Nuclear star formation in the hotspot galaxy NGC 2903

We present high-resolution near-infrared imaging obtained using adaptive op tics and HST/NICMOS, and ground-based spectroscopy of the hotspot galaxy NG C 2903. Our near- infrared resolution imaging enables us to resolve the inf rared hotspots into individual young stellar clusters or groups of these. T he spatial distribution of the stellar clusters is not coincident with that of the bright H II regions, as revealed by the HST/NICMOS Pa alpha image. Overall, the circumnuclear star formation in NGC 2903 shows a ring-like mor phology with an approximate diameter of 625 pc. The star formation properties of the stellar clusters and HII regions have been studied using the photometric and spectroscopic information in conjunc tion with evolutionary synthesis models. The population of bright stellar c lusters shows a very narrow range of ages, 4-7 x 10(6) yr after the peak of star formation, or absolute ages 6.5-9.5 x 10(6) yr (for the assumed short -duration Gaussian bursts), and luminosities similar to the clusters found in the Antennae interacting galaxy. This population of young stellar cluste rs accounts for some 7-12 per cent of the total stellar mass in the central 625 pc of NGC 2903. The H II regions in the ring of star formation have lu minosities close to that of the supergiant H II region 30 Doradus, they are younger than the stellar clusters, and they will probably evolve into brig ht infrared stellar clusters similar to those observed today. We find that the star formation efficiency in the central regions of NGC 2903 is higher than in normal galaxies, approaching the lower end of infrared luminous gal axies.