Mid-IR Spectroscopy of Submm Galaxies: Extended Star Formation in High-z Galaxies

Ultra-luminous infrared galaxies (ULIRGs; L > 1012 L_) are quite rare in the local universe, but seem to dominate the co-moving energy density at z > 2. Many are opticallyfaint, dust-obscured galaxies that have been identified only relatively recently by the detection of their thermal dust emission redshifted into the sub-mm wavelengths. These submm galaxies (SMGs) have been shown to be a massive objects (M_ ∼ 1011 M_) undergoing intense starformation( SFRs ∼ 102 − 103 M_ yr−1 ) and the likely progenitors of massive ellipticals today. However, the AGN contribution to the far-IR luminosity had for years remained a caveat to these results. We used the Spitzer Infrared Spectrograph (IRS) to investigate the energetics of 24 radio-identified and spectroscopically-confirmed SMGs in the redshift range of 0.6 < z < 3.2. We find emission from Polycyclic Aromatic Hydrocarbons (PAHs) – which are associated with intense star-formation activity – in > 80% of our sample and find that the median mid-IR spectrum is well described by a starburst component with an additional power-law continuum representing < 32% AGN contribution to the far-IR luminosity. We also find evidence for a more extended distribution of warm dust in SMGs compared to the more compact nuclear bursts in local ULIRGs and starbursts, suggesting that SMGs are not simple high-redshift analogs of local ULIRGs or nuclear starbursts, but have star formation which resembles that seen in less-extreme star-forming environments at z ∼ 0.