Probing dark matter haloes of spiral galaxies at poorly explored distances using satellite kinematics

Aims: 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 of satellite galaxies. In this pilot stage, we observed seven late-type spiral galaxies of about the same luminosity MR ~ -22 (and approximately the same mass). We investigate the kinematics of these galaxies, and the radial and angular distribution of their satellites. Methods: Using VIMOS at the VLT, we carried out a spectroscopic survey in seven 14' × 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. Results: We find (a) that the dark matter haloes of luminous spirals (MR ~ -22) have virial radii of ~400 kpc and virial masses of 3.5 × 1012 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.