2006ApJ...640..204A

We present mid-infrared spectra of the nearby ultraluminous infrared galaxy NGC 6240 taken with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. The spectrum of NGC 6240 is dominated by strong fine-structure lines, rotational H₂lines, and polycyclic aromatic hydrocarbon (PAH) emission features. The H₂line fluxes suggest molecular gas at a variety of temperatures. A simple two-temperature fit to the S(0) through S(7) lines implies a mass of ∼6.7x10⁶ M_☉at T∼957 K and ∼1.6x10⁹ M_☉ at T∼164 K, or about 15% of the total molecular gas mass in this system. Notably, we have detected the [Ne V] 14.3 µm emission line, with a flux of 5x10^–14 ergs/cm2/s, providing the first direct detection of the buried active galactic nucleus (AGN) in the mid-infrared. Modeling of the total spectral energy distribution (SED) from near- to far-infrared wavelengths requires the presence of a hot dust (T∼700 K) component, which we also associate with the buried AGN. The small [Ne V]/[Ne II] and [Ne V]/IR flux ratios, the relative fraction of hot dust emission, and the large 6.2 µm PAH equivalent width (EQW), are all consistent with an apparent AGN contribution of only 3%-5% to the bolometric luminosity. However, correcting the measured [Ne V] flux by the extinction implied by the silicate optical depth and our SED fitting suggests an intrinsic fractional AGN contribution to the bolometric luminosity of ∼20%-24% in NGC 6240, which lies within the range implied by fits to the hard X-ray spectrum.