Three dynamically distinct stellar populations in the halo of M49
  M49 (NGC 4472) is the dominant galaxy in subcluster B of the Virgo Cluster,
and a benchmark for studying the build-up of the extended halos of brightest
group galaxies in the outskirts of galaxy clusters. We investigate the
kinematics in the outer halo of M49, look for substructures, and describe the
transition to the surrounding intra-group light. As kinematic tracers we use
planetary nebulae (PNe), combining kinematics from the extended Planetary
Nebula Spectrograph (PN.S) early-type galaxy survey with our recent deep
photometric sample. We study the position-velocity-plane for bright and faint
PN populations out to 95 kpc radius, and employ a multi-Gaussian model for the
velocity distribution to identify stellar populations with distinct kinematics
and histories. We report the detection of stellar-kinematic substructure
associated with the interaction of M49 with the dwarf irregular galaxy VCC
1249. We find two kinematically distinct PN populations associated with the
main M49 halo and the extended intra-group light (IGL). These have velocity
dispersions $\sigma_{halo}\simeq 170$ km/s and $\sigma_{IGL} \simeq400$ km/s at
10-80 kpc radii. The overall luminosity profile and velocity dispersion at
$\sim80$ kpc are consistent with a flat circular velocity curve extrapolated
from X-ray observations. The dispersion of the PNe associated with the IGL
joins onto that of the satellite galaxies in subcluster B at $\sim100$ kpc
radius. This is the first time that the transition from halo to IGL is observed
based on the velocities of individual stars. Therefore the halo of M49,
consisting of at least three distinct components, has undergone an extended
accretion history within its parent group potential. The blue colours of the
IGL component are consistent with a population of stars formed in low-mass
galaxies at redshift $\sim0.5$ that has since evolved passively, as suggested
by other data.
