The effects of gasdynamics, cooling, star formation, and numerical resolution in simulations of cluster formation

We present the analysis of a suite of simulations of a Virgo-mass galaxy cl uster. Undertaken within the framework of standard cold dark matter cosmolo gy, these simulations were performed at differing resolutions and with incr easingly complex physical processes, with the goal of identifying the effec ts of each on the evolution of the cluster. We focus on the cluster at the present epoch and examine properties including the radial distributions of density, temperature, entropy, and velocity. We also map "observable" proje cted properties such as the surface mass density, X-ray surface brightness, and Sunyaev-Zeldovich signature. We identify significant differences betwe en the simulations, which highlights the need for caution when comparing nu merical simulations to observations of galaxy clusters. While resolution af fects the inner density profile in dark matter simulations, the addition of a gaseous component, especially one that cools and forms stars, affects th e entire cluster. For example, in simulations with gasdynamics but no cooli ng, improving the gravitational force resolution from 200 to 14 kpc increas es the X-ray luminosity and emission-weighted temperature by factors of 2.9 and 1.6, respectively, and it changes the form of the X-ray surface bright ness and temperature profiles. At the higher resolution, a simulation that includes cooling and star formation converts 30% of the cluster baryons int o stars and produces a massive central galaxy that substantially alters the cluster potential well. This cluster has 20% higher X-ray luminosity and 3 0% higher emission-weighted temperature than the corresponding cluster in t he no-cooling simulation. Its properties are reasonably close to those of o bserved X-ray dominant (XD) clusters, with conversion of cooled gas into st ars greatly reducing the observational conflicts found by Suginohara & Ostr iker in simulations with cooling but no star formation. We conclude that bo th resolution and included physical processes play important roles in simul ating the formation and evolution of galaxy clusters. Therefore, physical i nferences drawn from simulations that do not include gaseous components tha t can cool and form stars present a poor representation of reality.