Annu. Rev. Astron. Astrophys. 2004. 42: 603-683 Copyright © 2004 by Annual Reviews. All rights reserved

1.1. What is a Bulge? Classical and Physical Morphology

Before we proceed, we need to be clear on what we mean by a "bulge". This will indicate why, for some galaxies, we use the term "pseudobulge".

Renzini (1999) clearly states the canonical interpretation of Hubble-Sandage-de Vaucouleurs classifications: "It appears legitimate to look at bulges as ellipticals that happen to have a prominent disk around them [and] ellipticals as bulges that for some reason have missed the opportunity to acquire or maintain a prominent disk." We adopt this point of view. However, as observations improve, we discover more and more features that make it difficult to interpret every example of what we used to call a "bulge" as an elliptical living in the middle of a disk. This leads authors to agonize, "Are bulges of early-type and late-type spirals different? Are their formation scenarios different? Can we talk about bulges in the same way for different types of galaxies?" (Fathi & Peletier 2003).

We will conclude that early- and late-type galaxies generally do make their dense central components in different ways. This is not recognized in classical morphology, because it defines classification bins - deliberately and with good reason - without physical interpretation. Sandage and Bedke (1994) describe how, in the early stages of investigating a subject, a classifier should look for "natural groups" (Morgan 1951) of objects with similar features. Sandage emphasizes that it is important not to be led astray by preconceptions: "The extreme empiricist claims that no whiff of theory may be allowed into the initial classification procedures, decisions, and actions." Nevertheless, some choice of which features to consider as important and which to view as secondary must be made. After all, the goal is to understand the physics, and the exercise is useful only if classification bins at least partly isolate unique physics or order galaxies by physically relevant parameters. The Hubble-Sandage-de Vaucouleurs classification scheme has done these things remarkably well.

However, it is reasonable to expect that improved understanding of galaxies will show that the classification missed some of the physics. Also, some features of galaxies could not be observed well enough in the photographic era to be included. These include high-surface-brightness disky substructures in galaxy centers. Consistent with physical morphology as discussed in Kormendy (1982a), we wish to distinguish components in galaxies that have different origins.

At the level of detail that we nowadays try to understand, the time has passed when we can make effective progress by defining morphological bins with no guidance from a theory. Disks, bulges, and bars were different enough that we could do this. Afterward, robust conclusions could be reached, e.g., about the relative timescales of collapse and star formation (Eggen, Lynden-Bell, & Sandage 1962). But even inner rings and spiral arms - which are not subtle - do not scream the appropriate message, which is that spiral arms are details that would disappear quickly and without a trace if the driving mechanism switched off, whereas we will see that rings are a permanent rearranging of disk material. Inner rings are, in this sense, more fundamental than spiral arms. Years ago, people commonly reacted badly to a classification as complicated as (R)SB(r)b. The reason, we believe, was that the phenomenology alone did not sell itself. People did not see why this level of detail was important. Now, we will show that every letter in the above classification has a clearcut meaning in terms of formation physics. This is the goal of physical morphology.

We adopt the view that bulges are ellipticals living in the middle of a disk. Ellipticals formed via mergers (Toomre 1977a, Schweizer 1990). Therefore we do not use the name "bulge" for every central component that is in excess of the inward extrapolation of an exponential fitted to the disk brightness profile. If the evidence suggests that such a component formed by secular processes, we call it a "pseudobulge". In practice, we cannot be certain about formation mechanisms. If the component in question is very E-like, we call it a bulge, and if it is disk-like, we call it a pseudobulge. We comment on the difficulty in classifying intermediate cases in sections 4, 7, 9.1.

Finally, we comment on one of the biggest problems in this subject. It is exceedingly easy to get lost in the details. Many of the papers that we review interpret observations or simulations in much more detail than we will do here. For example, it is common for observers to distinguish nuclei, nuclear bars, nuclear disks, nuclear spiral structure, boxy bulges, exponential bulges (sometimes more than one per galaxy, if the brightness profile is piecewise exponential), and star formation rings. We will discuss all of these features, because they are central to the developing picture of what secular evolution can accomplish. But we will consider them all to be features of pseudobulges, because the evidence is that all of them are built by secular evolution out of disk material. In the same way, global spiral structure, flocculent spiral structure, and no spiral structure in S0 galaxies are all features of disks.