Mid-Infrared Spectroscopy of two luminous submillimeter galaxies at z ∼ 2.8

Using the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope, we have obtained rest-frame mid-infrared spectroscopy of two bright submillimeter galaxies. SMM J02399-0136 at z = 2.81 shows a superposition of polycyclic aromatic hydrocarbon (PAH) emission features and a mid-infrared continuum, indicating significant and roughly equal contributions to its bolometric luminosity from star formation and from a Compton-thick active galactic nucleus. We derive a new redshift of z = 2.80 for MM J154127+6616 from the IRS spectrum and find this object is dominated by starburst PAH emission. The rest-frame mid- to far-infrared spectral energy distributions are consistent with these submillimeter galaxies being scaled-up versions of local ultraluminous infrared galaxies. The mid-infrared spectra support the scenario that submillimeter galaxies are sites of extreme star formation and represent a key phase in the formation of massive galaxies.