Detection of the Buried AGN in NGC 6240 with the Infrared Spectrograph on The Spitzer Space Telescope

Details Detection of the Buried AGN in NGC 6240 with the Infrared Spectrograph on The Spitzer Space Telescope Detection of the Buried AGN in NGC 6240 with the Infrared Spectrograph on The Spitzer Space Telescope

2005-11-13

arxiv.org/abs/astro-ph/0511381v1

Astrophys.J.640:204-217,2006

Astronomy and Astrophysics

Astrophysics

18 pages, 4 figures, accepted for publication in The Astrophysical Journal

Scientific paper

10.1086/500040

We present mid-infrared spectra of the nearby, Ultraluminous Infrared Galaxy NGC 6240 taken with the IRS on Spitzer. The spectrum of NGC 6240 is dominated by strong fine-structure lines, rotational H2 lines, and PAH emission features. The H2 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 6.7E6 solar masses at T=957K and 1.6E9 solar masses at T=164K, or about 15% of the total molecular gas mass in this system. Noteably, we have detected the [NeV] 14.3 micron emission line, with a flux of 5E-14 erg/cm^2/s, providing the first direct detection of the buried AGN in the mid-infrared. Modelling of the total SED from near to far-infrared wavelengths requires the presence of a hot dust (T=700K) component, which we also associate with the buried AGN. The small [NeV]/[NeII] and [NeV]/IR flux ratios, the relative fraction of hot dust emission, as well as the large 6.2 micron PAH EQW, are all consistent with an apparent AGN contribution of only 3-5% to the bolometric luminosity. However, correcting the measured [NeV] 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 about 20-24% in NGC 6240, which lies within the range implied by fits to the hard X-ray spectrum.

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