Color Magnitude Relation and Morphology of Low-Redshift ULIRGs in SDSS
  We present color-magnitude and morphological analysis of 54 low-redshift
ULIRGs, a subset of the IRAS 1Jy sample (Kim & Sanders, 1998), in the SDSS. The
ULIRGs are on average 1 magnitude brighter in M0.1r than the SDSS galaxies
within the same redshift range. The majority of the ULIRGs (~87%) have the
colors typical of the blue cloud, and only 4 sources (~7%) are located in the
red sequence. While ULIRGs are popularly thought to be precursors to a QSO
phase, we find few (~6%) in the "green valley" where the majority of the X-ray
and IR selected AGNs are found, and none of which harbors an AGN. For the 14
previously spectroscopic identified AGNs (~28%), we perform PSF subtractions
and find that on average the central point sources contribute less than one
third to the total luminosity, and that their high optical luminosities and
overall blue colors are apparently the result of star formation activity of the
host galaxies. Visual inspection of the SDSS images reveals a wide range of
disturbed morphologies. A detailed morphology analysis using Gini and M20
coefficients shows that slightly less than one half (~42% in g band) of the
ULIRGs are located in the region where most local mergers are found. The
heterogeneous distribution of ULIRGs in the G-M20 space is qualitatively
consistent with the results found by numerical simulations of disk-disk
mergers. Our study also shows that the measured morphological parameters are
systematically affected by the SNR and thus the merging galaxies can appear at
various regions in the G-M20 space. In general, our results reinforce the view
that ULIRGs contain young stellar populations and are mergers in progress. Our
study provides a uniform comparison sample for studying ULIRGs at higher
redshifts such as Spitzer mid-IR selected ULIRGs at z=1~2 and submm galaxies.
