STRONG POLYCYCLIC AROMATIC HYDROCARBON EMISSION FROM z ≈ 2 ULIRGs

Using the Infrared Spectrograph on board the Spitzer Space Telescope, we present low-resolution (64 < lambda/delta lambda < 124), mid-infrared (20-38 mu m) spectra of 23 high-redshift ULIRGs detected in the Bootes field of the NOAO Deep Wide-Field Survey. All of the sources were selected to have (1) f(v)(24 mu m) > 0.5mJy; (2) R-[24] > 14 Vega mag; and (3) a prominent rest frame 1.6 mu m stellar photospheric feature redshifted into Spitzer's 3-8 mu m IRAC bands. Of these, 20 show emission from polycyclic aromatic hydrocarbons (PAHs), usually interpreted as signatures of star formation. The PAH features indicate redshifts in the range 1.5 < z < 3.0, with a mean of < z > = 1.96 and a dispersion of 0.30. Based on local templates, these sources have extremely large infrared luminosities, comparable to that of submillimeter galaxies. Our results confirm previous indications that the rest-frame 1.6 mu m stellar bump can be efficiently used to select highly obscured star-forming galaxies at z approximate to 2, and that the fraction of starburst-dominated ULIRGs increases to faint 24 mu m flux densities. Using local templates, we find that the observed narrow redshift distribution is due to the fact that the 24 mu m detectability of PAH-rich sources peaks sharply at z = 1.9. We can analogously use observed spectral energy distributions to explain the broader redshift distribution of Spitzer-detected ULIRGs that are dominated by an active galactic nucleus (AGN). Finally, we conclude that z approximate to 2 sources with a detectable 1.6 mu m stellar opacity feature lack sufficient AGN emission to veil the 7.7 mu m PAH band.