Double Bars, Inner Disks, and Nuclear Rings in Early-Type Disk Galaxies

We present results from an imaging survey of an unbiased sample ofthirty-eight early-type (S0--Sa), low-inclination, optically barred galaxies inthe field. Our goal was to find and characterize central stellar and gaseousstructures: secondary bars, inner disks, and nuclear rings. Bars inside barsare surprisingly common: at least one quarter of the sample galaxies (possiblyas many as 40%) are double-barred, with no preference for Hubble type or thestrength of the primary bar. A typical secondary bar is ~12% of the size of itsprimary bar and 240--750 pc in radius. We see no significant effect ofsecondary bars on nuclear activity. We also find kiloparsec-scale inner disksin at least 20% of our sample, almost exclusively in S0 galaxies. These disksare on average 20% the size of their host bar, and show a wider range ofrelative sizes than do secondary bars. Nuclear rings are present in about athird of our sample. Most are dusty, sites of current or recent star formation,or both; such rings are preferentially found in Sa galaxies. Three S0 galaxies(15% of the S0's) appear to have purely stellar nuclear rings, with no evidencefor dust or recent star formation. The fact that these central stellarstructures are so common indicates that the inner regions of early-type barredgalaxies typically contain dynamically cool and disklike structures. This isespecially true for S0 galaxies, where secondary bars, inner disks, and/orstellar nuclear rings are present at least two thirds of the time. (abridged)Comment: LaTeX, 15 pages, 7 EPS figures; to appear in The Astronomical Journal (July 2002