ARlogo Annu. Rev. Astron. Astrophys. 2004. 42: 603-683
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This paper reviews internal processes of secular evolution in disk galaxies. We concentrate on one important consequence: the buildup of dense central components that look like classical, i.e., merger-built bulges but that were made slowly by disks out of disk material. These are called pseudobulges. Our discussion updates a review by Kormendy (1993).

The relative importance of the different physical processes of galaxy evolution (Figure 1) changes as the Universe expands. Rapid processes that happen in discrete events are giving way to slow, ongoing processes.

Figure 1

Figure 1. Morphological box (Zwicky 1957) of processes of galactic evolution updated from Kormendy (1982a). Processes are divided vertically into fast (top) and slow (bottom). Fast evolution happens on a free-fall ("dynamical") timescale, tdyn ~ (G rho)-1/2, where rho is the density of the object produced and G is the gravitational constant. Slow means many galaxy rotation periods. Processes are divided horizontally into ones that happen purely internally in one galaxy (left) and ones that are driven by environmental effects such as galaxy interactions (right). The processes at center are aspects of all types of galaxy evolution. This paper is about the internal and slow processes at lower-left.

At early times, galactic evolution was dominated by a combination of dissipative collapse (Eggen, Lynden-Bell, & Sandage 1962; Sandage 1990) and mergers (Toomre 1977a) of galaxies that virialized out of the density fluctuations of cold dark matter. These are the top processes in Figure 1. The evolution timescale was short, on the order of the dynamical time of an individual halo, tdyn ~ (1 / G rho)1/2, where rho is the mean density and G is the gravitational constant (Binney & Tremaine 1987, Equation 2-30). The processes were violent. Many present-day galaxies owe their properties to this violence. Because mergers scramble disks and induce dissipation and starbursts, they are thought to make classical bulges and elliptical galaxies. Here, we do not review classical bulges other than to contrast them to pseudobulges. Most work on galaxy evolution in the past 25 years has concentrated on hierarchical clustering and mergers. As the Universe expands, and as galaxy clusters virialize and acquire large internal velocities, mergers get less common (Toomre 1977a; Conselice et al. 2003).

In the distant future, internal secular processes will become dominant. By these, we mean slow processes, ones that have timescales that are much longer than the dynamical time tdyn. To be interesting, they must operate over long times. Some secular processes, such as disk heating via stellar enounters with molecular clouds, are well known (Spitzer & Schwarzschild 1951, 1953). But star-star relaxation is exceedingly slow almost everywhere in almost every galaxy. Therefore, relevant secular processes generally involve the interactions of individual stars or gas clouds with collective phenomena such as bars, oval distortions, spiral structure, and triaxial dark matter halos. Also important are the interactions of these collective phenomena with each other. Given that hierarchical clustering continues today, has there been time for secular processes to have any significant effect? A clue that the answer is "yes" is provided by galaxies with superthin - and fragile - disks but apparently no bulges (e.g., de Vaucouleurs 1974a; Goad & Roberts 1981; Matthews, Gallagher, & van Driel 1999b; Freeman 2000; van der Kruit et al. 2001). It may seem counterintuitive to use bulgeless galaxies to argue that there has been time for secular evolution to make pseudobulges, but it is quite remarkable that such galaxies can exist within the paradigm of hierarchical clustering. They again provide us with an "existence proof". They show that some galaxies have suffered no major merger violence since the onset of star formation in the disk (Toth & Ostriker 1992). So there has been time for secular evolution to be important in at least some galaxies. Given that mergers make bulges and ellipticals, these tend to be late-type galaxies. Actually, since the latest-type galaxies are (pseudo)bulgeless, secular evolution is likely to be most important in intermediate-late type galaxies, i.e., Sbcs. But even some S0 and Sa galaxies contain pseudobulges. Secular processes have received less attention than galaxy mergers.

Still, this subject has made rapid progress. Many reviews discuss secular evolution in barred and oval galaxies (Kormendy 1979a, b; 1981, 1982a; Norman 1984; Combes 1991, 1998, 2000, 2001; Bosma 1992; Martinet 1995; Friedli & Benz 1995; Pfenniger 1996a, b, 2000; Sellwood & Debattista 1996; Buta 1995, 1999, 2000; Athanassoula 2002; Maciejewski 2003; Wada 2003; Sellwood & Shen 2004; and especially Sellwood & Wilkinson 1993, hereafter SW93, and Buta & Combes 1996). Discussions of pseudobulge formation include, besides the above, Pfenniger & Norman (1990); Courteau (1996b); Wyse, Gilmore, & Franx (1997); Carollo, Ferguson, & Wyse (1999); Kormendy & Gebhardt (2001); Balcells (2002); and especially Kormendy (1993) and Carollo (2003).

Theory and observations point to a variety of processes that redistribute energy in disks. Section 2 reviews evidence that bars and ovals rearrange disk gas into outer rings, inner rings, and central mass concentrations. Crunching gas makes stars (Schmidt 1959); this star formation produces a central stellar subsystem that has the high density and steep density gradient of a bulge but that was not formed by galaxy mergers.

Secular evolution is not confined to barred and oval galaxies. Bars can self-destruct by building up the central mass concentration, so secular evolution may have happened even if no bar is seen today. Global spiral structure also makes galaxies evolve, albeit more slowly than do bars.

These processes are manifestations of very general dynamical principles. Disks spread in radius - the inner parts shrink and the outer parts expand - because this lowers the total energy for fixed total angular momentum (Lynden-Bell & Kalnajs 1972; Tremaine 1989). Two-dimensional spreading by angular momentum transport is as fundamental for rotation-dominated disks as is three-dimensional spreading by energy transport in the core collapse of ellipsoidal systems dominated by random motions. The reason is the same, too. Self-gravitating systems have negative specific heats, so increasing the central density by flinging away the periphery lowers the total energy (Lynden-Bell & Wood 1968; Binney & Tremaine 1987). What makes evolution important in some systems and not in others? The determining factor is whether any evolution process is fast enough. Core collapse requires short relaxation times. Galaxy disks have long relaxation times, so their evolution is interesting only if they have an alternative to relaxation. Non-axisymmetries provide the engine for rapid evolution.

We will argue that pseudobulges are one result of this evolution. Of key importance is the observation that they retain some memory of their disky origin. We review this subject in detail (Section 4), because it is central to any conclusion that evolution has happened, and because it is the only way that we can recognize pseudobulges.

Next, we review observations of gas content and star formation rates. From these, we estimate pseudobulge growth times. These prove to be in good agreement with stellar population ages. So the picture of pseudobulge growth from rearranged disk gas is internally consistent.

The purpose of this paper is to connect up the large number of disparate results in this subject into a well developed and (as we hope to show) a well supported paradigm. Still, many questions remain unanswered. We especially need a better understanding of the relative importance of mergers and secular evolution as a function of galaxy type and luminosity. We hope that this paper will provide a concrete context that will allow efficient progress in this subject.

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