Inflow velocities of cold flows streaming into massive galaxies at high redshifts

We study the velocities of the accretion along streams from the cosmic web into massive galaxies at high redshift with the help of three different suites of AMR hydrodynamical cosmological simulations. The results are compared to free-fall velocities and to the sound speeds of the hot ambient medium. The sound speed of the hot ambient medium is calculated using two different methods to determine the medium's temperature. We find that the simulated cold stream velocities are in violent disagreement with the corresponding free-fall profiles. The sound speed is a better albeit not always correct description of the cold flows' velocity. Using these calculations as a first order approximation for the gas inflow velocities vinflow = 0.9 vvir is given. We conclude from the hydrodynamical simulations as our main result that the velocity profiles for the cold streams are constant with radius. These constant inflow velocities seem to have a "parabola-like" dependency on the host halo mass in units of the virial velocity that peaks at Mvir = 10^{12} Msun and we also propose that the best fit functional form for the dependency of the inflow velocity on the redshift is a square root power law relation: vinflow is proportional to sqrt(z + 1) vvir.