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 recombination lines of hydrogen, helium, and a prominent CO band absorption 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 star formation activity. The relative strength of several recombination lines 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 population model is used for interpreting the observational properties of the galaxy. We show that the Br gamma equivalent width and the strength of the CO band at 2.3 mu m provide a simple tool for determining the age of a starburst event. A chi 2 analysis is presented which allows 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 starburst started between 12 and 27 Myr ago. Given the dynamical mass observed 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 recently derived IMFs in the solar neighborhood and the Magellanic Clouds.