Detection of the Buried Active Galactic Nucleus in NGC 6240 with the Infrared Spectrograph on theSpitzer Space Telescope

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 H 2 lines, and polycyclic aromatic hydrocarbon (PAH) emission features. The H 2 line 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 ~6.7 × 10 6 M ☉ at T ~ 957 K and ~1.6 × 10 9 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 5 × 10 -14 ergs cm -2 s -1 , 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.