Mapping the Galactic halo. III. Simulated observations of tidal streams

We have simulated the evolution of tidal debris in the Galactic halo in order to guide our ongoing survey to determine the fraction of halo mass accreted via satellite infall. Contrary to naive expectations that the satellite debris will produce a single narrow velocity peak on a smooth distribution, there are many different signatures of substructure, including multiple peaks and broad but asymmetrical velocity distributions. Observations of the simulations show that there is a high probability of detecting the presence of tidal debris with a pencil-beam survey of 100 deg². In the limiting case of a single 10⁷ M_⊙ satellite contributing 1% of the luminous halo mass the detection probability is a few percent using just the velocities of 100 halo stars in a single 1 deg² field. The detection probabilities scale with the accreted fraction of the halo and the number of fields surveyed. There is also surprisingly little dependence of the detection probabilities on the time since the satellite became tidally disrupted, or on the initial orbit of the satellite, except for the time spent in the survey volume.