Probing dark matter haloes of spiral galaxies at poorly explored
  distances using satellite kinematics
  We present the results of a pilot project designed to study the distribution
of dark matter haloes out to very large radii in spiral galaxies. As dynamical
probe we use their rotation curves and the motions population of satellite
galaxies. In this pilot stage, we observed seven late-type spiral galaxies of
about the same luminosity M_R ~ -22 (and approximately the same mass). We
investigate the kinematics of these galaxies, and the radial and angular
distribution of their satellites. Using VIMOS at the VLT, we carried out a
spectroscopic survey in seven 14' x 14' fields each around a late-type isolated
spiral galaxy. We obtained radial velocities and spatial distributions for 77
candidate satellites. After removing the interlopers, we are left with 61 true
satellites. In combination with the rotation curves of the primary galaxies,
satellites are used to probe the gravitational potential of the primaries and
derive the dark matter halo properties by means of standard mass modeling
techniques. We find (a) that the dark matter haloes of luminous spirals (M_R ~
-22) have virial radii of ~400 kpc and virial masses of 3.5 x 10^12 Msun; (b)
that the radial velocity and angular distributions of the satellites around the
primaries are isotropic; and (c) that the resulting mass distribution is in
good agreement with that found in the optical regions of spirals and described
by the universal rotation curve of spirals once extrapolated to large radii.
The results obtained in this pilot phase of the project are already interesting
and limited only by small number statistics. The full project involving an
order of magnitude more targets, would very likely provide us with a definitive
picture of the dark matter distribution around spirals out to their virial
radii and beyond.
