Collisional versus collisionless matter: a one-dimensional analysis of gravitational clustering

Abstract

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. Initial Gaussian random density perturbations with a power-law spectrum $P(k) \propto k^n$ 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 $n$ and only for $n=-1$ the amplitude of baryonic density fluctuations is larger than that in the dark component. This result is also confirmed by the behaviour of the bias factor, defined as the ratio between the r.m.s of baryonic and dark matter fluctuations at different scales: while for $n=1,\ 3$ it is always less than unity except at very large scales where it tends to one, for $n=-1$ it is above 1.4 at all scales. All simulations show also that there is not an exact correspondence between the positions of largest peaks in dark and baryonic components, as confirmed by a cross-correlation analysis. The final temperatures depend on the initial spectral index: the highest values are obtained for $n=-1$ and are in proximity of high density regions.