K-band spectroscopy of an obscured massive stellar cluster in the Antennae galaxies (NGC 4038/4039) with NIRSPEC

Andrea Gilbert
James Graham
Ian McLean
Eric Becklin

Note: imported from RIT’s Digital Media Library running on DSpace to RIT Scholar Works in February 2013. Also archived in: arXiv:astro-ph/0002357 v1 17 Feb 2000


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 (12−18µm) ISO image presented by Mirabel et al. (1998). We obtained high signal-to-noise 2.03−2.45 µm spectra of the nucleus and the obscured star cluster at R ∼ 1900. The cluster is very young (age ∼ 4 Myr), massive (M ∼ 16 × 106 M⊙), and compact (density ∼ 115 M⊙ pc−3 within a 32 pc half-light radius), assuming a Salpeter IMF (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 FUV field powers a clumpy photodissociation region with densities nH ∼ > 105 cm−3 on scales of ∼ 200 pc, radiating LH21−0 S(1) = 9600 L⊙. (Refer to PDF file for exact formulas).