Ultra-luminous Infrared Galaxies in Sloan Digital Sky Survey Data
  Release 6
  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 AGN 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 PG QSOs, and
most Type I ULIRGs follow the M_{BH}-sigma relation. However, some ULIRGs with
larger Eddington ratio deviate from this relation, even the line width of the
[OIII] narrow line (NL) core or the [SII] 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}-sigma relation and the Eddington ratio supports that
the evolution of Type I ULIRGs is probably followed by the building up of the
M_{BH}-sigma relation and the evolution to the QSO phase.
