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Symposium 241

Abstract details

The physical conditions of the ionized gas and the stellar populations in circumnuclear starbursts
Johan H. Knapen, Marc Sarzi, Lisa M. Mazzuca

Abstract
We present an emission-line diagnostic analysis of integral-field spectroscopic observations that cover the circumnuclear ring-like regions in a small number of spiral galaxies. We concentrate on the specific case of the Sa galaxy NGC 7742, which hosts a spectacular circumnuclear starburst ring and nuclear regions characterized by low-ionization emission. The gas in the ring rotates in the opposite sense to the stars in the galaxy, suggesting a recent merging or acquisition event. The combination of integral-field measurements for the H$\alpha+$N[{\sc ii}] emission lines from DensePak and the H$\beta$ and [O{\sc iii}] emission from Sauron allow the construction of diagnostic diagrams that highlight the transition from star formation in the ring to excitation by high-velocity shocks or by a central AGN towards the center. DensePak measurements for the [S{\sc ii}] line ratio reveal very low gas densities in the ring, $N_{\rm e}~<~100 \,{\rm cm}^{-3}$, characteristic of massive H{\sc ii} regions. Comparison with MAPPINGS~III models for starbursts with low gas densities shows that the ring is of roughly solar metallicity. This suggests that the gas in the circumnuclear ring originated in a stellar system capable of substantially enriching the gas metallicity through sustained star formation. We suggest that NGC~7742 cannibalised a smaller galaxy rich in metal-poor gas, and that star formation episodes in the ring have since increased the metallicity to its present value. We also give a preview of our results from a similar analysis of the nuclear region of NGC~4314, and outline the promise of using the techniques developed for other galaxies.