Progress on bulge formation is dominated by two conceptual advances. This section revisits secular evolution in disk galaxies. This is a major addition that complements our picture of galaxy evolution by hierarchical clustering. I begin here because all further discussion depends on the resulting realization that the dense central components in galaxies come in two varieties with different formation processes, classical and pseudo bulges. Section 3 discusses the second conceptual advance, the discovery of a new channel for the formation of classical bulges. This is the formation at high z of unstable clumps in gas-rich disks; they sink to the center along with lots of disk gas and starburst and relax violently. In this way, bulge formation proceeds largely as it does during major mergers. This leads to a discussion of the merger formation of both bulges and ellipticals in Section 4.
Our pictures of the merger formation of classical bulges and ellipticals and the secular growth of pseudobulges out of disks both got their start in the late 1970s. The importance of major mergers (Toomre & Toomre 1972; Toomre 1977) in a hierarchically clustering universe (White & Rees 1978) got a major boost from the realization that CDM halos make galaxy collision cross sections much bigger than they look. This subject “took off” and rapidly came to control our formation paradigm. Secular evolution is a more difficult subject – slow processes are hard to study – and it did not get a similar boost from the CDM revolution. However, the earliest papers on the subject come from the same time period: e.g., Kormendy (1979a) emphasized the importance of slow interactions between nonaxisymmetric galaxy components; Kormendy (1979b) first pointed out the existence of surprisingly disky bulges; Combes & Sanders (1981) showed that boxy pseudobulges are edge-on bars. Kormendy (1981, 1982) reviewed and extended the results on disky bulges. This subject did not penetrate the galaxy formation folklore; rather, it remained a series of active but unconnected “cottage industries” for the next two decades. Nevertheless, by the 1990s, the concept–if not yet the name– of disky pseudobulges was well established (see Kormendy 1993 for a review), and the idea that boxy bulges are edge-on bars was well accepted (see Athanassoula 2005 for a more recent and thorough discussion). I hope it is fair to say that the comprehensive review by Kormendy & Kennicutt (2004) has helped to convert this subject into a recognized paradigm – it certainly is so in this book – although it is still not as widely understood or taken into account as is hierarchical clustering.
Kormendy & Kennicutt (2004) remains up-to-date and comprehensive on the basic results and on observations of prototypical pseudobulges. However, new reviews extend and complement it. Kormendy & Fisher (2005, 2008) and Kormendy (2008, 2012) provide the most important physical argument that was missing in Kormendy & Kennicutt (2004): Essentially all self-gravitating systems evolve toward more negative total energies (more strongly bound configurations) by processes that transport kinetic energy or angular momentum outward. In this sense, the secular growth of pseudobulges in galaxy disks is analogous to the growth of stars in protostellar disks, the growth of black holes in black hole accretion disks, the sinking of Jupiters via the production of colder Neptunes in protoplanetary disks, core collapse in globular clusters, and the evolution of stars into red (super)giants with central proto white dwarfs, neutron stars, or stellar-mass black holes. All of these evolution processes are related. So secular disk evolution and the growth of pseudobulges is very fundamental, provided that some process redistributes angular momentum in the disk. My Canary Islands Winter School lectures (Kormendy 2012) are an up-to-date observational review that includes environmental secular evolution. Sellwood (2014) provides an excellent theoretical review.
Boxy pseudobulges are discussed in four chapters of this book; I concentrate on disky pseudobulges. Fisher & Drory (2015) review the distinction between classical and pseudo bulges from a purely phenomenological point of view. That is, they intercompare observational diagnostics to distinguish between the two bulge types with no reference to physical interpretation. This is useful, because it gives relatively unbiased failure probabilities for each diagnostic. They are not wholly independent, of course, because they are intercompared. But they are independent enough in execution so that we get a sufficient estimate of the failure probability when they are combined by multiplying the individual failure probabilities.
Kormendy & Kennicutt (2004), Kormendy (2012), and KH13 strongly advocate the use of as many bulge classification criteria as possible. The reason is that any one criterion has a non-zero probability of failure. Confusion in the literature (e.g., Graham 2011) results from the fact that some authors use a single classification criterion (e.g., Sérsic index) and so get results that conflict with those derived using multiple criteria. But we have long known that most classical bulges have n ≥ 2, that most pseudobulges have n < 2, and that there are exceptions to both criteria. No-one should be surprised that Sérsic index sometimes fails to correctly classify a bulge. This is the point that Fisher & Drory (2015) make quantitative.
Fisher & Drory (2015) show that the failure probability of each classification criterion that they test is typically 10–20%. A few criteria are completely robust (if B / T ≳ 0.5, then the bulge is classical) and a few are less reliable (star formation rate cannot be used for S0s). But, by and large, it is reasonable to conclude that the use of M criteria, each with failure probability єm, results in a classification with a failure probability of order the product of the individual failure probabilities, Π1m єm. This becomes very small very quickly as M grows even to 2 and especially to M > 2. For example, essentially all bulge-pseudobulge classifications in KH13 were made using at least two and sometimes as many as five criteria.
Fisher & Drory (2015) also contribute new criteria that become practical as new technology such as intergral-field spectroscopy gets applied to large samples of galaxies. These are incorporated into an enlarged list of classification criteria below.
A shortcoming of Fisher & Drory's approach is that it is applied without regard to galaxy Hubble types. But we know that both many S0s and many Sbcs contain pseudobulges, but the latter all tend to be star-forming whereas the former generally are not. This is one reason for their conclusion (e.g.) that high star formation rate near the galaxy center robustly implies a pseudobulge, but no star formation near the center fails to prove that the bulge is classical. Classification criteria that involve gas content and star formation rate cannot be applied to S0 galaxies. Application to Sas is also fragile. Fortunately, most criteria do work for early-type galaxies.
2.1. Enlarged List of Bulge-Pseudobulge Classification Criteria
Kormendy & Kennicutt (2004), Kormendy (2012), and Fisher & Drory (2015) together provide the following improved list of (pseudo)bulge classification criteria. I note again: The failure rate for individual criteria ranges from 0% to roughly 25%. Therefore the use of more criteria quickly gives much more reliable results.
It is important to emphasize that classical and pseudo bulges can occur together. Fisher & Drory (2015) review examples of dominant pseudobulges that have small central classical bulges. And some giant classical bulges contain nuclear disks (e.g., NGC 3115: Kormendy et al. 1996b; NGC 4594: Kormendy et al. 1996a).
Criterion (9) for boxy pseudobulges works only for edge-on and near-edge-on galaxies. In face-on galaxies, it is easy to identify the elongated parts of bars, but they also have rounder, denser central parts, and these are not easily distinguished from classical bulges (Athanassoula 2015; Laurikainen & Salo 2015). So the above criteria almost certainly fail to find some pseudobulges in face-on barred galaxies.
2.2. Secular Evolution in Disk Galaxies: Applications
Progress in many subjects depends on a full integration of the picture of disk secular evolution into our paradigm of galaxy evolution. Examples include the following: