Subarcsecond mid-infrared observations of NGC 6240: Limitations of active galactic nucleus-starburst power diagnostics

In order to examine the relative importance of powerful starbursts and Compton-thick active galactic nuclei (AGNs) in NGC 6240, we have obtained mid-infrared images and low-resolution spectra of the galaxy with subarcsecond spatial resolution using the Keck telescopes. Despite the high spatial resolution (similar to 200 pc) of our data, no signature of the hidden AGNs has been detected in the mid-infrared. The southern nucleus, which we show provides 80%-90% of the total 8-25 mu m luminosity of the system, has a mid-infrared spectrum and a mid-/far-infrared spectral energy distribution consistent with starbursts. At the same time, however, it is also possible to attribute up to 60% of the bolometric luminosity to an AGN, consistent with X-ray observations, if the AGN is heavily obscured and emits mostly in the far-infrared. This ambiguity arises because the intrinsic variation of properties among a given galaxy population (e.g., starbursts) introduces at least a factor of a few uncertainty even into the most robust AGN-starburst diagnostics. We conclude that with present observations it is not possible to determine the dominant power source in galaxies when AGN and starburst luminosities are within a factor of a few of each other.