Infrared Spectroscopy of a Massive Obscured Star Cluster in the Antennae Galaxies (NGC 4038/9) with NIRSPEC

We present infrared spectroscopy of the Antennae Galaxies (NGC 4038/4039)with NIRSPEC at the W. M. Keck Observatory. We imaged the star clusters in thevicinity of the southern nucleus (NGC 4039) in 0.39" seeing in K-band usingNIRSPEC's slit-viewing camera. The brightest star cluster revealed in thenear-IR (M_K(0) = -17.9) is insignificant optically, but coincident with thehighest surface brightness peak in the mid-IR (12-18 micron) ISO imagepresented by Mirabel et al. (1998). We obtained high signal-to-noise 2.03 -2.45 micron spectra of the nucleus and the obscured star cluster at R ~ 1900. The cluster is very young (4 Myr old), massive (16e6 M_sun), and compact(density ~ 115 M_sun pc^(-3) within a 32 pc half-light radius), assuming aSalpeter IMF (0.1 - 100 M_sun). Its hot stars have a radiation fieldcharacterized by T_eff ~ 39,000 K, and they ionize a compact H II region withn_e ~ 1e4 cm^(-3). The stars are deeply embedded in gas and dust (A_V ~ 9-10mag), and their strong FUV field powers a clumpy photodissociation region withdensities n_H >= 1e5 cm^(-3) on scales of up to 200 pc, radiating L[H_2 1-0S(1)] = 9600 L_sun.