|Annu. Rev. Astron. Astrophys. 2004. 42:
Copyright © 2004 by Annual Reviews. All rights reserved
4.4. Bars Within Bars
Figure 14 shows galaxies that have a secondary bar interior to the main bar. The inner bar is, in fact, the "bulge" - its surface brightness increases rapidly toward the center, far above the inward extrapolation of the disk brightness profile. However, bars are disk phenomena. Seeing a nuclear bar is strong evidence that a galactic center is dominated by a pseudobulge.
Figure 14. Bars within bars. Each galaxy image is rotated so that the main bar is horizontal. Contour levels are close together at large radii and widely spaced in the nuclear bars. NGC 3081 and NGC 1433 have inner rings. NGC 1291 is also shown in Figure 2, NGC 3081 and NGC 3945 in Figure 5, and NGC 6782 in Figure 8. The images are courtesy Ron Buta.
The nuclear bar in NGC 1291 was seen as long ago as Evans (1951). de Vaucouleurs (1975) saw nuclear bars in four of the six galaxies illustrated in Figure 14: NGC 1291, NGC 1433 (see also Sandage & Brucato 1979), NGC 1543 (see de Vaucouleurs 1959), and NGC 3081.
Other early examples are NGC 1326 (de Vaucouleurs 1974b), NGC 2859, NGC 3945, NGC 7743 (Kormendy 1979b), NGC 1543 (Sandage & Brucato 1979), NGC 1317 (Schweizer 1980), and NGC 2950 (Kormendy 1981, 1982a, b). Kormendy concluded: "triaxial SB bulges and bars rotate rapidly and are therefore dynamically similar. Both are different from elliptical galaxies, which rotate slowly." We return to these points in Section 4.6.
The number of known examples grew rapidly as work on barred galaxies accelerated (Jarvis et al. 1988; Buta 1990; Buta & Crocker 1993; Shaw et al. 1993b, 1995; Wozniak et al. 1995; Friedli et al. 1996; Elmegreen et al. 1996; Jungwiert, Combes, & Axon 1997; Mulchaey et al. 1997; Erwin & Sparke 1999, 2003; Márquez et al. 1999; Martini & Pogge 1999; Colina & Wada 2000; Greusard et al. 2000; Rest et al. 2001). Erwin (2004) has compiled a catalog, and Friedli (1996) and Erwin (2004) provide reviews.
Recent studies focus on larger and more representative samples and therefore yield better estimates of what fraction of SB galaxies contain nuclear bars. Erwin & Sparke (2002) find nuclear bars in 26 ± 7 % of their sample of 38 SB galaxies. They remark that the true fraction could be as large as 40 %; they could not detect nuclear bars in the (many) objects that have central dust. As in the previous section, pseudobulge features are surprisingly common. The galaxies in the above survey are S0 - Sa; these are the Hubble types that are most likely to contain classical bulges.
Laine et al. (2002) analyze HST NICMOS H-band images of a matched sample of Seyfert and non-Seyfert galaxies. The sample is slightly biased toward early Hubble types but otherwise is representative. They find that 28 ± 5 % of their barred galaxies have a nuclear bar. They also find several indications that nuclear and main bars have a different origin, most notably that main bar sizes are proportional to the scale length of the disk while nuclear bar sizes are uncorrelated with the size of the disk and almost always smaller than ~ 1.6 kpc in radius. Nuclear bars and nuclear star-forming rings have similar size distributions when normalized by the galactic diameter D25. They argue plausibly that this means that nuclear bar radii, like nuclear ring radii, are bounded approximately by ILR (see also Pfenniger & Norman 1990; Friedli & Martinet 1993).
Observations like these support the cononical hypothesis that nuclear bars form when infalling disk gas builds up a central, cold, and disky system that is sufficiently self-gravitating to become barred. How this happens is not known. One possibility is that a cold nuclear disk suffers its own bar instability, independent of that of the main bar (Friedli & Martinet 1993; Combes 1994).
A good sign that we understand the essence of nuclear bar dynamics is the observation (Figure 14) that inner bars are oriented randomly with respect to main bars (Buta & Crocker 1993; Friedli & Martinet 1993; Shaw et al. 1995; Wozniak et al. 1995; Friedli et al. 1996; Erwin & Sparke 2002). This can be understood within the dynamical framework of Section 2.2. At small radii, (r) - (r) / 2 reaches a high peak in galaxies that have such high central mass concentrations. A bar's pattern speed p seeks out approximately the local angular velocity - / 2 at which closed ILR orbits precess. Therefore, the pattern speeds of inner bars are almost certainly much higher than those of main bars 4 Similarly, because - / 2 decreases outward, the pattern speeds of spiral arms are likely to be slower than those of bars (Sellwood 1985; Sparke & Sellwood 1987; Sellwood & Sparke 1988; Sellwood & Wilkinson 1993). This accounts for the comment in Section 2.1 that the spiral arms of SB(r) galaxies "often [begin] downstream from the ends of the bar" (Sandage & Bedke 1994). (see Pfenniger & Norman 1990, Friedli & Martinet 1993, Buta & Combes 1996, and Maciejewski & Sparke 2000 for further discussion). Kinematic decoupling of main and nuclear bars is observed by Emsellem et al. (2001) and Corsini et al. (2003).
Shlosman, Frank, & Begelman (1989) suggest that bars within bars are a primary way to transport gas farther inward than the gravitational torque of the main bar can achieve. To fuel nuclear activity in galaxies, they envisage a hierarchy of bars within bars. Triple bars have been seen (Friedli 1996 and Erwin & Sparke 1999 provide reviews).
NGC 4736 is an example of a nuclear bar in an unbarred but oval galaxy (Block et al. 1994; Möllenhoff, Matthias, & Gerhard 1995). It emphasizes again the similarity between bars and ovals as engines for secular evolution.
4 Similarly, because - / 2 decreases outward, the pattern speeds of spiral arms are likely to be slower than those of bars (Sellwood 1985; Sparke & Sellwood 1987; Sellwood & Sparke 1988; SW93). This accounts for the comment in Section 2.1 that the spiral arms of SB(r) galaxies "often [begin] downstream from the ends of the bar" (Sandage & Bedke 1994). Back.