  We report the detection of 6.2um polycyclic aromatic hydrocarbon (PAH) and
rest-frame 4-7um continuum emission in the z=4.055 submillimeter galaxy GN20,
using the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope.
This represents the first detection of PAH emission at z>4. The strength of the
PAH emission feature is consistent with a very high star formation rate of
~1600Msun/yr. We find that this intense starburst powers at least ~1/3 of the
faint underlying 6um continuum emission, with an additional, significant (and
perhaps dominant) contribution due to a power-law-like hot dust source, which
we interpret to likely be a faint, dust-obscured active galactic nucleus (AGN).
The inferred 6um AGN continuum luminosity is consistent with a sensitive upper
limit on the hard X-ray emission as measured by the Chandra X-Ray Observatory
if the previously undetected AGN is Compton-thick. This is in agreement with
the finding at optical/infrared wavelengths that the galaxy and its nucleus are
heavily dust-obscured. Despite the strong power-law component enhancing the
mid-infrared continuum emission, the intense starburst associated with the
photon-dominated regions that give rise to the PAH emission appears to dominate
the total energy output in the infrared. GN20 is one of the most luminous
starburst galaxies known at any redshift, embedded in a rich protocluster of
star-forming galaxies. This investigation provides an improved understanding of
the energy sources that power such exceptional systems, which represent the
extreme end of massive galaxy formation at early cosmic times.
