MASSIVE STAR-FORMATION IN THE INFRARED-BRIGHT GALAXY NGC-972

We present the results of optical broadband and Ha imaging studies of the infrared-bright galaxy NGC 972. The broadband images and B/R color map reveal the peculiar morphology and complex dust distribution. Mas sive star formation activity in this galaxy is evident from the contin uum-subtracted Ha image, which shows the presence of circumnuclear act ivity and disk star formation within a radius of 3.4 kpc. The circumnu clear star-forming regions are distributed in a ring of radius 630 pc and closely associated with an inner dust ring. Aperture photometry ha s been performed on the individual H II regions, and we estimate an ag e less than or equal to 5.4 Myr for the nuclear starburst using evolut ionary synthesis models. The Ha luminosity of the nucleus is comparabl e to that of starburst nuclei. The emission-line ratios of NGC 972 are also indicative of a nuclear starburst, which is powered by photoioni zation by a large number of hot, massive stars. The enhancement of low -ionization lines compared with normal H II regions can be attributed to the influence of dust on the thermal properties of the nebula or to contributions by shocks from supernova remnants. The star formation r ate is estimated as 0.32 M-. yr(-1) for the nucleus and 2.1-2.7 M-. yr (-1) in the inner 3.6 kpc of the galaxy.