Type I ultraluminous infrared galaxies: Transition stage from ULIRGs to QSOs

We examine whether the ultraluminous infrared galaxies that contain a type 1 Seyfert nucleus (a type I ULIRG) are in the transition stage from ULIRGs to quasi-stellar objects (QSOs). To investigate this issue, we compare the black hole (BH) mass, the bulge luminosity, and the far-infrared luminosity among type I ULIRGs, QSOs, and elliptical galaxies. As a result, we find the following results: (1) The type I ULIRGs have systematically smaller BH masses in spite of the comparable bulge luminosity relative to QSOs and elliptical galaxies. (2) The far-infrared luminosity of most type I ULIRGs is larger than the Eddington luminosity. We show that the above results do not change significantly for three type I ULIRGs for which we can estimate the visual extinction from the column density. Also, for all eight type I ULIRGs, we investigate the effect of uncertainties of BH mass measurements and our sample bias to make sure that our results are not altered even if we consider the above two effects. In addition, Anabuki recently revealed that their X-ray properties are similar to those of the narrow-line Seyfert 1 galaxies. These would indicate that active galactic nuclei (AGNs) with a high mass accretion rate exist in type I ULIRGs. On the basis of all of these findings, we conclude that it would be a natural interpretation that type I ULIRGs are the early phase of BH growth, namely, the missing link between ULIRGs and QSOs. Moreover, by comparing our results with a theoretical model of a coevolution scenario of a QSO BH and a galactic bulge, we show clearly that this explanation could be valid.