|Annu. Rev. Astron. Astrophys. 2004. 42:
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How general are the results of the previous section? We reviewed the effects of bars on disks as the most clearcut example of internal secular evolution. But we do not mean to create the impression that such evolution is important only in the ~ 1/3 of all disk galaxies that look barred at optical wavelengths. In this section, we first review evidence that many apparently unbarred galaxies clearly show bars in the infrared. With the previous section as a guide, we then argue that similar evolution happens in unbarred but oval galaxies and at slower rates in global-pattern spirals. In fact, any nonaxisymmetry in the gravitational potential plausibly rearranges disk gas.
3.1. Many "Unbarred" Galaxies Show Bars In the Infrared
Near-infrared images penetrate dust absorption and are insensitive to the low-M / L frosting of young stars in Sb - Sm disks. We then see the underlying old stars that trace the mass distribution. The most important revelation is that bars are hidden in many galaxies that look unbarred in the optical (Block & Wainscoat 1991; Spillar et al. 1992; Mulchaey & Regan 1997; Mulchaey et al. 1997; Seigar & James 1998; Knapen et al. 2000; Eskridge et al. 2000, 2002; Block et al. 2001; Laurikainen & Salo 2002; Whyte et al. 2002). About two-thirds of all spiral galaxies look barred in the infrared. Quantitative measures of bar strengths based on infrared images (Buta & Block 2001; Block et al. 2001; Laurikainen & Salo 2002) should prove useful in gauging the consequences for secular evolution.
Some bars are weak in amplitude. But secular evolution can be more important than this suggests, because many of bars are embedded in oval disks (Section 3.2) that contribute at least as much to the nonaxisymmetric potential as do the bars. NGC 1068 is one example (Scoville et al. 1988; Thronson et al. 1989; Pompea & Rieke 1990); for others, see Hackwell & Schweizer (1983); Block et al. (2002); Jarrett et al. (2003).
Hidden bars are the first reason why the results of Section 2 are relevant to more than just the galaxies that look barred at optical wavelengths.