A NEAR-INFRARED SPECTROSCOPIC STUDY OF THE LUMINOUS MERGER NGC-3256 .1. CONSTRAINTS ON THE INITIAL MASS FUNCTION OF THE STARBURST

Near-infrared spectroscopy of the central regions of the merger galaxy NGC 3256 is presented. The central 3'' of the nucleus show strong rec ombination lines of hydrogen, helium, and a prominent CO band absorpti on at 2.3 mu m. These features suggest there is a large population of OB stars and red supergiants associated with a vigorous episode of sta r formation activity. The relative strength of several recombination l ines and the slope of the 2.2 mu m continuum indicate that the nucleus of NGC 3256 is affected by an extinction A(k) = 0.6 mag. A stellar po pulation model is used for interpreting the observational properties o f the galaxy. We show that the Br gamma equivalent width and the stren gth of the CO band at 2.3 mu m provide a simple tool for determining t he age of a starburst event. A chi 2 analysis is presented which allow s both the age of the burst and the slope of the initial mass function to be determined in a unique way. The analysis shows that the starbur st started between 12 and 27 Myr ago. Given the dynamical mass observe d in the nucleus of NGC 3256, our analysis yields an IMF index [psi(m) proportional to m(-alpha)]alpha < 2.2 which is shallower than recentl y derived IMFs in the solar neighborhood and the Magellanic Clouds.