XRay Clusters in a Cold Dark Matter + Lambda Universe: A Direct, LargeScale, HighResolution, Hydrodynamic Simulation
Abstract
A new, threedimensional, shockcapturing, hydrodynamic code is utilized to determine the distribution of hot gas in a CDM + {LAMBDA} model universe. Periodic boundary conditions are assumed: a box with size 85 h^1^ Mpc, having cell size 0.31 h^1^ Mpc, is followed in a simulation with 270^3^ = 10^7.3^ cells. We adopt {OMEGA} = 0.45, λ = 0.55, h = H/100 km s^1^ Mpc^1^ = 0.6, and then, from COBE and light element nucleosynthesis, σ_8_ = 0.77, {OMEGA}_b_= 0.043. We identify the Xray emitting clusters in the simulation box, compute the luminosity function at several wavelength bands, the temperature function and estimated sizes, as well as the evolution of these quantities with redshift. This open model succeeds in matching local observations of clusters in contrast to the standard {OMEGA} = 1, CDM model, which fails. It predicts an order of magnitude decline in the number density of bright (hv = 210 keV) clusters from z = 0 to z = 2 in contrast to a slight increase in the number density for standard {OMEGA} = 1, CDM model. This COBEnormalized CDM + {LAMBDA} model produces approximately the same number of Xray clusters having L_x_ > 10^43^ ergs s^1^ as observed. The background radiation field at 1 keV due to clusters is 10% of the observed background which, after correction for numerical effects, again indicates that the model is consistent with observations. The number density of bright clusters increases to z ~ 0.20.5 and then declines, but the luminosity per typical cluster decreases monotonically with redshift, with the result that the number density of bright clusters shows a broad peak near z = 0.5, and then a rapid decline as z approaches 3. The most interesting point which we find is that the temperatures of clusters in this model freeze out at later times (z <= 0.3), while previously we found in the CDM model that there was a steep increase during the same interval of redshift. Equivalently, we find that L^*^ of the Schechter fits of cluster luminosity functions peaks near z = 0.3 in this model, while in the CDM model it is a monotonically decreasing function of redshift. Both trends should be detectable even with a relatively "soft" Xray instrument such as ROSAT, providing a powerful discriminant between {OMEGA} = 1 and {OMEGA} < 1 models. Detailed computations of the luminosity functions in the range L_x_ = 10^40^10^44^ ergs s^1^ in various energy bands are presented for both cluster cores (r <= 0.5 h^1^ Mpc) and total luminosities (r < 1 h^ 1^ Mpc). These are to be used for comparison with ROSAT and other observational data sets. They show the above noted negative evolution. We find little dependence of core radius on cluster luminosity and the dependence of temperature on luminosity log kT_x_ = A + B log L_x_, which is slightly steeper (B = 0.32 +/ 0.01) than indicated by observations (B = 0.265 +/ 0.035), but within observational errors. In contrast, the standard {OMEGA} = 1 model predicted temperatures which were significantly too high. The mean luminosityweighted temperature is 1.8 keV, dramatically lower (by a factor of 3.5) than that found in the {OMEGA} = 1 model, and the evolution far slower (30% vs. 50%) than in the {OMEGA} = 1 model to redshift z = 0.5. A modest average temperature gradient in clusters is found with temperatures dropping to 90% of central values at 0.4 h^1^ Mpc and to 60% of central values at 0.9 h^1^ Mpc. Examining the ratio of gastototal mass in the clusters, we find a slight antibias [b = 0.9 or ({OMEGA}_gas_/{OMEGA}_tot_)_cl_ = 0.083+/ 0.007], which is consistent with observations [({OMEGA}_gas_/{OMEGA}_tot_)_obs_ = 0.097 +/ 0.019 for the Coma cluster for the given value of h, cf., White 1991].
 Publication:

The Astrophysical Journal
 Pub Date:
 July 1994
 DOI:
 10.1086/174297
 arXiv:
 arXiv:astroph/9404012
 Bibcode:
 1994ApJ...429....4C
 Keywords:

 Astronomical Models;
 Cosmology;
 Dark Matter;
 Galactic Clusters;
 Galactic Evolution;
 X Ray Sources;
 Brightness Temperature;
 Computerized Simulation;
 Cosmic X Rays;
 Distribution Functions;
 Hydrodynamic Equations;
 Luminosity;
 Three Dimensional Models;
 Astrophysics;
 COSMOLOGY: THEORY;
 COSMOLOGY: DARK MATTER;
 GALAXIES: CLUSTERING;
 GALAXIES: EVOLUTION;
 HYDRODYNAMICS;
 METHODS: NUMERICAL;
 RADIATION MECHANISMS: NONTHERMAL;
 XRAYS: GALAXIES;
 Astrophysics
 EPrint:
 32p plaintex to appear in The Astrophysical Journal, July 1, 1994