Shape, Spin, and Baryon Fraction of Clusters in the MareNostrum Universe

The MareNostrum Universe is one of the largest cosmological SPH simulation done so far. It consists of $1024^3$ dark and $1024^3$ gas particles in a box of 500 $h^{-1}$ Mpc on a side. Here we studythe shapes and spins of the dark matter and gas components of the 10,000 mostmassive objects extracted from the simulation as well as the gas fraction inthose objects. We find that the shapes of objects tend to be prolate both inthe dark matter and gas. There is a clear dependence of shape on halo mass, themore massive ones being less spherical than the less massive objects. The gasdistribution is nevertheless much more spherical than the dark matter, althoughthe triaxiality parameters of gas and dark matter differ only by a few percentand it increases with cluster mass. The spin parameters of gas and dark mattercan be well fitted by a lognormal distribution function. On average, the spinof gas is 1.4 larger than the spin of dark matter. We find a similar behaviorfor the spins at higher redshifts, with a slightly decrease of the spin ratiosto 1.16 at $z=1.$ The cosmic normalized baryon fraction in the entire clustersample ranges from $Y_b = 0.94$, at $z=1$ to $Y_b = 0.92$ at $z=0$. At bothredshifts we find a slightly, but statistically significant decrease of $Y_b$with cluster mass.Comment: 7 pages, 6 figures. Accepted for publication in The Astrophysical Journa