2000ApJ...533L..57G

We present infrared spectroscopy of the Antennae galaxies (NGC 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 NIRSPEC's slit-viewing camera. The brightest star cluster revealed in the near-IR [M_K(0)≃-17.9] is insignificant optically but is coincident with the highest surface brightness peak in the mid-IR (12-18 µm) Infrared Space Observatory image presented by Mirabel et al. We obtained high signal-to-noise ratio 2.03-2.45 µm spectra of the nucleus and the obscured star cluster at R∼1900. The cluster is very young (∼4 Myr), massive (M∼16x10⁶ M_☉), and compact (with a density of ∼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∼39,000 K, and they ionize a compact H II region with n_e∼10⁴ cm^–3. The stars are deeply embedded in gas and dust (A_V∼9-10 mag), and their strong far-ultraviolet field powers a clumpy photodissociation region with densities n_H≳10⁵ cm^–3 on scales of ∼200 pc, radiating L_H2_1–0S(1)=9600 L_☉.