Mid-Infrared Spectroscopic Properties of Ultra-Luminous Infrared Quasars

We analyse mid-infrared (MIR) spectroscopic properties for 19 ultra-luminous infrared quasars (IR QSOs) in the local universe based on the spectra from the Infrared Spectrograph on board the Spitzer Space Telescope. The MIR properties of IR QSOs are compared with those of optically-selected Palomar-Green QSOs (PG QSOs) and ultraluminous infrared galaxies (ULIRGs). The average MIR spectral features from ∼5 to 30μm, including the spectral slopes, 6.2μmPAH emission strengths and [NeII] 12.81μm luminosities of IR QSOs, differ from those of PG QSOs. In contrast, IR QSOs and ULIRGs have comparable PAH and [NeII] luminosities. These results are consistent with IR QSOs being at a transitional stage from ULIRGs to classical QSOs. We also find that the colour index α(30, 15) is a good indicator of the relative contribution of starbursts to AGNs for all QSOs. Correlations between the [NeII] 12.81μm and PAH 6.2μm luminosities and those between the [NeII], PAH with 60μm luminosities for ULIRGs and IR QSOs indicate that both [NeII] and PAH luminosities are approximate star formation rate indicators for IR QSOs and starburst-dominated galaxies; the scatters are, however, quite large (∼ 0.7 to 0.8 dex). Finally the correlation between the EW (PAH 6.2μm) and outflow velocities suggests that star formation activities are suppressed by feedback from AGNs and/or supernovae.