Dominik A. Riechers, Alexandra Pope, Emanuele Daddi, Lee Armus, Christopher L. Carilli, Fabian Walter, Jacqueline Hodge, Ranga-Ram Chary, Glenn E. Morrison, Mark Dickinson, Helmut Dannerbauer, David Elbaz
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 ~1800Msun/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). 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. By comparing the 6um AGN continuum luminosity to an upper limit on the hard X-ray emission as measured by the Chandra X-Ray Observatory, we also find evidence that the previously undetected AGN in this source is Compton-thick, consistent with the finding at optical/infrared wavelengths that the galaxy and its nucleus are heavily dust-obscured. 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.
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http://arxiv.org/abs/1306.5235
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