Spitzer Mid - IR Spectroscopy of Infrared - Luminous Galaxies at z ∼ 1 −

I discuss Spitzer GO programs aimed at studying the mid-IR spectral properties of IR-luminous galaxies at high z. I present the IRS lowresolution, mid-IR spectra of a sample of 52 sources from our GO1 medium size program. We found that ∼ 75% of the sample are at z ∼ 1.5−2.6, with redshifts determined based on features including PAH emission and silicate absorption in the mid-IR spectra. This dataset revealed the first direct evidence that PAH features are already a significant component of a dusty galaxy’s spectrum as early as z = 2. Among the 52 targets, one third are starbursts at z ∼ 2 with strong PAH emission, and two thirds are AGN-dominated systems with strong silicate absorption and weak or no PAH emission. The bulk of the sample are dusty, infrared-luminous galaxies at z ∼ 1−3 with estimated Lbol ∼ 10 L⊙. At such a high luminosity, a significant fraction of our sample have dust-obscured, strong AGN components. I will discuss the implication for 24μm-selected, 1mJy or brighter samples. I also present the ground-based near-IR/optical spectroscopy and 1.2mm continuum followup observations of this sample. The Keck spectroscopic redshifts confirm the redshift measurements based on the mid-IR spectra. Since half of our sample are optically faint sources with R ≥ 25.5mag (Vega), our results demonstrate the potential of using mid-infrared spectroscopy, especially the aromatic and silicate features produced by dust grains, to directly probe optically faint and infrared-luminous populations at high redshift.