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 4038/9) 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 [MK(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 ~ 16 × 106 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 Teff ~ 39,000 K, and they ionize a compact H II region with ne ~ 104 cm-3. The stars are deeply embedded in gas and dust (AV ~ 9-10 mag), and their strong far-ultraviolet field powers a clumpy photodissociation region with densities nH ≳ 105 cm-3 on scales of ~200 pc, radiating L = 9600 L☉.