Satellites of simulated galaxies: survival, merging and their relationto the dark and stellar haloes

We study the population of satellite galaxies formed in a suite of N-body/gasdynamical simulations of galaxy formation in a Lambda cold dark matter universe. The simulations resolve nearly 10 most luminous satellites around each host, and probe systems up to six or seven magnitudes fainter than the primary. We find little spatial or kinematic bias between the dark matter and the satellite population. The radius containing half of all satellites is comparable to the half-mass radius of the dark matter component, and the velocity dispersion of the satellites is a good indicator of the virial velocity of the halo; sigma(sat)/V-vir similar to 0.9 +/- 0.2. Applied to the Local Group, this result suggests that the virial velocity of the Milky Way and M31 might be substantially lower than the rotation speed of their disc components; we find V-vir(MW) similar to 109 +/- 22 km s(-1) and V-vir(M31) similar to 138 +/- 35 km s(-1), respectively, compared to V-rot(MW) similar to 220 km s(-1) and V-rot(M31) similar to 260 km s(-1). Although the uncertainties are large, it is intriguing that both estimates are significantly lower than expected from some semi-analytic models, which predict a smaller difference between V-vir and V-rot. The detailed kinematics of simulated satellites and dark matter are also in good agreement: both components show a steadily decreasing velocity dispersion profile and a mild radial anisotropy in their velocity distribution. By contrast, the stellar halo of the simulated galaxies, which consists predominantly of stellar debris from disrupted satellites, is kinematically and spatially distinct from the population of surviving satellites. This is because the survival of a satellite as a self-bound entity depends sensitively on mass and on time of accretion and surviving satellites are significantly biased toward low-mass systems that have been recently accreted by the galaxy. Our results support recent proposals for the origin of the systematic differences between stars in the Galactic halo and in Galactic satellites: the elusive 'building blocks' of the Milky Way stellar halo were on average more massive, and were accreted (and disrupted) earlier than the population of dwarfs that has survived self-bound until the present.