Star Formation in Polar-Ring Galaxies

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Polar-ring galaxies are systems with two nearly orthogonal rotational axes, and are therefore clearly the result of galaxy-galaxy interactions. The host galaxies of polar rings are typically gas-poor early-type systems. The rings themselves, however, are typically gas-rich, blue, star-forming systems. They have traditionally been modelled as the remnants of accreted dwarf irregular galaxies. Published results on the molecular gas content, and nebular abundances argue against this model. We present H-alpha imaging data for three polar-ring galaxies, and derive their HII region luminosity functions. In all three galaxies, the HII region luminosity functions are much steeper than found for any dwarf galaxy. The slopes are generally closer to those of Sa galaxies. Thus the HII region LFs of polar rings join the ISM abundances in resembling the very earliest-type spirals galaxies, rather than their supposed dwarf irregular precursors.