Morphology, Star Formation, and Kinematics of Nuclear Rings

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This thesis presents a detailed optical study with the goal of better understanding the elusive physical nature of nuclear rings. We use H-alpha imaging of the central kilo-parsec region of a large sample of spiral galaxies to verify that nuclear rings occur primarily in spiral types Sa-Sbc. Late-type galaxies have a patchy and more diffuse circumnuclear appearance in H-alpha. From the parent sample, we identify 22 nuclear rings and analyze the HII regions that comprise them. We derive the ages throughout each nuclear ring, and find that the stellar content is consistently very young, with ages ranging from 1 Myr to 10 Myrs. Approximately half of the rings contain azimuthal age gradients that encompass at least 25% of the ring. Two-thirds of the galaxies containing a nuclear ring and a bar show a link between the youngest HII region(s) and the location along the ring where the bar dust lanes merge. We show that regions of enhanced star formation, as seen in nuclear rings, correspond to regions with (1) the strongest H-alpha emission, (2) high luminosities of order 10^40 erg/s - 10^42 erg/s, (3) low residual velocities of order 10 km/s, and (4) low velocity dispersions ranging from 20 km/s- 50 km/s. The lack of strong non-circular motions in the rings, coupled with a direct relationship between the position angles and ellipticities of the rings and those of their host galaxies, indicate the rings are in the same plane as the disk and are circular. We find a correlation between the largest velocity deviations (just outside of the rings) and the location of the bar minor axis, where the dominant family of stellar orbits transitions from the x1 family to the x2 family. Lastly, we apply two-dimensional line ratio diagnostics to separate the physical environments of the nuclear ring and an AGN present in NGC 7742. Results indicate very low gas densities in the nuclear ring, and show the transition from star formation in the ring to excitation by high-velocity shocks or by a central AGN towards the center. Comparison to starburst population models reveals that the HII regions comprising the ring are of roughly solar metallicity.