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 IR 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 ultra-luminous infrared galaxies (ULIRGs). The average MIR spectral features from ∼5 to 30 μm, including the spectral slopes, 6.2 μm polycyclic aromatic hydrocarbon (PAH) emission strengths and [Ne ii] 12.81 μm luminosities of IR QSOs, differ from those of PG QSOs. In contrast, IR QSOs and ULIRGs have comparable PAH and [Ne ii] 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 active galactic nuclei (AGNs) for all QSOs. Correlations between the [Ne ii] 12.81 μm and PAH 6.2 μm luminosities and those between the [Ne ii], PAH with 60 μm luminosities for ULIRGs and IR QSOs indicate that both [Ne ii] and PAH luminosities are approximate star-formation rate indicators for IR QSOs and starburst-dominated galaxies; the scatters are, however, quite large (∼0.7–0.8 dex). Finally, the correlation between the equivalent widths (EW; PAH 6.2 μm) and outflow velocities suggests that star-formation activities are suppressed by feedback from AGNs and/or supernovae.