Dual Halos and Formation of Early-Type Galaxies
  We present a determination of the two-dimensional shape parameters of the
blue and red globular cluster systems (GCSs) in a large number of elliptical
galaxies and lenticular galaxies (early-type galaxies, called ETGs). We use a
homogeneous data set of the globular clusters in 23 ETGs obtained from the
HST/ACS Virgo Cluster Survey. The position angles of both blue and red GCSs
show a correlation with those of the stellar light distribution, showing that
the major axes of the GCSs are well aligned with those of their host galaxies.
However, the shapes of the red GCSs show a tight correlation with the stellar
light distribution as well as with the rotation property of their host
galaxies, while the shapes of the blue GCSs do much less. These provide clear
geometric evidence that the origins of the blue and red globular clusters are
distinct and that ETGs may have dual halos: a blue (metal-poor) halo and a red
(metal-rich) halo. These two halos show significant differences in metallicity,
structure, and kinematics, indicating that they are formed in two
distinguishable ways. The red halos might have formed via dissipational
processes with rotation, while the blue halos are through accretion.
