Mid-Infrared Spectroscopic Properties of Ultra-Luminous Infrared Quasars

TL;DR

This study investigates the mid-infrared spectral features of 19 local ultra-luminous infrared quasars, comparing them with other galaxy types to understand their transitional nature and star formation activity.

Contribution

It provides the first detailed MIR spectral analysis of IR QSOs, revealing their similarities to ULIRGs and differences from PG QSOs, and introduces the color index alpha(30, 15) as a starburst indicator.

Findings

IR QSOs have MIR features distinct from PG QSOs but similar to ULIRGs.

PAH and [NeII] luminosities correlate with star formation rates.

Star formation appears suppressed by AGN or supernova feedback.

Abstract

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 ~ 5 to 30um, including the spectral slopes, 6.2um PAH emission strengths and [NeII] 12.81um 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] 12.81um and PAH 6.2um luminosities and those between the [NeII], PAH with 60um 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 6.2um) and outflow velocities suggests that star formation activities are suppressed by feedback from AGNs and/or supernovae.