The inner parts of some spiral galaxies have higher star-formation rates than usual and this star formation is frequently arranged in a ring or pseudo-ring pattern around their nuclei. In general, circumnuclear star-forming regions (CNSFRs), also referred to as "hotspots," are alike luminous and large disk H ii regions, but look more compact and have higher peak surface brightness (Kennicut et al. 1989). In many cases they contribute substantially to the UV emission of the entire nuclear region (e.g. Colina et al. 2002). Their Ha luminosities overlap with those of H ii galaxies, being typically higher than 1039ergs-1, which indicates that they are relatively massive ionizing star clusters. These regions are expected to have a high metallicity corresponding to their position near the galactic bulge. They have considerable weight in the determination of abundance gradients, which in turn are widely used to constrain chemical-evolution models and constitute excellent laboratories to study how star formation proceeds in high-metallicity environments.

The Metal-rich Universe, eds. G. Israelian and G. Meynet. Published by Cambridge University Press. © Cambridge University Press 2008.

4,500 5,000

4,500 5,000

7,000 9,000

9,200 9,400

Figure 26.1. Left panel: the blue spectrum of region 4 in NGC 2903. Right panel: the red spectrum of region 3 in nGc 3351.

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