Minor Merger Origin for the Circumnuclear Starburst in NGC 7742

We present an emission-line diagnostic analysis of integral-fieldspectroscopic observations that cover the central kiloparsec of NGC 7742. ThisSa galaxy hosts a spectacular nuclear starburst ring and nuclear regionscharacterized by low-ionization emission. The gas in the ring rotates in theopposite sense to the stars in the galaxy, suggesting a recent merging oracquisition event. The combination of integral-field measurements for theH-alpha + [NII] emission lines from DensePak and the H-beta and [OIII] emissionfrom Sauron allow the construction of diagnostic diagrams that highlight thetransition from star formation in the nuclear ring to excitation byhigh-velocity shocks or by a central AGN towards the center. DensePakmeasurements for the [SII] line ratio reveal very low gas densities in thenuclear ring, Ne < 100 per cm^3, characteristic of massive HII regions.Comparison with MAPPINGS III models for starbursts with low gas densities showthat the ring is of roughly solar metallicity. This suggests that the gas inthe nuclear ring originated in a stellar system capable of substantiallyenriching the gas metallicity through sustained star formation. We suggest thatNGC 7742 cannibalised a smaller galaxy rich in metal-poor gas, and that starformation episodes in the ring have since increased the metallicity to itspresent value.Comment: 4 figure