A Hubble Space Telescope Study of Star Formation in the Inner Resonance Ring of NGC 3081

Abstract

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 R₁ 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 +/-60° of the bar axis and follow the Hα 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.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS 5-26555.