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 [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_solar), and compact (with a density of ~115 M_solar pc^-3 within a 32 pc half-light radius), assuming a Salpeter initial mass function (0.1-100 M_solar). 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_H21-0S(1)=9600 L_solar. Data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.