Bars are among the most common morphological features of disk-shaped galaxies. Unlike spiral arms, bars cross the "spiral-S0 divide" in the Hubble sequence and are abundant among spirals (at the 50-70% level) when both SAB and SB types are considered (de Vaucouleurs 1963; Sellwood & Wilkinson 1993). The bar fraction has cosmological significance (Sheth et al. 2008), and many estimates of the nearby galaxy bar fraction have been made from both optical and IR studies (see Buta et al. 2010a for a summary of recent work).
Bars are fairly well-understood features of galaxy morphology that have been tied to a natural instability in a rotationally supported stellar disk (see review by Sellwood & Wilkinson 1993). The long-term maintenance of a bar in a mostly isolated galaxy is thought to depend on how effectively it transfers angular momentum to other galaxy components, such as the halo (Athanassoula 2003). Bars are thought to be transient features that, in spiral galaxies, may dissolve and regenerate several times over a Hubble time (Bournaud & Combes 2002). Alternatively, bars may be long-lived density wave modes that drive secular evolution of both the stellar and gaseous distributions (Zhang & Buta 2007; Buta & Zhang 2009). The possible secular evolution of bars in S0 galaxies is discussed by Buta et al. (2010b). Bars are also thought to drive spiral density waves (Kormendy & Norman 1979; Buta et al. 2009; Salo et al. 2010).
The actual morphology of bars shows interesting variations that merit further study. The family classifications SAB and SB indicate some measure of bar strength, but do not allude to the varied appearances of bars even among those only classified as SB. Regular bars, such as those illustrated in Figure 8, are the conventional types that define the SB class. Figure 22 shows "ansae"-type bars, referring to bars which have "handles" or bright enhancements at the ends. Martinez-Valpuesta et al. (2007) carried out a statistical study and found that ansae are present in 40% of early-type barred galaxies and are very rare for types later than stage Sb. Ansae are usually detectable in direct images, but their visibility can be enhanced using unsharp-masking (all the right frames for each galaxy in Figure 22). Morphologically, ansae may be small round enhancements like those seen in NGC 5375 and 7020, but in some cases, ansae are approximately linear enhancements, giving the bar a parallelogram appearance as in NGC 7098, or curved arcs, giving the bar a partial ring appearance as in NGC 1079. Color index maps in the dVA show that ansae are generally as red as the rest of the bar, indicating the features are stellar dynamical in origin, rather than gas-dynamical. Nevertheless, ansae made of star-forming regions are known. Martinez-Valpuesta et al. (2007) illustrate the case of NGC 4151, a well-known Seyfert 1 galaxy with a strong bar-like inner oval. The appearance of this galaxy's ansae in the 1.65µm H-band is shown in the lower right frames of Figure 22, where the ansae are seen to have irregular shapes compared to the others shown.
Figure 22. Examples showing ansae bar morphologies as compared to one mostly non-ansae bar. For each galaxy, the left frame is the full image while the right frame is an unsharp masked image, both in units of mag arcsec-2. The galaxies are: (left to right): Row 1 - NGC 5375 (SDSS g) and 7020 (I) (both round ansae type); Row 2 - NGC 7098 (I, linear, partly wavy ansae) and NGC 1079 (Ks, curved ansae); Row 3 - NGC 4643 (I, mostly non-ansae type with trace of ring arcs at bar ends) and NGC 4151 (OSUBSGS H, irregular ansae).
Another subtlety about bars is their general boxy character. Athanassoula et al. (1990) showed that generalized ellipses fit the projected isophotes of bars better than do normal ellipses. For 11 or 12 SB0 galaxies examined in this study, a significant degree of boxiness was found near the bar semi-major axis radius.
The unsharp-masked image of NGC 7020 in Figure 22 shows an X-shaped pattern in the inner regions that is the likely signature of a significantly three-dimensional bar. NGC 1079 and 5375 shows hints of similar structure. The X-pattern is expected to be especially evident in edge-on galaxies which show the extended vertical structure of bars. Many examples have been analyzed (Bureau et al. 2006; see also the dVA). Buta et al. (2010a) have suggested that edge-on bars recognized from the X-pattern be denoted "SBx."
The cause of bar ansae is uncertain. In simulations, Martinez-Valpuesta et al. (2006) found that ansae form late, after a second bar-buckling episode in a disk model with a live axisymmetric halo, and appear as density enhancements in both the face-on and edge-on views.
Figure 23 shows three examples of galaxies having oval inner disks. These features are described in detail by Kormendy and Kennicutt (2004 = KK04), who present both photometric and kinematic criteria for recognizing them. The images in Figure 23 are optical and have been cleaned of foreground and background objects, and have also been deprojected based on the mean shape and major axis position angle of faint outer isophotes. In all three cases, the presence of an outer ring allows clear recognition of the oval shape, assuming that the inner and outer structures are in the same plane. The upper panels of Figure 23 show the full morpholopy with the outer rings, while the lower panels show the bright oval inner disks. The shapes of the oval disks are varied and range from axis ratio 0.84 for NGC 4736 to 0.55 for NGC 1808. The most striking example is NGC 4941, whose oval disk includes a bright, normal-looking spiral pattern with isophotal axis ratio 0.68. Many other examples are provided by KK04.
Figure 23. Examples of oval disk galaxies. Left to right: NGC 4736 (B), NGC 4941 (B), and NGC 1808 (V). The images are cleaned of foreground and background objects and have been deprojected and rotated so that the major axis of the oval is horizontal. The upper panels show the ovals imbedded within outer rings, while the lower panels focus on the ovals alone.
The ovals appear to play a bar-like role in these galaxies. The outer rings may be resonant responses to the nonaxisymmetric potential of the ovals, which clearly harbor a great deal of mass in spite of the mildness of their departures from axisymmetry. As noted by KK04, oval disk galaxies are expected to evolve secularly in much the same manner as typical barred galaxies. On the other hand, ovals themselves could be products of bar secular evolution. Laurikainen et al. (2009) found that the near-IR bar fraction in S0 galaxies is significantly less than that in S0/a or early-type spiral galaxies (also found by Aguerri et al. 2009 in the optical), while the oval/lens fraction is higher, suggesting that some ovals/lenses might be dissolved bars. Further evidence that bars might be dissolving in some galaxies is the detection of extremely weak bars in residual images of visually nonbarred S0 galaxies where a two-dimensional decomposition model has been subtracted. Such a bar is detected in the SA0° galaxy NGC 3998 (Laurikainen et al. 2009). Aguerri et al. (2009) suggest that central concentration is a key factor in bar evolution, and that a unimodal distribution of bar strengths argues against the idea that bars dissolve and reform (Bournaud & Combes 2002).
Regular barred galaxies also often include an oval bounded by an elongated inner ring. The deprojected blue light images of two examples are shown in Figure 24. NGC 1433 has a very strong normal bar and one of the most intrinsically elongated inner rings known. The inner ring lies at the edge of an oval which is more conspicuous at longer wavelengths. In NGC 1433, the inner ring, the oval, and the bar are all aligned parallel to each other. The situation is different in ESO 565-11, whose bright oval is strongly misaligned with a prominent bar, but similar to NGC 1433, the oval is bounded by an inner pseudoring. The suggestion in this case is that the bar and the oval are independent patterns.
Figure 24. Examples of two galaxies having highly a elongated inner ring at the boundary of a broad oval. Left to right: NGC 1433, ESO 565-11 (both B-band). Each galaxy also has a prominent bar which is aligned with the oval and inner ring in NGC 1433, but misaligned with these features in ESO 565-11. ESO 565-11 also has a highly elongated, large nuclear ring of star-forming regions.