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 andCompton-thick AGNs in NGC 6240, we have obtained mid-infrared images andlow-resolution spectra of the galaxy with sub-arcsecond spatial resolutionusing the Keck Telescopes. Despite the high spatial resolution (~200 pc) of ourdata, 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 umluminosity of the system, has a mid-infrared spectrum and a mid-/far-infraredspectral energy distribution consistent with starbursts. At the same time,however, it is also possible to attribute up to 60% of the bolometricluminosity to an AGN, consistent with X-ray observations, if the AGN is heavilyobscured and emits mostly in the far-infrared. This ambiguity arises becausethe intrinsic variation of properties among a given galaxy population (e.g.,starbursts) introduces at least a factor of a few uncertainty even into themost robust AGN-starburst diagnostics. We conclude that with presentobservations it is not possible to determine the dominant power source ingalaxies when AGN and starburst luminosities are within a factor of a few ofeach other.Comment: 27 pages, 7 figures, 2 tables; Accepted for publication in A