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5.1. Bars

5.1.1. Morphology and Photometry

Bars are distinguished from other triaxial features such as elliptical galaxies and oval disks by their larger ellipticities and by the qualitative difference between their major- and minor-axis brightness profiles (Fig. 40). Along the major axis the surface brightness is nearly constant interior to a sharp outer edge, while along the minor axis the profile is steep, as in an r1/4 law. The long axis of a bar has a profile similar to that of an oval disk. However, oval disks have axial ratios b/a ~ 0.8 (compared to b/a ~ 0.2 for a bar), and they have outer cutoffs along both principal axes. It is possible that all of these differences are more quantitative than qualitative; i.e., that bars, ellipticals and oval disks represent different extremes of a continuum of properties. However, there are two reasons why I prefer to treat bars, ovals and ellipticals as different. First, there is growing evidence that there are significant, qualitative differences in the dynamics of these objects. Some of this evidence is discussed below. Second, there appear to be few transition objects. In particular, it is important to note that barred ellipticals are not observed: whenever a galaxy has a bar, it also has a disk. This association with high-angular-momentum material foreshadows the difference between bars and ellipticals found in section 5.1.2: bars rotate more rapidly than ellipticals.

The distinction between bars and oval disks is not well understood (section 5.3). However, there is one sense in which bars and ovals are nearly equivalent. They produce very similar non-axisymmetric forces (bars are more elongated than ovals, but they contain a smaller fraction of the mass). As a result, secular evolution in gas dynamics proceeds similarly in barred and oval galaxies (sections 5.2 and 5.4). Since many ovals are classified SA (section 2.5.1), the fraction of all disk galaxies which are triaxial enough so that secular evolution may be important is larger than the 65% (de Vaucouleurs 1963) which are classified SB or SAB.

Little detailed photometry of bars is available. In particular, there is too little data for studies of characteristic parameters. In published photometry, the fraction of light in the bar ranges from 0% to ~ 32% (Crane 1975; Benedict 1976; Okamura 1978). These fractions underestimate the local perturbations produced by the bar because much of the light and mass of a galaxy are at large radii.

The vertical structure of a bar has been studied in detail only in NGC 4762 (Tsikoudi 1977, 1980; Burstein 1979d). Generally it is difficult to recognize an edge-on bar. However, in NGC 4762 the series of plateaus seen in the major-axis profile are plausibly identified with the nested ovals of nearly constant brightness seen in SB(lens)0 galaxies. Tsikoudi's and Burstein's results then suggest that any bar is part of the thin disk. More photometry is needed, but the morphology of edge-on galaxies (at least half of which are barred) also suggests that bars are quite flat.

Published photometry and isophotometry therefore allow us to estimate the three-dimensional shapes of bars. Typically the principal axes appear to have length scales in the proportion 1:2:10. Extremes are estimated to be ~ 1 : 3 : 15 and 1:1 1/2:4. If these values are confirmed by detailed photometry, then, in the words of M. Schwarzschild (1980b), bars are "ferociously triaxial".

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