We present a new diagnostic diagram for mid-infrared spectra of infrared galaxies based on the equivalent width of the 6.2 µm PAH emission feature and the strength of the 9.7 µm silicate feature. Based on the position in this diagram we classify galaxies into 9 classes ranging from continuum-dominated AGN hot dust spectra and PAH-dominated starburst spectra to absorption-dominated spectra of deeply obscured galactic nuclei. We find that galaxies are systematically distributed along two distinct branches: one of AGN and starburst-dominated spectra and one of deeply obscured nuclei and starburst-dominated spectra. The separation into two branches likely reflects a fundamental difference in the dust geometry in the two sets of sources: clumpy versus nonclumpy obscuration. Spectra of ULIRGs are found along the full length of both branches, reflecting the diverse nature of the ULIRG family.

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We present R $ 600, 10Y37 m spectra of 53 ultraluminous infrared galaxies (ULIRGs), taken using the Infrared Spectrograph on board Spitzer. The spectra show fine-structure emission lines of neon, oxygen, sulfur, silicon, argon, chlorine, iron, and phosphorous; molecular hydrogen lines, and C 2 H 2 , HCN, and OH À absorption features. We employ diagnostics based on the fine-structure lines, the polycyclic aromatic hydrocarbon (PAH) features and the 9.7 m silicate absorption feature, to show that the infrared emission from most ULIRGs is powered mostly by star formation, with only $20% of ULIRGs hosting an AGN with a greater IR luminosity than the starburst. The detection of [Ne v] k14.32 in just under half the sample, however, implies that an AGN contributes significantly to the mid-IR flux in $42% of ULIRGs. The starbursts and AGNs in ULIRGs appear more extincted, and for the starbursts more compact than those in lower luminosity systems. The excitations and electron densities in the narrow-line regions of ULIRGs appear comparable to those of starbursts with LP10 11.5 L , although the NLR gas in ULIRGs may be more dense. We show that the [Ne ii] k12.81 + [Ne iii] k15.56 luminosity correlates with both infrared luminosity and the luminosity of the 6.2 and 11.2 m PAH features, and derive a calibration between PAH luminosity and star formation rate. Finally, we show that ULIRGs with silicate absorption strengths S sil of 0:8 P S sil P 2:4 are likely to be powered mainly by star formation, but that ULIRGs with S sil P 0:8, and possibly those with S sil k 2:4, contain an IR-luminous AGN.

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The SEDs and IR spectra of a remarkable sample of obscured AGNs selected in the MIR are modeled with recent clumpy torus models. The sample contains 21 AGNs at z ¼ 1:3Y3 discovered in the largest Spitzer surveys (SWIRE, NDWFS, and FLS) by means of their extremely red IR to optical colors. All sources show the 9.7 m silicate feature in absorption and have extreme MIR luminosities [L(6 m) ' 10 46 ergs s À1 ]. The IR SEDs and spectra of 12 sources are well reproduced with a simple torus model, while the remaining nine sources require foreground extinction from a cold dust component to reproduce both the depth of the silicate feature and the NIR emission from hot dust. The best-fit torus models show a broad range of inclinations. Based on the unobscured QSO MIR luminosity function (Brown and coworkers) and on a color-selected sample of AGNs, we estimate the surface densities of obscured and unobscured QSOs with L(6 m) > 10 12 L and z ¼ 1:3Y3:0 to be about 17Y22 and 11.7 deg À2 , respectively. Overall we find that $35%Y41% of luminous QSOs are unobscured, 37%Y40% are obscured by the torus, and 23%Y25% are obscured by a cold absorber detached from the torus. These fractions are consistent with a decrease of the torus covering fraction at large luminosities as predicted by receding torus models. An FIR component is observed in eight objects with luminosity greater than 3:3 ; 10 12 L , implying SFRs of 600Y3000 M yr À1 . In the whole sample, the average contribution from a starburst component to the bolometric luminosity, as estimated from the PAH 7.7 m luminosity in the composite IR spectra, is 20% of the total bolometric luminosity.