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

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

Sensitivity of galaxy cluster dark energy constraints to halo modeling uncertainties

We perform a sensitivity study of dark energy constraints from galaxy cluster surveys to uncertainties in the halo mass function, bias and the mass-observable relation. For a set of idealized surveys, we evaluate cosmological constraints as priors on sixteen nuisance parameters in the halo modeling are varied. We find that surveys with a higher mass limit are more sensitive to mass-observable uncertainties while surveys with low mass limits that probe more of the mass function shape and evolution are more sensitive to mass function errors. We examine the correlations among nuisance and cosmological parameters. Mass function parameters are strongly positively (negatively) correlated with Omega_DE (w). For the mass-observable parameters, Omega_DE is most sensitive to the normalization and its redshift evolution while w is more sensitive to redshift evolution in the variance. While survey performance is limited mainly by mass-observable uncertainties, the current level of mass function error is responsible for up to a factor of two degradation in ideal cosmological constraints. For surveys that probe to low masses (10^13.5 h^-1 M_sun), even percent-level constraints on model nuisance parameters result in a degradation of ~ sqrt{2} (2) on Omega_DE (w) relative to perfect knowledge.Comment: 13 pages, 5 figures, accepted by PR

Galaxy Tracers and Velocity Bias

This paper examines several methods of tracing galaxies in N-body simulations and their effects on the derived galaxy statistics, especially measurements of velocity bias. Using two simulations with identical initial conditions, one following dark matter only and the other following dark matter and baryons, both collisionless and collisional methods of tracing galaxies are compared to one another and against a set of idealized criteria. None of the collisionless methods proves satisfactory, including an elaborate scheme developed here to circumvent previously known problems. The main problem is that galactic overdensities are both secularly and impulsively disrupted while orbiting in cluster potentials. With dissipation, the baryonic tracers have much higher density contrasts and much smaller cross sections, allowing them to remain distinct within the cluster potential. The question remains whether the incomplete physical model introduces systematic biases. Statistical measures determined from simulations can vary significantly based solely on the galaxy tracing method utilized. The two point correlation function differs most on sub-cluster scales with generally good agreement on larger scales. Pairwise velocity dispersions show less uniformity on all scales addressed here. All tracing methods show a velocity bias to varying degrees, but the predictions are not firm: either the tracing method is not robust or the statistical significance has not been demonstrated. Though theoretical arguments suggest that a mild velocity bias should exist, simulation results are not yet conclusive.Comment: ApJ, in press, 23 pages, plain TeX, 8 of 13 figures included, all PostScript figures (4.8 MB) available via anonymous ftp from ftp://astro.princeton.edu/summers/tracers . Also available as POPe-616 on http://astro.princeton.edu/~library/prep.htm

Confusion of Diffuse Objects in the X-ray Sky

Most of the baryons in the present-day universe are thought to reside in intergalactic space at temperatures of 10^5-10^7 K. X-ray emission from these baryons contributes a modest (~10%) fraction of the ~ 1 keV background whose prominence within the large-scale cosmic web depends on the amount of non-gravitational energy injected into intergalactic space by supernovae and AGNs. Here we show that the virialized regions of groups and clusters cover over a third of the sky, creating a source-confusion problem that may hinder X-ray searches for individual intercluster filaments and contaminate observations of distant groups.Comment: accepted to ApJ Letters, 7 pages, 3 figure