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

Abstract

We present infrared spectroscopy of the Antennae galaxies (NGC 4038/9)with the near-infrared spectrometer (NIRSPEC) at the W. M. KeckObservatory. We imaged the star clusters in the vicinity of the southernnucleus (NGC 4039) with 0.39'' seeing in the K band using NIRSPEC'sslit-viewing camera. The brightest star cluster revealed in the near-IR[M_{K}(0)~=-17.9] is insignificant optically but is coincidentwith the highest surface brightness peak in the mid-IR (12-18 {μ}m)Infrared Space Observatory image presented by Mirabel et al. We obtainedhigh signal-to-noise ratio 2.03-2.45 {μ}m spectra of the nucleus andthe obscured star cluster at R~1900. The cluster is very young (~4 Myr),massive (M~16x10^{6} M_{solar}), and compact (with adensity of ~115 M_{solar} pc^{-3} within a 32 pchalf-light radius), assuming a Salpeter initial mass function (0.1-100M_{solar}). Its hot stars have a radiation field characterizedby T_{eff}~39,000 K, and they ionize a compact H II region withn_{e}~10^{4} cm^{-3}. The stars are deeplyembedded in gas and dust (A_{V}~9-10 mag), and their strongfar-ultraviolet field powers a clumpy photodissociation region withdensities n_{H}{\gt}~10^{5} cm^{-3} on scales of~200 pc, radiating L_{H 2}1-0S(1) =9600L_{solar}. Data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation.