R. G. Morris

Improved constraints on dark energy from Chandra X-ray observations of the largest relaxed galaxy clusters

We present constraints on the mean matter density, � m, dark energy density, � DE, and the dark energy equation of state parameter, w, using Chandra measurements of the X-ray gas mass fraction (fgas )i n 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 measurem = 0.28 ± 0.06 (68 per cent confidence limits, using standard priors on the Hubble constant, H0, and mean baryon density, � b h 2 ). Analysing the data for all 42 clusters, employ- ing only weak priors on H0 andb h 2 , we obtain a similar result onm and a detection of the effects of dark energy on the distances to the clusters at ∼99.99 per cent confidence, with � DE = 0.86 ± 0.21 for a non-flatCDM 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 measurem = 0.28 ± 0.06 and w =− 1.14 ± 0.31. Combining the fgas data with independent constraints from cosmic mi- crowave background and SNIa studies removes the need for priors onb h 2 and H0 and leads to tighter constraints: � m = 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 as- sociated with instrument calibration, cluster physics and data modelling. The measured small systematic scatter, tight constraint onm 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.

Cosmology and astrophysics from relaxed galaxy clusters – II. Cosmological constraints

We present cosmological constraints from measurements of the gas mass fraction,

Cool X-ray emitting gas in the core of the Centaurus cluster of galaxies

f_{gas}

Azimuthally Resolved X-Ray Spectroscopy to the Edge of the Perseus Cluster

, for massive, dynamically relaxed galaxy clusters. Our data set consists of Chandra observations of 40 such clusters, identified in a comprehensive search of the Chandra archive, as well as high-quality weak gravitational lensing data for a subset of these clusters. 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, significantly reduces systematic uncertainties compared to previous work. Our data for the first time constrain the intrinsic scatter in

Tracing Gas Motions in the Centaurus Cluster

f_{gas}

X-ray bright active galactic nuclei in massive galaxy clusters – I. Number counts and spatial distribution

,

Cosmology and astrophysics from relaxed galaxy clusters – III. Thermodynamic profiles and scaling relations

(7.4pm2.3)

X-ray bright active galactic nuclei in massive galaxy clusters – III. New insights into the triggering mechanisms of cluster AGN

% in a spherical shell at radii 0.8-1.2

X-ray bright active galactic nuclei in massive galaxy clusters – II. The fraction of galaxies hosting active nuclei

r_{2500}

Optical and Sunyaev-Zel'dovich observations of a new sample of distant rich galaxy clusters in the ROSAT All Sky

, consistent with the expected variation in gas depletion and non-thermal pressure for relaxed clusters. From the lowest-redshift data in our sample we obtain a constraint on a combination of the Hubble parameter and cosmic baryon fraction,