Spherical collapse models with clustered dark energy

We investigate the clustering effect of dark energy (DE) in the formation of galaxy clusters using the spherical collapse model. Assuming a fully clustered DE component, the spherical overdense region is treated as an isolated system which conserves the energy separately for both matter and DE inside the spherical region. Then, by introducing a parameter r to characterize the degree of DE clustering, which is defined by the nonlinear density contrast ratio of matter to DE at turnaround in the recollapsing process, i.e. r≡δ_de,ta ^NL∕δ_m,ta ^NL, we are able to uniquely determine the spherical collapsing process and hence obtain the virialized overdensity Δ_vir through a proper virialization scheme. Estimation of the virialized overdensities from current observation on galaxy clusters suggests that 0.5<r<0.8 at 1σ level for the clustered DE with w<−0.9. Also, we compare our method to the linear perturbation theory that deals with the growth of DE perturbation at early times. While both results are consistent with each other, our method is practically simple and it shows that the collapse process is rather independent of initial DE perturbation and its evolution at early times.