaseous in-fall.
However, it has become clear that many galaxies host bulges having quite different properties that are now described as a pseudobulges. They have more nearly exponential light distributions (Andredakis & Sanders (1994); Fisher & Drory (2008)), exhibit quite a high degree of rotation that has a roughly cylindrical flow pattern in 3D (Kormendy & Illingworth 1982), and are generally flatter than are classical bulges. Kormendy & Kennicutt (2004), updated in Kormendy (2012), gave a more detailed description of how a pseudobulge can be distinguished from a classical bulge.
The observed properties of pseudobulges strongly suggest a different formation mechanism and it seems highly likely that they formed through internal evolution from the disk (Kormendy & Kennicutt 2004), and that this evolution is more rapid in galaxies with a higher gas fraction (Kormendy 2012). Their basic idea is that pseudobulge formation is mediated by a bar, which first forms and buckles, as described in Section 5, and then dissolves into a dynamically hot, but flattened and rotationally supported bulgelike structure.
Kormendy (2012) proposed that a CMC of both stars and gas causes the bar to dissolve and create a pseudobulge. Gas is indeed driven inward by bars (Section 5.5) and simulations (Section 5.6) show that bars dissolved by massive CMCs do indeed form thickened, rotationally supported, near axisymmetric structures that resemble pseudobulges. The mass fraction in the dense central concentrations required to cause the bar to dissolve entirely is very high (e.g. Debattista et al. (2006) their simulation NG5). The more concentrated the mass the more efficiently it destroys the bar (Shen & Sellwood 2004), but the mass required is far larger than that suggested for any supermassive black hole (e.g. Gültekin et al. 2009). Large gas concentrations spread over an area of few hundred parsecs in radius are observed (e.g. Sakamoto et al. 1999; Sheth et al. 2005), but again are nowhere near massive enough.
However, the gas in the nuclear region forms stars at a vigorous rate (Section 7.1), with presumably a significant fraction of the mass being locked into long-lived stars that are gravitationally bound to the region where they formed. Kormendy (2012) therefore proposed that the stars built up in the nuclear region over a protracted period, together with the gas, eventually reach the combined mass required to dissolve the bar. Kormendy developed this proposal at length in his review, to which I refer the intersted reader for the full picture. If his plausible idea is correct, it once again implies that significant secular evolution is mediated by the behavior of gas.
No simulation has yet tested this suggestion, however. Previous studies of bar dissolution have created the central mass rather quickly, giving the bar little time to adjust as the mass grows, and further simulations of more gradual growth are needed to confirm that dissolution can eventually occur. The numerical task is particularly challenging for several reasons: (a) The evolution must be followed for a long period while the orbit time-scales in the very center are short. (b) Gas would have to be accreted continuously to the bar region, and the subsequent inflow rate should not be exaggerated by numerical viscosity (Section 5.5). (c) The halo would need to modeled self-consistently to follow bar growth through dynamical friction (Section 6).
Two other methods that might dissolve a bar were discussed in Section 5.6: Bournaud et al. (2005) and Combes (2008) suggest that the angular momentum added to bars as they drive gas inwards can weaken or destroy them. While more work on this scenario is needed, the inflow requirements are severe, and the consequence would not be so different from the build-up of a CMC. It is also noteworthy that some possible interactions between a bar and a strong spiral can weaken or destroy the bar. The mechanism and the conditions under which this behavior can occur also require further study, but the process may prove useful in this context, especially as strong spirals are most likely to arise in gas-rich outer disks.
Bars could also be destroyed in minor mergers, of course. But to make a pseudobulge, the perturber would have to be dense enough to not be tidally disrupted before reaching the bar, but not so massive as to destroy the cylindrical flow pattern and/or shallow inner radial light profile. This degree of fine-tuning makes the explanation seem untenable to account for the observed high frequency of galaxies that seem to host pseudobulges (Kormendy et al. 2010; Fisher & Drory 2011).
Other mechanisms for pseudobulge formation have been proposed. Guedes et al. (2013) found that the pseudobulge in their simulations was formed at an early stage through mergers, although its subsequent development was still mediated by a bar. Okamoto (2013) argued for an early starburst origin. However, these ideas may be inconsistent with a broad range of ages among the stars of pseudobulges (Fisher et al. 2009).
Kormendy (2012) also highlighted the lens component seen in some barred galaxies, which he argues is the intermediate case in which the bar is dissolving, while a lens in an unbarred galaxy is a fully dissolved bar (see also Combes 2008). He therefore suggested that bar dissolution could be gradual, else we would not observe many transition cases. More moderate mass concentrations do cause bars to weaken and to become more oval (Section 5.6), but no author has commented, as far as this reviewer is aware, that the weakened bar in a simulation inhabits a lenslike structure. Nevertheless, lenses are established features of galaxies that seem most likely to have been created through disk evolution. The fact that we do not yet have a satisfactory explanation for their origin is part of the reason why galaxy evolution remains so fascinating.