Evidence for double-barred galaxies - galaxies where a smaller, secondary bar is nested inside a larger, primary bar - dates back to the mid-1970s, when de Vaucouleurs (1975) pointed out three examples (NGC 1291, 1326, and 1543). These and a handful of other candidates identified in the 1980s (e.g., Kormendy 1982) remained isolated observational curiosities for some years.
Theoretical interest was sparked by Shlosman et al. (1989), who suggested that a system of nested bars could be an especially effective way to transfer gas from galactic (kpc) scales down to near-nuclear (sub-100 pc) scales and thereby feed active nuclei. They also sketched a possible formation scenario, in which the gas inflow driven by a large-scale bar gave rise to a central concentration which became dynamically decoupled and unstable, leading in turn to the formation of an independently rotating inner bar.
The early 1990s saw the first attempts to define samples of double bars and to investigate some of their properties, particularly in work by Buta & Crocker (1993) and Friedli & Martinet (1993); this probably marks the point at which double bars became recognized as a distinct category of galaxy. This led in turn to imaging surveys specifically aimed at identifying and characterizing double-barred galaxies, starting with Wozniak et al. (1995) and Friedli et al. (1996), and then with larger samples by Jungwiert et al. (1997), Erwin & Sparke (2002), and Laine et al. (2002). Kinematic studies of double-barred systems are also becoming more common (e.g., Emsellem et al. 2001, Schinnerer et al. 2001, Moiseev et al. 2004); see the contribution by Alexei Moiseev in this volume.
The same period saw the first detailed attempts to characterize and model double-barred galaxies - and their formation - theoretically Pfenniger & Norman (1990), Friedli & Martinet (1993), Combes (1994). More recent work has focused on the question of whether and under what circumstances self-consistent orbital structures supporting two bars can exist (e.g., Maciejewski & Sparke 1997, Maciejewski & Sparke 2000, El-Zant & Shlosman 2003, Maciejewski & Athanassoula 2008), and on hydrodynamical and N-body modeling of double-bar formation (e.g. Rautiainen & Salo 1999, Englmaier & Shlosman 2004, Debattista & Shen 2007). (See the contributions by Witold Maciejewski and Juntai Shen in this volume for more details.)
In this review, I will focus on the current observational status of double-barred galaxies, with a particular emphasis on what know about them as a population, and what we can tell about the inner bars, both structurally and dynamically. I will also consider the question, first raised by Shlosman et al. (1989), of whether double-barred galaxies can promote nuclear activity.
Figure 1. NGC 2950, an example of a double-barred galaxy. Left and middle panels show R-band isophotes, with the outer bar (left panel) and inner bar (middle panel) indicated by arrows. Right panel: unsharp mask of the same image, highlighting the ends of the inner bar. Although the two bars are (almost) perpendicular in this galaxy, inner and outer bars are found at all possible relative orientations (see Figure 6).