OBSERVATIONS AND MODELING OF THE NUCLEAR STARBURST IN NGC-6946

Near-infrared images in both broad and narrow bands, plus long-slit an d aperture spectroscopy at moderate and high resolution have been obta ined for NGC 6946. We have used these data and data from the literatur e to derive a set of observational constraints on the stellar populati on in the nucleus. We estimate an ultraviolet flux of 10(52) ionizing photons s(-1). A thorough examination of the gas and stellar motions p uts a 2 sigma upper limit on the dynamical mass in the central 8.5('') (which corresponds to 230 pc at an assumed distance of 5.5 Mpc) of si milar to 3 x 10(8) M(.). A detailed study of broadband colors, emissio n-line ratios, and the 10 mu m silicate absorption feature indicates t hat the extinction to the starburst is high, with A(v) = 10.4 mag. An estimate of the supernova rate from both nonthermal radio emission and [Fe II] 1.644 mu m emission from gas excited in fast shocks agrees we ll with our models. The current rate of supernova explosions in the nu cleus is similar to 6 x 10(-3) yr(-1). The bolometric luminosity of th e starburst region is greater than 2.2 x 10(9) L(.). In addition to th e diagnostic features we have used here, the infrared spectra show a w ealth of stellar features and weaker emission lines, including atomic absorption lines and several H-2 emission lines. We have measured the lines of H-2 and compared the ratios to models to determine the excita tion mechanism.The properties of NGC 6946 have been fitted with a star burst model. We find that the starburst probably had a duration of sim ilar to 20 million years and that the star formation is likely to be b iased toward high-mass stars compared with those in the local neighbor hood.