  In order to investigate the importance of pre-equilibrium dynamics on
relativistic heavy-ion collision observables, we match a highly non-equilibrium
early evolution stage, modeled by free-streaming partons generated from the
Monte Carlo Kharzeev-Levin-Nardi (MC-KLN) and Monte Carlo Glauber (MC-Glb)
models, to a locally approximately thermalized later evolution stage described
by viscous hydrodynamics, and study the dependence of final hadronic transverse
momentum distributions, in particular their underlying radial and anisotropic
flows, on the switching time between these stages. Performing a 3-parameter fit
of the measured values for the average transverse momenta $\langle p_\perp
\rangle$ for pions, kaons and protons as well as the elliptic and triangular
flows of charged hadrons $v_{2,3}^\mathrm{ch}$, with the switching time
$\tau_s$, the specific shear viscosity $\eta/s$ during the hydrodynamic stage,
and the kinetic decoupling temperature $T_\mathrm{dec}$ as free parameters, we
find that the preferred "thermalization" times $\tau_s$ depend strongly on the
model of the initial conditions. MC-KLN initial conditions require an earlier
transition to hydrodynamic behavior (at $\tau_s \approx$ 0.13 fm/$c$) ,
followed by hydrodynamic evolution with a larger specific shear viscosity
$\eta/s\approx$ 0.2, than MC-Glb initial conditions which prefer switching at a
later time ($\tau_s\approx$ 0.6 fm/$c$) followed by a less viscous hydrodynamic
evolution with $\eta/s\approx$ 0.16. These new results including
pre-equilibrium evolution are compared to fits without a pre-equilbrium stage
where all dynamic evolution before the onset of hydrodynamic behavior is
ignored. In each case, the quality of the dynamical descriptions for the
optimized parameter sets, as well as the observables which show the strongest
constraining power for the thermalization time, are discussed.
