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 (NCC 4038/9) with the near-infrared spectrometer (NIRSPEC) at the W. M. Keck Observatory. We imaged the star clusters in the vicinity of the southern nucleus (NGC 4039 ) with 0".39 seeing in the K band using NDRSPEC's slit-viewing camera. The brightest star cluster revealed in the near-IR [M-K(0) = 17.9] is insignifi cant optically but is coincident with the highest surface brightness peak i n the mid-IR (12-18 mu m) Infrared Space Observatory image presented by Mir abel et al. We obtained high signal-to-noise ratio 2.03-2.45 mu m spectra o f the nucleus and the obscured star cluster at R similar to 1900. The clust er is very young (similar to 4 Myr), massive (M similar to 16 x 10(6) M.), and compact (with a density of similar to 115 M. pc(-3) within a 32 pc half -light radius), assuming a Salpeter initial mass function (0.1-100 M.). Its hot stars have a radiation field characterized by T-eff similar to 39,000 K, and they ionize a compact H (II) region with n(e) similar to 10(4) cm(-3 ). The stars are deeply embedded in gas and dust (A(v) similar to 9-10 mag) , and their strong far-ultraviolet field cm powers a clumpy photodissociati on region with densities n(H) greater than or similar to 10(5) cm(-3) on sc ales of similar to 200 pc, radiating L-H2 (1-OS(1)) = 9600 L..