http://hdl.handle.net/2027.42/57483 2013-05-25T22:14:14Z http://hdl.handle.net/2027.42/79631 The Art of Physics Akerlof, Carl W. 31st Distinguished Senior Faculty Lecture, presented in the Rackham Amphitheatre on February 8, 2011, 4:10 p.m. 2011-02-08T00:00:00Z http://hdl.handle.net/2027.42/60635 Previrialization Evrard, August E.; Crone, Mm 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. 1992-07-20T00:00:00Z http://hdl.handle.net/2027.42/60634 The L-X-T relation and intracluster gas fractions of X-ray clusters Arnaud, M.; Evrard, August E. 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. 1999-05-21T00:00:00Z http://hdl.handle.net/2027.42/60633 The merger in abell 576: A line-of-sight bullet cluster? Dupke, R. A.; Mirabal, N.; Bregman, J. N.; Evrard, August E. 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. 2007-10-20T00:00:00Z