Perturbed dark energy: classical scalar field versus tachyon

The evolution of scalar linear perturbations is studied in gauge-invariant approach for 2-component models with nonrelativistic matter and minimally coupled scalar fields, the potentials of which were constructed for either constant dark energy equation of state (EoS) parameter $w$ or its adiabatic sound speed $c_a^2$ equal to zero. The numerical solutions show that such fields are almost smoothed out on subhorizon scales. However they cause the scale dependent suppression of the nonrelativistic mater density perturbations and the decay of gravitational potential, which can be used for choice of the dark energy model. We discuss 2 types of the Lagrangian: classical and tachyonic ones. As our results show, the fields with $w=const$ are almost indistinguishable, while for fields with $c_a^2=0$ the difference of dark energy effective sound speeds $c_s^2$, which is caused by the shape of Lagrangian, affects the evolution of perturbations significantly. We present also the transfer functions for both components.