A Hubble Space Telescope study of star formation in the inner resonance ring of NGC 3081

We present Hubble Space Telescope Wide Field Planetary Camera 2 images of the inner regions of NGC 3081, an absolute magnitude M-B = - 20.0 early-type barred spiral having four well-defined resonance rings: a nuclear ring, an inner ring, an outer R-1 ring, and an outer R-2' pseudoring. Here we focus on a photometric study of the inner ring, a feature likely associated with an inner 4: 1 resonance near the ends of the bar. The ring is notable for its high contrast and sharp definition, which is due to a significant degree of active star formation. The ring is also notable for its significant intrinsic elongation and parallel alignment with the bar. These characteristics influence the way star-forming sites are distributed around the ring. The ring is lined by numerous blue sources, many of which appear to be slightly diffuse compared with the stellar point-spread function. These blue sources are strongly concentrated within +/-60degrees of the bar axis and follow the Halpha distribution well. The blue sources are much larger than typical Galactic open or globular clusters and may represent young massive clusters like the "populous clusters'' of the LMC and objects seen previously mainly in intermediate- to late-type spiral galaxies. We also present an analysis of the integrated light of the inner ring, to deduce information on its star formation history. A profile analysis is used to separate the ring from the background old disk starlight. High-resolution Fourier analysis is used to search for wavelength-dependent phase shifts along the ring to determine if star-forming sites stay in the ring as they age. The results give an intriguing picture of a galaxy in an advanced evolutionary state where periodic orbits are clearly manifested in the morphology.