Massive Star Formation in the Infrared-bright Galaxy NGC 972

Abstract

We present the results of optical broadband and Hα 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. Massive star formation activity in this galaxy is evident from the continuum-subtracted Hα image, which shows the presence of circumnuclear activity and disk star formation within a radius of 3.4 kpc. The circumnuclear star-forming regions are distributed in a ring of radius 630 pc and closely associated with an inner dust ring. Aperture photometry has been performed on the individual H ii regions, and we estimate an age <=5.4 Myr for the nuclear starburst using evolutionary synthesis models. The Hα luminosity of the nucleus is comparable to that of starburst nuclei. The emission-line ratios of NGC 972 are also indicative of a nuclear starburst, which is powered by photoionization 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 rate is estimated as 0.32 M_ȯ yr^-1 for the nucleus and 2.1-2.7 M_ȯ yr^-1 in the inner 3.6 kpc of the galaxy.