Abstract details

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.