229 research outputs found
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 , with the
result that low 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 at radii where the density contrast is
between and . 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 .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 in the
loitering model. Further, in the loitering scenario, nonlinear growth of
perturbation occurs more recently () 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/ 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, ).
Using measures of X--ray morphology such as the axial ratio and centroid
shifting, we demonstrate that clusters evolved in the two low 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
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