| Abstract | Ultra-luminous infrared galaxies (ULIRGs) are interesting objects with dramatic properties. Many efforts have been made to understand the physics of their luminous infrared emission and evolutionary stages. However, a large ULIRG sample is still needed to study the properties of their central black holes (BHs), the BH-host galaxy relation, and their evolution. We identified 308 ULIRGs from the Sloan Digital Sky Survey Data Release 6, and classified them into the NL ULIRGs (with only narrow emission lines) and the Type I ULIRGs (with broad emission lines). About 56% of ULIRGs in our total sample show interaction features, and this percentage is 79% for redshift z < 0.2. Optical identifications of these ULIRGs show that the active galactic nucleus percentage is at least 49%, and the percentage increases with the infrared luminosity. We found 62 Type I ULIRGs, and estimated their BH masses and velocity dispersions from their optical spectra. Together with known Type I ULIRGs in the literature, a sample of 90 Type I ULIRGs enables us to make a statistical study. We found that the BH masses of Type I ULIRGs are typically smaller than those of Palomar-Green QSOs, and most Type I ULIRGs follow the M BH-σ relation. However, some ULIRGs with a larger Eddington ratio deviate from this relation, even though the line width of the [O III] narrow-line (NL) core or the [S II] line was used as the surrogate of velocity dispersion. This implies that at least some ULIRGs are probably still in the early evolution stage toward QSOs. The anti-correlation between the mass deviation from the M BH-σ relation and the Eddington ratio supports that the evolution of Type I ULIRGs is probably followed by the building up of the M BH-σ relation and the evolution to the QSO phase. |
