Collisional versus Collisionless Matter: a One-dimensional Analysis of Gravitational Clustering

We present the results of a series of one-dimensional N-body and hydrodynamical simulations which have been used for testing the different clustering properties of baryonic and dark matter in an expanding background. The numerical code is based on the piecewise parabolic method. Initial Gaussian random density perturbations with a power-law spectrum P(k)proportional to k(n) are assumed. We analyse the distribution of density fluctuations and thermodynamical quantities for different spectral indices n and discuss the statistical properties of clustering in the corresponding simulations. At large scales the final distribution of the two components is very similar while at small scales the dark matter presents a lumpiness which is not found in the baryonic matter. The amplitude of density fluctuations in each component depends on the spectral index n but that of the baryonic matter is always larger than the one in the dark component. This result is also confirmed by the behaviour of the bias factor, defined as the ratio between the rms of baryonic and dark matter fluctuations at different scales, which is larger than unity in all the models we have considered. The final temperatures depend on the initial spectral index: the highest values (10(8) K) are obtained for n = -1, and are in proximity to high-density regions. In the other models, the typical post-shock temperatures are smaller (10(5)-10(7) K).