This is the first of a series of papers in which we derive simultaneous constraints on cosmological parameters and X-ray scaling relations using observations of the growth of massive, X-ray ...flux-selected galaxy clusters. Our data set consists of 238 cluster detections from the ROSAT All-Sky Survey, and incorporates follow-up observations of 94 of those clusters using the Chandra X-ray Observatory or ROSAT. Here we describe and implement a new statistical framework required to self-consistently produce simultaneous constraints on cosmology and scaling relations from such data, and present results on models of dark energy. In spatially flat models with a constant dark energy equation of state, w, the cluster data yield Ωm= 0.23 ± 0.04, σ8= 0.82 ± 0.05 and w=−1.01 ± 0.20, incorporating standard priors on the Hubble parameter and mean baryon density of the Universe, and marginalizing over conservative allowances for systematic uncertainties. These constraints agree well and are competitive with independent data in the form of cosmic microwave background anisotropies, type Ia supernovae, cluster gas mass fractions, baryon acoustic oscillations, galaxy redshift surveys and cosmic shear. The combination of our data with current microwave background, supernova, gas mass fraction and baryon acoustic oscillation data yields Ωm= 0.27 ± 0.02, σ8= 0.79 ± 0.03 and w=−0.96 ± 0.06 for flat, constant w models. The combined data also allow us to investigate evolving w models. Marginalizing over transition redshifts in the range 0.05–1, we constrain the equation of state at late and early times to be respectively w0=−0.88 ± 0.21 and wet=−1.05+0.20−0.36, again including conservative systematic allowances. The combined data provide constraints equivalent to a Dark Energy Task Force figure of merit of 15.5. Our results highlight the power of X-ray studies, which enable the straightforward production of large, complete and pure cluster samples and admit tight scaling relations, to constrain cosmology. However, the new statistical framework we apply to this task is equally applicable to cluster studies at other wavelengths.
This is the second in a series of papers studying the astrophysics and cosmology of massive, dynamically relaxed galaxy clusters. The data set employed here consists of Chandra observations of 40 ...such clusters, identified in a comprehensive search of the Chandra archive for hot (kT ... 5 keV), massive, morphologically relaxed systems, as well as high-quality weak gravitational lensing data for a subset of these clusters. Here we present cosmological constraints from measurements of the gas mass fraction, ..., for this cluster sample. By incorporating a robust gravitational lensing calibration of the X-ray mass estimates, and restricting our measurements to the most self-similar and accurately measured regions of clusters, we significantly reduce systematic uncertainties compared to previous work. Our data for the first time constrain the intrinsic scatter in fgas, 7.4 ± 2.3 per cent in a spherical shell at radii 0.8-1.2 ... (~1/4 of the virial radius), consistent with the expected level of variation in gas depletion and non-thermal pressure for relaxed clusters. From the lowest redshift data in our sample, five clusters at z < 0.16, we obtain a constraint on a combination of the Hubble parameter and cosmic baryon fraction, ... = 0.089 ± 0.012, that is insensitive to the nature of dark energy. Combining this with standard priors on h and ... provides a tight constraint on the cosmic matter density, ... = 0.27 ± 0.04, which is similarly insensitive to dark energy. Using the entire cluster sample, extending to z > 1, we obtain consistent results for ... and interesting constraints on dark energy: ... for non-flat ...CDM (cosmological constant) models, and w = -0.98 ± 0.26 for flat models with a constant dark energy equation of state. Our results are both competitive and consistent with those from recent cosmic microwave background, Type Ia supernova and baryon acoustic oscillation data. We present constraints on more complex models of evolving dark energy from the combination of fgas data with these external data sets, and comment on the possibilities for improved fgas constraints using current and next-generation X-ray observatories and lensing data. (ProQuest: ... denotes formulae/symbols omitted.)
We present constraints on the mean matter density, Ωm, dark energy density, ΩDE, and the dark energy equation of state parameter, w, using Chandra measurements of the X-ray gas mass fraction (fgas) ...in 42 hot (kT > 5 keV), X-ray luminous, dynamically relaxed galaxy clusters spanning the redshift range 0.05 < z < 1.1. Using only the fgas data for the six lowest redshift clusters at z < 0.15, for which dark energy has a negligible effect on the measurements, we measure Ωm= 0.28 ± 0.06 (68 per cent confidence limits, using standard priors on the Hubble constant, H0, and mean baryon density, Ωb h2). Analysing the data for all 42 clusters, employing only weak priors on H0 and Ωb h2, we obtain a similar result on Ωm and a detection of the effects of dark energy on the distances to the clusters at ∼99.99 per cent confidence, with ΩDE= 0.86 ± 0.21 for a non-flat ΛCDM model. The detection of dark energy is comparable in significance to recent type Ia supernovae (SNIa) studies and represents strong, independent evidence for cosmic acceleration. Systematic scatter remains undetected in the fgas data, despite a weighted mean statistical scatter in the distance measurements of only ∼5 per cent. For a flat cosmology with a constant dark energy equation of state, we measure Ωm= 0.28 ± 0.06 and w=−1.14 ± 0.31. Combining the fgas data with independent constraints from cosmic microwave background and SNIa studies removes the need for priors on Ωb h2 and H0 and leads to tighter constraints: Ωm= 0.253 ± 0.021 and w=−0.98 ± 0.07 for the same constant-w model. Our most general analysis allows the equation of state to evolve with redshift. Marginalizing over possible transition redshifts 0.05 < zt < 1, the combined fgas+ CMB + SNIa data set constrains the dark energy equation of state at late and early times to be w0=−1.05 ± 0.29 and wet=−0.83 ± 0.46, respectively, in agreement with the cosmological constant paradigm. Relaxing the assumption of flatness weakens the constraints on the equation of state by only a factor of ∼2. Our analysis includes conservative allowances for systematic uncertainties associated with instrument calibration, cluster physics and data modelling. The measured small systematic scatter, tight constraint on Ωm and powerful constraints on dark energy from the fgas data bode well for future dark energy studies using the next generation of powerful X-ray observatories, such as Constellation-X.
This is the fourth in a series of papers studying the astrophysics and cosmology of massive, dynamically relaxed galaxy clusters. Here, we use measurements of weak gravitational lensing from the ...Weighing the Giants project to calibrate Chandra X-ray measurements of total mass that rely on the assumption of hydrostatic equilibrium. This comparison of X-ray and lensing masses measures the combined bias of X-ray hydrostatic masses from both astrophysical and instrumental sources. While we cannot disentangle the two sources of bias, only the combined bias is relevant for calibrating cosmological measurements using relaxed clusters. Assuming a fixed cosmology, and within a characteristic radius (r
2500) determined from the X-ray data, we measure a lensing to X-ray mass ratio of 0.96 ± 9 per cent (stat) ± 9 per cent (sys). We find no significant trends of this ratio with mass, redshift or the morphological indicators used to select the sample. Our results imply that any departures from hydrostatic equilibrium at these radii are offset by calibration errors of comparable magnitude, with large departures of tens-of-percent unlikely. In addition, we find a mean concentration of the sample measured from lensing data of
$c_{200} = 3.0_{-1.8}^{+4.4}$
. Anticipated short-term improvements in lensing systematics, and a modest expansion of the relaxed lensing sample, can easily increase the measurement precision by 30–50 per cent, leading to similar improvements in cosmological constraints that employ X-ray hydrostatic mass estimates, such as on Ωm from the cluster gas mass fraction.
This is the second in a series of papers in which we derive simultaneous constraints on cosmology and X-ray scaling relations using observations of massive, X-ray flux-selected galaxy clusters. The ...data set consists of 238 clusters with 0.1–2.4 keV luminosities >2.5 × 1044 h−270 erg s−1, and incorporates follow-up observations of 94 of those clusters using the Chandra X-ray Observatory or ROSAT (11 were observed with both). The clusters are drawn from three samples based on the ROSAT All-Sky Survey: the ROSAT Brightest Cluster Sample (78/37 clusters detected/followed-up), the ROSAT-ESO Flux-Limited X-ray sample (126/25) and the bright sub-sample of the Massive Cluster Survey (34/32). Our analysis accounts self-consistently for all selection effects, covariances and systematic uncertainties. Here we describe the reduction of the follow-up X-ray observations, present results on the cluster scaling relations, and discuss their implications. Our constraints on the luminosity–mass and temperature–mass relations, measured within r500, lead to three important results. First, the data support the conclusion that excess heating of the intracluster medium (or a combination of heating and condensation of the coldest gas) has altered its thermodynamic state from that expected in a simple, gravitationally dominated system; however, this excess heat is primarily limited to the central regions of clusters (r < 0.15r500). Secondly, the intrinsic scatter in the centre-excised luminosity–mass relation is remarkably small, being bounded at the <10 per cent level in current data; for the hot, massive clusters under investigation, this scatter is smaller than in either the temperature–mass or YX–mass relations (10–15 per cent). Thirdly, the evolution with redshift of the scaling relations is consistent with the predictions of simple, self-similar models of gravitational collapse, indicating that the mechanism responsible for heating the central regions of clusters was in operation before redshift 0.5 (the limit of our data) and that its effects on global cluster properties have not evolved strongly since then. Our results provide a new benchmark for comparison with numerical simulations of cluster formation and evolution.
The size increase of a nanoscale material is commonly associated with the increased stability of its ordered phases. Here we give a counterexample to this trend by considering the formation of the ...defect-free L1
ordered phase in AgPt nanoparticles, and showing that it is better stabilized in small nanoparticles (up to 2.5 nm) than in larger ones, in which the ordered phase breaks in multiple domains or is interrupted by faults. The driving force for the L1
phase formation in small nanoparticles is the segregation of a monolayer silver shell (an Ag-skin) which prevents the element with higher surface energy (Pt) from occupying surface sites. With increasing particle size, the Ag-skin causes internal stress in the L1
domains which cannot thus exceed the critical size of ~2.5 nm. A multiscale modelling approach using full-DFT global optimization calculations and atomistic modelling is used to interpret the findings.
This is the fourth of a series of papers in which we derive simultaneous constraints on cosmological parameters and X-ray scaling relations using observations of the growth of massive, X-ray ...flux-selected galaxy clusters. Our data set consists of 238 clusters drawn from the ROSAT All-Sky Survey and incorporates extensive follow-up observations using the Chandra X-ray Observatory. Here we examine the constraints on neutrino properties that are enabled by the precise and robust constraint on the amplitude of the matter power spectrum at low redshift available from our data. In combination with cluster gas mass fraction, cosmic microwave background, supernova and baryon acoustic oscillation data, and incorporating conservative allowances for systematic uncertainties, we limit the species-summed neutrino mass, Mν, to < 0.33 eV at 95.4 per cent confidence in a spatially flat, cosmological constant (ΛCDM) model. In a flat ΛCDM model where the effective number of neutrino species, Neff, is allowed to vary, we find Neff= 3.4+0.6−0.5 (68.3 per cent confidence, incorporating a direct constraint on the Hubble parameter from Cepheid and supernova data). We also obtain results with additional degrees of freedom in the cosmological model, in the form of global spatial curvature (Ωk) and a primordial spectrum of tensor perturbations (r and nt). The results are not immune to these generalizations; however, in the most general case we consider, in which Mν, Neff, curvature and tensors are all free, we still obtain Mν < 0.70 eV and Neff= 3.7 ± 0.7 (at, respectively, the same confidence levels as above). These results agree well with recent work using independent data and highlight the importance of measuring cosmic structure and expansion at low as well as high (z∼ 1100) redshifts. Although our cluster data extend to redshift z= 0.5, the direct effect of neutrino mass on the growth of structure at late times is not yet detected at a significant level.
Abstract
Although neural-network-based emulators enable efficient parameter estimation in 21 cm cosmology, the accuracy of such constraints is poorly understood. We employ nested sampling to fit mock ...data of the global 21 cm signal and high-
z
galaxy ultraviolet luminosity function (UVLF) and compare for the first time the emulated posteriors obtained using the global signal emulator
globalemu
to the “true” posteriors obtained using the full model on which the emulator is trained using
ARES
. Of the eight model parameters we employ, four control the star formation efficiency (SFE) and thus can be constrained by UVLF data, while the remaining four control UV and X-ray photon production and the minimum virial temperature of star-forming halos (
T
min
) and thus are uniquely probed by reionization and 21 cm measurements. For noise levels of 50 and 250 mK in the 21 cm data being jointly fit, the emulated and “true” posteriors are consistent to within 1
σ
. However, at lower noise levels of 10 and 25 mK,
globalemu
overpredicts
T
min
and underpredicts
γ
lo
, an SFE parameter, by ≈3
σ
–4
σ
, while the “true”
ARES
posteriors capture their fiducial values within 1
σ
. We find that jointly fitting the mock UVLF and 21 cm data significantly improves constraints on the SFE parameters by breaking degeneracies in the
ARES
parameter space. Our results demonstrate the astrophysical constraints that can be expected for global 21 cm experiments for a range of noise levels from pessimistic to optimistic, as well as the potential for probing redshift evolution of SFE parameters by including UVLF data.
We present a quantitative study of the X-ray morphology of galaxy clusters, as a function of their detection method and redshift. We analyze two separate samples of galaxy clusters: a sample of 36 ...clusters at selected in the X-ray with the ROSAT PSPC 400 deg2 survey, and a sample of 90 clusters at selected via the Sunyaev-Zel'dovich (SZ) effect with the South Pole Telescope. Clusters from both samples have similar-quality Chandra observations, which allow us to quantify their X-ray morphologies via two distinct methods: centroid shifts (w) and photon asymmetry ( ). The latter technique provides nearly unbiased morphology estimates for clusters spanning a broad range of redshift and data quality. We further compare the X-ray morphologies of X-ray- and SZ-selected clusters with those of simulated clusters. We do not find a statistically significant difference in the measured X-ray morphology of X-ray and SZ-selected clusters over the redshift range probed by these samples, suggesting that the two are probing similar populations of clusters. We find that the X-ray morphologies of simulated clusters are statistically indistinguishable from those of X-ray- or SZ-selected clusters, implying that the most important physics for dictating the large-scale gas morphology (outside of the core) is well-approximated in these simulations. Finally, we find no statistically significant redshift evolution in the X-ray morphology (both for observed and simulated clusters), over the range of to , seemingly in contradiction with the redshift-dependent halo merger rate predicted by simulations.
Recent measurements of the cosmic microwave background (CMB) by the Planck Collaboration have produced arguably the most powerful observational evidence in support of the standard model of cosmology, ...i.e. the spatially flat ...CDM paradigm. In this work, we perform model selection tests to examine whether the base CMB temperature and large scale polarization anisotropy data from Planck 2015 (P15; Planck Collaboration XIII) prefer any of eight commonly used one-parameter model extensions with respect to flat ...CDM. We find a clear preference for models with free curvature, ..., or free amplitude of the CMB lensing potential, A sub( L). We also further develop statistical tools to measure tension between data sets. We use a Gaussianization scheme to compute tensions directly from the posterior samples using an entropy-based method, the surprise, as well as a calibrated evidence ratio presented here for the first time. We then proceed to investigate the consistency between the base P15 CMB data and six other CMB and distance data sets. In flat ...CDM we find a 4.8... tension between the base P15 CMB data and a distance ladder measurement, whereas the former are consistent with the other data sets. In the curved ...CDM model we find significant tensions in most of the cases, arising from the well-known low power of the low-l multipoles of the CMB data. In the flat ...CDM+A sub( L) model, however, all data sets are consistent with the base P15 CMB observations except for the CMB lensing measurement, which remains in significant tension. This tension is driven by the increased power of the CMB lensing potential derived from the base P15 CMB constraints in both models, pointing at either potentially unresolved systematic effects or the need for new physics beyond the standard flat ...CDM model. (ProQuest: ... denotes formulae/symbols omitted.)
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