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Off-Axis Cluster Mergers: Effects of a Strongly Peaked Dark Matter Profile

P. M. Ricker and C. L. Sarazin

Ap. J. 561 621 (2001)
astro-ph/0107210


Abstract

We present a parameter study of offset mergers between clusters of galaxies. Using the Eulerian hydrodynamics/N-body code COSMOS, we simulate mergers between nonisothermal, hydrostatic clusters with a steep central dark matter density profile and a beta-model gas profile. We constrain global properties of the model clusters using observed cluster statistical relationships. We consider impact parameters between zero and five times the dark matter scale radius and mass ratios of 1:1 and 1:3. The morphological changes, relative velocities, and temperature jumps we observe agree with previous studies using the King profile. We observe a larger jump in X-ray luminosity (~4-10x) than in previous work, and we argue that this increase is most likely a lower limit due to our spatial resolution. We emphasize that luminosity and temperature jumps due to mergers may have an important bearing on constraints on Omega derived from the observation of hot clusters at high redshift. Shocks are relatively weak in the cluster cores; hence they do not significantly increase the entropy there. Instead, shocks create entropy in the outer regions, and this high-entropy gas is mixed with the core gas during later stages of the merger. Ram pressure initiates mixing by displacing the core gas from its potential center, causing it to become convectively unstable. The resulting convective plumes produce large-scale turbulent motions with eddy sizes up to several hundred kpc. This turbulence is pumped by dark matter-driven oscillations in the gravitational potential. Even after nearly a Hubble time these motions persist as subsonic turbulence in the cluster cores, providing ~10% of the support against gravity. The dark matter oscillations are also reflected in the extremely long time following a merger required for the remnant to reach virial equilibrium. Constant-density gas cores are not destroyed by a merger, even if the dark matter density profile has a central cusp.

Physics included
  • ICM hydrodynamics (PPM) - 256x1282 grid
  • Dark matter (particle-mesh) - 1283 particles
  • Self-gravity (multigrid)
  • Boundary conditions - isolated (gravity), vacuum (hydro/N-body)
  • Grid - nonuniform; smallest zones are about 1/7 the NFW scale radius, box dimensions are about 7x5x5 cluster radii for 1:1 collisions, 9x7x7 for 1:3 collisions

Cluster initial conditions
  • Total density profile - NFW
  • Gas density profile - beta model (beta=2/3)
  • Gas temperature profile, DM velocity dispersion profile - from hydrostatic equilibrium
  • Cutoff radius - 10x NFW scale radius
  • Gas core radius - 1/2 NFW scale radius
  • Specify 2-10 keV emission-weighted gas temperature and total gas fraction (Mgas/Mtot inside cutoff radius); compute central gas density, core radius using virial relation for mass (Evrard, Metzler, and Navarro 1996) and observed L-T relation (Markevitch 1998)
  • Compute infall velocity under the assumption that clusters started from rest at 2x the turnaround radius for an overdensity containing their total mass; use extended Press-Schechter (Lacey and Cole 1993) to get most likely collapse redshift given M1 and M2. For this assumed CDM, h=0.6, Omega=1, Lambda=0.

Movie information
  • Movies are: (a) gas density (color) and gravitational potential (contours) (b) gas specific entropy (color) and gas velocity (arrows)
  • Units are 1015 Msun (mass), 1 Gyr (time), 1 h-1Mpc (length; h=0.6), keV (temperature)
  • Movies are made at two zoom levels: wider movies have about 3 h-1 Mpc on a side, narrower movies have about 1.5 h-1 Mpc on a side
  • Movies are in MPEG or Quicktime format

Simulation movies and images

Impact parameter
Mass
ratio
  zero 2rs 5rs
1:1
1:3

 
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