Theoretical studies and numerical simulations

People involved at OAB: Cappi, Lanzoni, Zamorani.

N-body simulations of massive DM halos and the formation and evolution of galaxy clusters

Given a sample of 15 massive ( $M=10^{14}\div 10^{15}M_{\odot}$) DM halos, obtained by means of high-resolution N-body simulations in a $\Lambda$CDM Universe (B. Lanzoni, PhD thesis), several studies on their properties and those of their galactic population have been achieved or are still in progress at the Bologna Observatory, and involve collaborations with other national and international institutes.

A detailed analysis of the properties of the substructures of DM halos has been performed by B. Lanzoni with G. De Lucia, G. Kauffmann, V. Springel, S. White (MPA, Garching, Germany), F. Stoehr (MPA), G. Tormen (Astronomy Dept., University of Padova), and N. Yoshida (Harvard-Smithsonian Center forAstrophysics, USA), who also studied a region of mean density. They find that the substructure mass function is almost independent of the mass of the parent halo and is well described by a power-law. Low-mass halos show steeper radial number density profiles of substructures than high-mass halos, and more massive substructures are preferentially located in the external regions of their parent halos. The mass accretion and merging histories of substructures is found to be largely independent of environment, and a significant fraction of the substructures residing in clusters at the present day were accreted at redshifts $z < 1$ (thus implying that a significant fraction of present-day ``passive'' cluster galaxies should have been still outside the cluster progenitor and more active at $z\sim1$). Clues on the physical origin of the scaling relations observed for nearby galaxy clusters and elliptical galaxies have been obtained by B. Lanzoni, A. Cappi, G. Zamorani, in collaboration with L. Ciotti (Astronomy Dept., University of Bologna) and G. Tormen (Astronomy Dept., University of Padova). Under the assumption of a reasonable trend of the mass-to-light ratio with the cluster total luminosity, the DM halos are found to reproduce not only the Fundamental Plane, but also the observed luminosity-radius and the luminosity-velocity dispersion relations. Therefore, the scaling relations of galaxy clusters can be explained as the result of the cosmological collapse of density fluctuations and the observed trend of the $M/L$ ratio, while the analogous scaling relations of elliptical galaxies seem to require a non negligible role of merging and gas dissipation during galaxy formation and evolution.

By applying the semi-analytical hierarchical model GalICs (Hatton et al. 2003) to the 15 massive DM halos, B. Lanzoni, in collaboration with J. Devriendt (Lyon Observatory, France), B. Guiderdoni and G. Mamon (IAP, Paris), is studying galaxy formation and evolution in clusters. The comparison with several observed properties of cluster galaxies (luminosity function, color-magnitude relation, morphological fractions, etc.), as well as with the model results obtained for the field (Hatton et al. 2003) allows us to obtain insights on how galaxies form and evolve, and on how and how much the environment affects their evolution and final properties.

The Fundamental Plane of Elliptical Galaxies

In collaboration with L. Ciotti, and by means of fully analytical galaxy models and Monte-Carlo numerical simulations, B. Lanzoni explored the importance of projection effects in producing the observed thickness of the edge-on Fundamental Plane. They find that the statistical contribution of projection effects to the observed rms thickness is only marginal, while $\sim90\%$ of it is due to intrinsic variations of galaxy properties.