Infrared Spectroscopy of a Massive Obscured Star Cluster in the Antennae Galaxies (NGC 4038/4039) with NIRSPEC
Abstract
We present infrared spectroscopy of the Antennae Galaxies (NGC 4038/4039) with NIRSPEC at the W. M. Keck Observatory. We imaged the star clusters in the vicinity of the southern nucleus (NGC 4039) in 0.39" seeing in 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 coincident with the highest surface brightness peak in the mid-IR (12-18 micron) ISO image presented by Mirabel et al. (1998). We obtained high signal-to-noise 2.03 - 2.45 micron spectra of the nucleus and the obscured star cluster at R ~ 1900. The cluster is very young (4 Myr old), massive (16e6 Msun), and compact (density ~ 115 Msun pc(-3) within a 32 pc half-light radius), assuming a Salpeter IMF (0.1 - 100 Msun). Its hot stars have a radiation field characterized by Teff ~ 39,000 K, and they ionize a compact H II region with ne ~ 1e4 cm(-3). The stars are deeply embedded in gas and dust (AV ~ 9-10 mag), and their strong FUV field powers a clumpy photodissociation region with densities nH >= 1e5 cm(-3) on scales of up to 200 pc, radiating L[H2 1-0 S(1)] = 9600 Lsun.
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