|Annu. Rev. Astron. Astrophys. 2004. 42:
Copyright © 2004 by Annual Reviews. All rights reserved
We see only snapshots of galaxy evolution, so it is difficult to study slow processes. 1 Mergers are an easier problem - transient phenomena such as tidal tails are readily recognizable (e.g., Toomre & Toomre 1972; Schweizer 1990). Why do we think that secular evolution is happening? We begin with an "existence proof" - a review of how n-body simulations account for the morphological features seen in barred galaxies. Our suggestion that pseudobulges are constructed out of rearranged disk gas is embedded in this larger picture of SB secular evolution.
Barred galaxies are reviewed in detail by Sellwood & Wilkinson (1993). They are a rich subject; whole conferences have been devoted to them (e.g., Buta, Crocker, & Elmegreen 1996; Sandqvist & Lindblad 1996). We cover only those subjects that relate to our theme.
2.1. Morphology of Barred Galaxies
Barred galaxy morphology is discussed by de Vaucouleurs (1959, 1963), Sandage (1961, 1975), Kormendy (1979b), Sandage & Bedke (1994), Buta & Crocker (1991), Buta & Combes (1996), and Buta (1995, 1999). We use these diagnostic features:
Barred spiral galaxies are divided into subclasses SB(s), in which the spiral arms begin at the ends of the bar, and SB(r), in which a complete "inner ring" of stars connects the ends of the bar. In the latter case, the spiral arms start somewhere on the ring, "often downstream from the ends of the bar" (Sandage & Bedke 1994). SB(r) and SB(s) galaxies are contrasted in Figure 6; additional SB(r) galaxies are shown in Figures 3 and 5, and additional SB(s) galaxies are shown in Figure 7.
Some barred and oval galaxies have "outer rings" (R) that are 2.2 ± 0.1 times the diameter of the bar or inner disk. Outer rings in barred and unbarred galaxies are similar (Fig. 2, 5). Inner and outer rings are different; there is no size overlap. Some galaxies contain both (Fig. 5).
In SB(s) galaxies, an almost-straight dust lane parallels the ridge line of the bar but is displaced slightly forward in the direction of galactic rotation. Such dust lanes are analogous to and connect up with the prominent dust lanes seen on the trailing side of the arms in global-pattern spirals. Examples are shown in Figures 6, 7, and 8. These dust lanes are almost never present in SB(r) galaxies (Sandage 1961). NGC 1512 in Figure 3 is a rare exception.
Many barred and oval galaxies have very active star formation near their centers, in what is conventionally identified as the bulge. Often the star formation is concentrated in a ring. Figures 3, 7, and 8 show examples.
Many barred galaxies have "bulges" that are themselves elongated into a structure resembling a bar. Examples are shown in Figure 14.
Many early-type SB galaxies contain a "lens" in the disk - a shelf of slowly decreasing surface brightness with a sharp outer edge. Lenses have intrinsic axial ratios of ~ 0.85; the bar usually fills the longest dimension. These properties are discussed in Kormendy (1979a, b, 1981, 1982a) and in Athanassoula et al. (1982). Lenses are sometimes seen in unbarred galaxies; NGC 1553 is the best example (Freeman 1975; Kormendy 1984). Lenses in early-type galaxies look similar to oval disks in late-type galaxies (Section 3.2); it is not clear whether or not they are physically similar. Lenses are illustrated in Figures 2 and 5.
These features can be understood at least qualitatively as results of secular evolution driven by nonaxisymmetric gravitational potentials. An exact correspondence between n-body simulations and observations cannot be expected, because real galaxies have a complicated interplay between gas, star formation, and energy feedback from massive young stars back into the interstellar medium. Such effects, along with the self-gravity of the gas, are often omitted from simulations and at best are included only approximately. Nevertheless, n-body simulations have been conspicuously successful in reproducing the structure of barred galaxies.
Figure 2. Prototypical outer rings in barred and unbarred galaxies. NGC 1291 is an (R)SB(lens)0/a galaxy - it has a bar embedded in a lens of the same major-axis diameter (see also Kormendy 1979b). NGC 4736 is classified (R)SA(r)ab. The purpose of this figure is to show how blue the outer rings are: they are dominated by young stars. Both rings also contain H I gas (van Driel et al. 1988; Bosma et al. 1977b). Sources: NGC 1291 - Buta, Corwin, & Odewahn (2003); NGC 4736 - NOAO.
Figure 3. NGC 1512, an SB(r)ab galaxy imaged with HST by Maoz et al. (2001). This figure (courtesy NASA and ESA) illustrates the stellar population of inner rings. As is common in intermediate-Hubble-type galaxies, the bar in NGC 1512 is made of old, red stars and the disk is made of young, blue stars. The point of this figure is that the inner ring has the same stellar population as the disk, not the bar. Also seen at center is a nuclear star formation ring that is shown at higher magnification in Figure 8 and the start of a well developed, curved dust lane (cf. Figures 6 - 8) that extends out of the field of view to the right. The corresponding dust lane on the other side is visible near the central ring but not at larger radii. The outer parts of NGC 1512 are illustrated by Sandage & Bedke (1994), who note that NGC 1512 is morphologically normal except for some distortion of its outer spiral structure (not shown here) by a tidal encounter with neighboring NGC 1510.
1 Mergers are an easier problem - transient phenomena such as tidal tails are readily recognizable (e. g., Toomre & Toomre 1972; Schweizer 1990). Back.