The distribution of star formation in the central regions of spiral galaxies

Abstract

Nuclear rings are characterized by their high star formation rates, and are believed to be the products of gas inflow towards the inner regions of spiral galaxies. We present first results from a statistical survey of these rings, aimed to provide an insight into their triggering mechanisms and star formation properties. We gathered photometric data for 22 nuclear star-forming rings from a larger H survey of nearby spirals, of which we highlight three interesting cases (NGC 1343, NGC 1530, and NGC 5953). The high-quality images reveal that the rings are comprised of several distinct star-forming clusters within a few kiloparsecs of the nucleus. We analyzed each nuclear ring to obtain morphological parameters such as ellipticity, position angle, and size. We then compute the equivalent widths of each H emitting (HII) region forming the nuclear ring. Using modern population synthesis models, we convert the equivalent widths into an estimate of the age of each cluster. In general, ages range from 1 Myr to 10 Myrs throughout the rings. We compare the ages to the positions along each ring to detect possible age-related patterns. Where a bar exists in the host galaxy, we determine the approximate intersection points of the bar to the ring to probe whether the youngest hotspots occur near this intersection. We find that three rings from the sample show age gradients or bisymmetries along the plane of the ring, and in one case the youngest cluster does indeed intersect with one of the bar's interaction points to the ring.