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

Fellow of the Pacific Institute of Mathematical Sciences 1998-1999, Dept. of Physics and Astronomy, University of Victoria, PO Box 3055, Victoria, B.C., V8W 3P6, Canada & Astronomy Dept., University of Washington, Box 351580, Seattle WA 98195-1580, U.S.A. Electronic mail: [email protected] Electronic mail: [email protected] Dept. of Physics and Astronomy, University of Victoria, PO Box 3055, Victoria, B.C., V8W 3P6, Canada Electronic mail: [email protected] Dept. of Physics and Astronomy, University of Massachusetts, Amherst, MA 01003, U.S.A. Electronic mail: [email protected] Astronomy Dept., University of Washington, Box 351580, Seattle WA 98195-1580, U.S.A. Electronic mail: [email protected] Department of Astronomy, Harvard University, 60 Garden Street, Cambridge MA 02138 Electronic mail: [email protected] Dept. of Astronomy, The Ohio State University, Columbus, OH 43210, U.S.A. Electronic mail: [email protected] We compare five simulations of the formation of a Virgo-mass galaxy cluster, in a cold dark matter universe with Ω = 1 and H0 = 50 km s−1 Mpc−1, 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 Sunyaev-Zel’dovich flux decrement. The dark-matter-only simulations, with gravitational softening lengths rg = 14 kpc and rg = 1.4 kpc, develop steep central density profiles, with ρ ∝ r−1.4. With rg = 14 kpc, 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 (∼ 2 Mpc) in to the center, peaking at T ∼ 1.4Tvir. However, reducing the gravitational resolution to rg = 200 kpc flattens the density and temperature profiles and reduces the X-ray luminosity and emission-weighted temperature by factors of 2.9 and 1.6, respectively. Adding radiative cooling, star formation, and supernova feedback, in a simulation with rg = 14 kpc, changes the cluster structure substantially. The dominant central galaxy doubles the total mass inside 40 kpc, drawing in the surrounding dark matter and producing a sharp peak in the X-ray surface brightness and gas temperature. The cluster 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. Furthermore, even high resolution simulations may predict incorrect cluster X-ray and lensing properties if they do not include radiative cooling and star formation. Subject headings: Galaxy Clusters; Numerical Simulations; Star Formation