1.2. FIRST FACT: COMPLEXITY IS THE RULE

All recent high-resolution studies consistently report a large complexity in the inner regions of at least half of the local disk galaxy population of all Hubble types. On scales smaller that 1 kpc, more than half of the galaxies host inner bars (within bars), dust or stellar or gaseous disks, spiral-like dust lanes, star-forming rings, spiral arms, a central cluster (Section 1.5), or simply irregular central emission. (e.g., Carollo et al. 1997a; Carollo, Stiavelli, & Mack 1998; Martini & Pogge 1999; Laine et al. 2002; Böker, Stanek, & van der Marel 2003).

Nuclear bars have received particular attention, as they are claimed to play an important role in feeding gas into the centers of galaxies (Shlosman et al. 1989), potentially building central nuclei and bulges, and fueling nuclear activity. Intimate links between bars and central starbursts, in particular circumnuclear star-forming rings, are supported by observations (Knapen, Pérez-Ramírez, & Laine 2002). Other studies point out, however, that, on the nuclear scales, stellar rings and inner disks inside large-scale bars of moderately inclined galaxies could be mistaken for secondary bars or even coexist with them, producing erroneous statistics for the occurrence of nuclear bars (Erwin & Sparke 1999). Still, bona fide secondary inner bars, typically about 250 pc-1 kpc in size (~ 12% the size of their primary bars) appear to be present in as many as 40% of all barred S0-Sa galaxies (Erwin & Sparke 1999). Larger samples of early-type galaxies confirm a high frequency of detection of bars-within-bars (Rest et al. 2001). This high frequency is interpreted to indicate that secondary nuclear bars are relatively long-lived structures. The presence or absence of secondary bars appears to have no significant effect on nuclear activity (as previously reported by, e.g., Regan & Mulchaey 1999). In contrast, nuclear spirals, dusty or star-forming nuclear rings, and off-plane dust are reported to be very often accompanied by LINER or Seyfert nuclei (Erwin & Sparke 1999; Martini & Pogge 1999).

Circumnuclear starburst rings have also been thoroughly investigated (e.g., Maoz et al. 1996, 2001). These rings appear to be a common mode of starbursts in relatively early-type disk galaxies, and are thought to be associated with inner-Linblad resonances. They contain super star clusters with total luminosities as high as M_V -15 mag (L_V 1.3 × 10⁸ L_V, ), radii of the order of a few parsecs, and masses in excess of 10⁴ M. These clusters are very similar to the super star clusters formed in merging systems (e.g., Whitmore et al. 1999; Hunter et al. 2000): they are bound systems, believed to evolve into stellar structures similar to globular clusters. The starburst rings are thought to be likely associated with bar-driven inflow. Schinnerer et al. (2002) report in the double-barred galaxy NGC 4303, which also hosts a circumnuclear star-forming ring (Colina & Arribas 1999), an extremely good agreement between the observed overall gas geometry and dynamical models for the gas flow in barred galaxies (Englmaier & Shlosman 2000). Observational evidence seems thus to be accumulating in support of the theoretical prediction that disk instabilities on large scales are major drivers of evolution on circumnuclear (and nuclear) galactic scales. Similar to nuclear bars, circumnuclear star-forming rings are found to coexist with AGNs but are not associated one-to-one with AGN activity.