Spitzer mid-infrared spectroscopy of infrared luminous galaxies at z ~ 2. III. far-IR to radio properties and optical spectral diagnostics

Abstract

We present the FIR, millimeter, and radio photometry and optical and NIR spectroscopy of a sample of 48 z ~ 1–3 Spitzer-selected ULIRGs with IRS MIR spectra. Our goals are to compute their bolometric emission and to determine both the presence and relative strength of their AGN and starburst components. We find that strong-PAH sources tend to have higher MIPS 160 μm and MAMBO 1.2 mm fluxes than weak-PAH sources. The depth of the 9.7 μm silicate feature does not affect MAMBO detectability. We fit the far-IR SEDs of our sample and find an average LIR ~ 7 × 10^(12)L_☉ for our z > 1.5 sources. Our spectral decomposition suggests that strong-PAH sources typically have ~20%-30% AGN fractions of L_(IR). The weak-PAH sources by contrast tend to have ≥70% AGN fractions, with a few sources having comparable contributions of AGN and starbursts. The optical line diagnostics support the presence of AGNs in the bulk of the weak-PAH sources. With one exception, our sources are narrow-line sources, show no obvious correspondence between the available optical extinction and the silicate feature depth, and, in two cases, show some evidence for outflows. Radio AGNs are present in both strong- and weak-PAH sources. This is supported by our sample's FIR-to-radio ratios (q) being consistently below the average value of 2.34 for local star-forming galaxies. We use survival analysis to include the lower limits given by the radio-undetected sources, arriving at (q) = 2.07 ± 0.01 for our z > 1.5 sample. In total, radio and, where available, optical line diagnostics support the presence of AGNs in 57% of the z > 1.5 sources, independent of IR-based diagnostics. For higher z sources, the AGN luminosities alone are estimated to be >10^(12) L_☉, which, supported by the available [O III] luminosities, implies that the bulk of our sources host obscured quasars.

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