The Effects of Gasdynamics, Cooling, Star Formation, and Numerical Resolution in Simulations of Cluster Formation

We compare five simulations of the formation of a Virgo-mass galaxy cluster, in a CDM universe with Omega=1 and h=0.5, in order to isolate the effects of numerical resolution and input physics on cluster structure. We examine density temperature, entropy, and radial velocity profiles and projected quantities such as surface mass density, X-ray surface brightness and temperature, and S-Z flux decrement. The dark-matter-only simulations, with gravitational softening lengths r_g=14 kpc and r_g=1.4kpc, develop steep central density profiles, with rho~r^-1.4. With r_g=14kpc, the addition of a non-radiative ("adiabatic") gas component does not alter the dark matter profile significantly. The gas is close to hydrostatic equilibrium, and its temperature rises steadily from the virial radius (~2Mpc) in to the center, peaking at T~1.4T_vir. However, reducing the gravitational resolution to r_g=200 kpc flattens the density and temperature profiles and reduces the X-ray luminosity andemission-weighted temperature by factors of 2.9 and 1.6, respectively. Adding radiative cooling, star formation, and supernova feedback, with r_g=14kpc, changes the cluster structure substantially. The dominant central galaxy doubles the total mass inside 40kpc, producing a cusp in the X-ray surface brightness profile. The cluster's X-ray luminosity and emission-weighted temperature rise by 20% and 30%, respectively, relative to the adiabatic case. We conclude that existing simulations that attempt to model the full cluster population in a large volume as opposed to "zooming in" on one cluster at a time, probably lack the resolution needed to calculate X-ray luminosities and temperatures. Even high resolution simulations may predict incorrect cluster X-ray and lensing properties if they do not include radiative cooling and star formation.