Simulations of Clusters of Galaxies

The degree of complexity and, to a somewhat lesser degree, realism in simulations has advanced rapidly in the past few years. The simplest approach - modeling a cluster as collisionless dark matter and collisonal, non--radiative gas is now fairly well established. One of the most fruitful results of this approach is the {\sl morphology--cosmology connection} for X-ray clusters. Simulations have provided the means to make concrete predictions for the X-ray morphologies of clusters in cosmologies with different $\Omega_o$, with the result that low $\Omega_o$ cosmologies fair rather poorly when compared to observations. Another result concerns the accuracy of \xray binding mass estimates. The standard, hydrostatic, isothermal model estimator is found to be accurate to typically better than $50\%$ at radii where the density contrast is between $10^2$ and $10^3$. More complicated approaches, which attempt to explicitly follow galaxy formation within the proto--cluster environment are slowly being realized. The key issue of {\sl dynamical biasing} of the galaxy population within a cluster is being probed, but conclusive answers are lacking. The dynamics of multi--phase gas, including conversion of cold, dense gas into stars and the feedback therefrom, is the largest obstacle hindering progress. An example demonstrating the state--of--the--art in this area is presented.Comment: to appear in Proceedings of the XIVth Moriond Astrophysics Meeting. 10 pages, uuencoded, compressed postscript file includes figures (~1 Mb after unpacked

Gas dynamic simulations of galaxy formation

Results are presented from a simulation modeling the formation of a group of galaxies in a 'standard' cold, dark matter universe with delta = 1, h sub 0 = 50 km/(s(Mpc)), baryon fraction omega sub b = 0.1 and spectrum normalization sigma sub 8 = 0.6 (bias parameter b = 1.7). Initial conditions are generated within a periodic box with comoving length 16 Mpc in a manner constrained to produce a small cluster of total mass approximately 10 exp 14 solar mass. Two sets of 643 particles are used to model the dark matter and baryon fluids. Each gas particle represents 1.08 x 10 exp -8 solar mass, implying an L* galaxy is resolved by approximately 1000 particles. The system is evolved self-consistently in three dimensions using the combined N-body/hydrodynamic scheme P3MSPH up to a final redshift z = 1. Evolving to the present is prohibited by the fact that the mean density in the simulated volume is above critical and the entire volume would be going nonlinear beyond this point, We are currently analyzing another run with somewhat poorer mass resolution which was evolved to the present

Clues to galaxy activity from rich cluster simulations

New simulations of rich cluster evolution are used to evaluate the first infall hypothesis of Gunn and Dressler - the idea that the enhanced fraction of active galaxies seen in high redshift clusters is due to a one-time burst of star formation triggered by the rapid rise in external pressure as a galaxy plows into the hot intracluster medium (ICM). Using three-dimensional simulations which contain both baryonic gas and collisionless dark material, local static pressure histories for test orbits of galaxies are generated and a simple trigger threshold based on dP/dt/P sub ISM is applied to define an active fraction of the population. The results lend qualitative and quantitative support to the first infall interpretation

The Lx-T Relation and Intracluster Gas Fractions of X-ray Clusters

We re-examine the X-ray luminosity-temperature relation using a nearly homogeneous data set of 24 clusters selected for statistically accurate temperature measurements and absence of strong cooling flows. The data exhibit a remarkably tight power-law relation between bolometric luminosity and temperature with a slope 2.88 \pm 0.15. With reasonable assumptions regarding cluster structure, we infer an upper limit on fractional variations in the intracluster gas fraction <(\delta\fgas/\fgas)^2)^1/2 \le 15%. Imaging data from the literature are employed to determine absolute values of fgas within spheres encompassing density contrast 500 and 200 with respect to the critical density. Comparing binding mass estimates based on the virial theorem (VT) and the hydrostatic, betamodel (BM), we find a temperature-dependent discrepancy in fgas between the two methods caused by sytematic variation of the outer slope parameter beta with temperature. There is evidence that cool clusters have a lower mean gas fraction that hot clusters, but it is not possible to assess the statistical significance of this effect in the present dataset. The temperature dependance of the ICM density structure, coupled with the increase of the gas fraction with T in the VT aproach, explains the steepening of the Lx-T relation. The small variation about the mean gas fraction within this majority sub-population of clusters presents an important constraint for theories of galaxy formation and supports arguments against an Einstein-deSitter universe based on the population mean gas fraction and primordial nucleosynthesis. The apparent trend of lower gas fractions and more extended atmospheres in low T systems are consistent with expectations of models incorporating the effects of galactic winds on the ICM. ABRIDGEDComment: 11 pages, 4 figures, uses mn.sty and epsf.sty, accepted for publication in MNRAS; minor modifications: discussion added on CF LX (Sec. 3.1);comparison with Allen & Fabian L-T results (Sec.3.1 & Sec.4.4); statistics precised (3.1), discussion clarified (Sec. 2.2,Sec. 4.4); slight mistake in the r-T and M-T relation calibration corrected and thus fgas in Fig.3, Fig 4, Tab 2 slightly change

Enrichment and heating of the intracluster medium by ejection from galaxies

Results of N-body + hydrodynamic simulations designed to model the formation and evolution of clusters of galaxies and intracluster gas are presented. Clusters of galaxies are the largest bound, relaxed objects in the universe. They are strong x-ray emitters; this radiation originates through thermal bremsstrahlung from a diffuse plasma filling the space between cluster galaxies, the intracluster medium or ICM. From observations, one can infer that the mass of the ICM is comparable to or greater than the mass of all the galaxies in the cluster, and that the ratio of mass in hot gas to mass in galaxies, M(sub ICM)/M(sub STARS), increases with the richness of the cluster. Spectroscopic studies of cluster x-ray emission show heavy element emission lines. While M(sub ICM)/M(sub STARS) is greater than or equal to 1 implies that most of the ICM is primordial in nature, the discovery of heavy elements indicates that some of the gas must have been processed through galaxies. Galaxy evolution thus directly impacts cluster evolution

The temperature-mass relation in magnetized galaxy clusters

We use cosmological, magneto-hydrodynamic simulations of galaxy clusters to quantify the dynamical importance of magnetic fields in these clusters. The set-up of initial magnetic field strengths at high redshifts is chosen such that observed Faraday-rotation measurements in low-redshift clusters are well reproduced in the simulations. We compute the radial profiles of the intracluster gas temperature and of the thermal and magnetic pressure in a set of clusters simulated in the framework of an Einstein-de Sitter and a low-density, spatially-flat CDM cosmological model. We find that, for a realistic range of initial magnetic field strengths, the temperature of the intracluster gas changes by less than $\approx5$.Comment: Accepted for publication in A&

Structure in a Loitering Universe

We study the formation of structure for a universe that undergoes a recent loitering phase. We compare the nonlinear mass distribution to that in a standard, matter dominated cosmology. The statistical aspects of the clustered matter are found to be robust to changes in the expansion law, an exception being that the peculiar velocities are lower by a factor of $\sim 3$ in the loitering model. Further, in the loitering scenario, nonlinear growth of perturbation occurs more recently ($z\sim 3-5$) than in the matter dominated case. Differences in the high redshift appearances of the two models will result but observable consequences depend critically on the chosen form, onset and duration of the loitering phase.Comment: 8 pages, (uses revtex.sty), 5 figures not included, available on request, UM AC 92-

The intracluster gas fraction in X-ray clusters: constraints on the clustered mass density

The mean intracluster gas fraction of X-ray clusters within their hydrostatic regions is derived from recent observational compilations of David, Jones & Forman and White & Fabian. At radii encompassing a mean density 500 times the critical value, the individual sample bi-weight means are moderately (2.4 sigma) discrepant; revising binding masses with a virial relation calibrated by numerical simulations removes the discrepancy and results in a combined sample mean and standard error (f) over bar(gas)(r(500)) = (0.060 +/- 0.003) h(-3/2). For hierarchical clustering models with an extreme physical assumption to maximize cluster gas content, this value constrains the universal ratio of total, clustered-to-baryonic mass Omega(m)/Omega(b) less than or equal to 23.1 h(3/2). Combining this with a maximal value of Omega(b), from primordial nucleosynthesis results in Omega(b) h(1/2) < 0.76. A more physically plausible approach based on low deuterium abundance inferences from quasar absorption spectra and accounting for baryons within cluster galaxies yields an estimate of Omega(m) h(2/3) = 0.30 +/- 0.07, With sources Of systematic error involved in the derivation providing approximately 30 per cent additional uncertainty. Other effects which could enhance the likelihood of the Einstein-de Sitter case Omega(m) = 1 are presented, and their observable signatures discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/60620/1/Evrard1997Intracluster.pd

A Morphology--Cosmology Connection for X--Ray Clusters

We employ N--body/$3D$ gas dynamic simulations of the formation of galaxy clusters to determine whether cluster X--ray morphologies can be used as cosmological constraints. Confirming the analytic expectations of Richstone, Loeb, \& Turner, we demonstrate that cluster evolution is sensitive to the cosmological model in which the clusters form. We further show that evolutionary differences are echoed in the gross morphological features of the cluster X--ray emission. We examine current--epoch X--ray images of models originating from the same initial density fields evolved in three different cosmologies: (i) an unbiased, low density universe with \Omega_o \se 0.2; (ii) an unbiased universe dominated by vacuum energy with \Omega_o \se 0.2 and \lambda_o \se 0.8 and (iii) a biased Einstein--deSitter model (\Omega \se 1, $\sigma_8=0.59$). Using measures of X--ray morphology such as the axial ratio and centroid shifting, we demonstrate that clusters evolved in the two low $\Omega_o$ models are much more regular, spherically symmetric, and centrally condensed than clusters evolved in the Einstein--deSitter model. This morphology--cosmology connection, along with the availability of a large body of cluster X--ray observations, makes cluster X--ray morphology both a powerful and a practical cosmological discriminant.Comment: (uuencoded, compressed postscript, 9 pages including figures), CFA-370