DSpace Collection: Astrophysics (Physics, Department of)

Previrialization

Sun, 19 Jul 1992 22:58:59 GMT

Title: Previrialization

Authors: Evrard, AE; Crone, Mm

Abstract: We report the results of N-body experiments employing Gaussian random initial conditions designed to address the effect of previrialization recently examined by Peebles. The central issue is whether the development of small-scale structure within a collapsing protocluster can significantly retard its collapse. A series of runs with progressively diminished small-scale power indicates that the collapse of objects is insensitive to the degree of clustering on small scales. We conclude that previrialization does not have significant impact on the formation epoch of galaxies and clusters arising from gravitational clustering of initially Gaussian random density fields.

The L-X-T relation and intracluster gas fractions of X-ray clusters

Thu, 20 May 1999 22:58:59 GMT

Title: The L-X-T relation and intracluster gas fractions of X-ray clusters

Authors: Arnaud, M; Evrard, AE

Abstract: 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 +/- 0.15. With reasonable assumptions regarding cluster structure, we infer an upper limit on fractional variations in the intracluster gas fraction [(delta f(gas)/f(gas))(2)](1/2) less than or equal to 15 per cent. A strictly homogeneous Ginga subset of 18 clusters places a more stringent limit of 9 per cent. Imaging data from the literature are employed to determine absolute values of f(gas) within spheres encompassing density contrasts delta(c) = 500 and 200 with respect to the critical density. Comparing binding mass estimates based on the virial theorem (VT) and the hydrostatic beta-model (BM), we find a temperature-dependent discrepancy in f(gas) between the two methods caused by systematic variation of the outer slope parameter beta with temperature. Mean values (for H-0 = 50 km s(-1) Mpc(-1)) range from (f) over bar(gas) = 0.10 for cool (T < 4 keV) clusters using the VT at delta(c) = 500 to 0.22 for hot (T > 4keV) clusters using the BM at delta(c) = 200. There is evidence that cool clusters have a lower mean gas fraction than hot clusters, but it is not possible to assess the statistical significance of this effect in the present data set. The T dependence of the intracluster medium (ICM) density structure, coupled with the increase of the gas fraction with T in the VT approach, explains the steepening of the L-X-T relation. The small variation about the mean gas fraction within this majority subpopulation of clusters presents an important constraint for theories of galaxy formation and supports arguments against an Einstein-de Sitter universe based on the population mean gas fraction and conventional, primordial nucleosynthesis. The apparent trend of lower gas fractions and more extended atmospheres in low-temperature systems is consistent with expectations of models incorporating the effects of galactic winds on the ICM.

The merger in abell 576: A line-of-sight bullet cluster?

Fri, 19 Oct 2007 22:58:59 GMT

Title: The merger in abell 576: A line-of-sight bullet cluster?

Authors: Dupke, RA; Mirabal, N; Bregman, JN; Evrard, AE

Abstract: Using a combination of Chandra and XMM-Newton observations, we confirmed the presence of a significant velocity gradient along the northeast-southwest direction in the intracluster gas of the cluster Abell 576. The results are consistent with a previous ASCA SIS analysis of this cluster. The error-weighted average over the ACIS-S3 and EPIC MOS1 and MOS2 spectrometers for the maximum velocity difference is > 3.3; 10(3) km s(-1) at the 90% confidence level, similar to the velocity limits estimated indirectly for the Bullet Cluster (1E 0657 -56). The probability that the velocity gradient is generated by standard random gain fluctuations with Chandra and XMM-Newton is < 0.1%. The regions of maximum velocity gradient are in CCD zones that have the lowest temporal gain variations. It is unlikely that the velocity gradient is due to Hubble distance differences between projected clusters (probability less than or similar to 0.01%). We mapped the distribution of elemental abundance ratios across the cluster and detected a strong chemical discontinuity using the abundance ratio of silicon to iron, equivalent to a variation from 100% SN Ia iron mass fraction in the west-northwest regions to 32% in the eastern region. The center of the cluster is located at the chemical discontinuity boundary, which is inconsistent with the radially symmetric chemical gradient found in some regular clusters, but consistent with a cluster merging scenario. We predict that the velocity gradient as measured will produce a variation of the cosmic microwave background (CMB) temperature toward the east of the core of the cluster that will be detectable by current and near-future bolometers. The measured velocity gradient opens up the possibility that this cluster is passing through a near line-of-sight merger stage where the cores have recently crossed.

Four measures of the intracluster medium temperature and their relation to a cluster's dynamical state

Sun, 31 Dec 2000 22:58:59 GMT

Title: Four measures of the intracluster medium temperature and their relation to a cluster's dynamical state

Authors: Mathiesen, BF; Evrard, AE

Abstract: We employ an ensemble of 24 hydrodynamic cluster simulations to create spatially and spectrally resolved images of quality comparable to Chandra's expected performance. Emission from simulation mass elements is represented using the XSPEC MEKAL program assuming 0.3 solar metallicity and the resulting spectra are fitted with a single-temperature model. Despite significant departures from isothermality in the cluster gas, single-temperature models produce acceptable fits to 20,000 source photon spectra. The spectral fit temperature T-s is generally lower than the mass-weighted average temperature T-m due to the influence of soft line emission from cooler gas being accreted as part of the hierarchical clustering process. The nature of this deviation depends on the bandpass used for spectral fitting. In a Chandra-like bandpass of 0.5 to 9.5 keV we find a nearly uniform fractional bias of (T-m - T-s)/T-s similar or equal to 20%, although smaller clusters sometimes demonstrate much greater deviations. If the minimum energy threshold is raised to 2 keV, however, the effect of line emission on the spectrum is greatly decreased and T-s becomes a nearly unbiased estimator of for smaller clusters. The fractional deviation in T-s relative to T-m is scale-dependent in this bandpass and follows the approximate relation (T-m - T-s)/T-s = 0.2 log(10) T-m. This results in an observed M-ICM-T-s relationship for the simulations with slope of about 1.6, intermediate between the virial relation M proportional to T-m(3/2) and the observed relation M-ICM proportional to T-2. Tracking each cluster in the ensemble at 16 epochs in its evolutionary history, we catalog merger events with mass ratios exceeding 10% in order to investigate the relationship between spectral temperature and proximity to a major merger event. Clusters that are very cool relative to the mean mass-temperature relationship lie preferentially close to a major merger, suggesting a viable observational method to cull a subset of dynamically young clusters from the general population.