Abstract
We performed spectroscopic observations for a large infrared quasi-stellar object (QSO) sample with a total of 25 objects. The sample was compiled from the QDOT redshift survey, the 1 Jy ultraluminous IRAS galaxy survey, and a sample obtained by a cross-correlation study of the IRAS Point-Source Catalogue with the ROSAT All-Sky Survey Catalogue. Statistical analyses of the optical spectra show that the vast majority of infrared QSOs have narrow permitted emission lines (with FWHM of Hβ less than 4000 km s-1) and more than 60% of them are luminous narrow-line Seyfert 1 galaxies. Two of the infrared QSOs are also classified as low-ionization broad absorption line (lo-BAL) QSOs. More than 70% of infrared QSOs are moderately or extremely strong Fe II emitters. This is the highest percentage of strong Fe II emitters in all subclasses of QSO Seyfert 1 samples. We found that the Fe II to Hβ line ratio is significantly correlated with the [O III] λ5007 peak and Hβ blueshift. Soft X-ray–weak infrared QSOs tend to have large blueshifts in permitted emission lines and significant Fe II 48, 49 (5100–5400 Å) residuals relative to the Boroson & Green Fe II template. If the blueshifts in permitted lines are caused by outflows, then they appear to be common in infrared QSOs. As the infrared-selected QSO sample includes both luminous narrow-line Seyfert 1 galaxies and lo-BAL QSOs, it could be a useful laboratory to investigate the evolutionary connection among these objects.
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