Context : 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 .

Aims : We investigate the kinematics in the outer halo of M49 , look for substructures , and describe the transition to the surrounding intra-group light .

Methods : 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 .

Results : 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 \mathrm { km } \mathrm { s } ^ { -1 } and \sigma _ { IGL } \simeq 400 \mathrm { km } \mathrm { s } ^ { -1 } at 10-80 kpc radii .
The overall luminosity profile and velocity dispersion at \sim 80 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 \sim 100 kpc radius .
This is the first time that the transition from halo to IGL is observed based on the velocities of individual stars .

Conclusions : 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 \sim 0.5 that has since evolved passively , as suggested by other data .