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 ultra-luminous infrared galaxies ( ULIRGs ) .
The average MIR spectral features from \sim 5 to 30 \mu m , including the spectral slopes , 6.2 \mu m PAH emission strengths and [ NeII ] 12.81 \mu m 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 \alpha ( 30 , 15 ) is a good indicator of the relative contribution of starbursts to AGNs for all QSOs .
Correlations between the [ NeII ] 12.81 \mu m and PAH 6.2 \mu m luminosities and those between the [ NeII ] , PAH with 60 \mu m 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 ( \sim 0.7 to 0.8 dex ) .
Finally the correlation between the EW ( PAH 6.2 \mu m ) and outflow velocities suggests that star formation activities are suppressed by feedback from AGNs and/or supernovae .