S. W. Allen

Constraints on dark energy from Chandra observations of the largest relaxed galaxy clusters

We present constraints on the mean matter density, {Omega}{sub m}, dark energy density, {Omega}{sub DE}, and the dark energy equation of state parameter, w, using Chandra measurements of the X-ray gas mass fraction (fgas) in 42 hot (kT > 5keV), X-ray luminous, dynamically relaxed galaxy clusters spanning the redshift range 0.05 < z < 1.1. Using only the fgas data for the 6 lowest redshift clusters at z < 0.15, for which dark energy has a negligible effect on the measurements, we measure {Omega}{sub m}=0.28{+-}0.06 (68% confidence, using standard priors on the Hubble Constant, H{sub 0}, and mean baryon density, {Omega}{sub b}h{sup 2}). Analyzing the data for all 42 clusters, employing only weak priors on H{sub 0} and {Omega}{sub b}h{sup 2}, we obtain a similar result on {Omega}{sub m} and detect the effects of dark energy on the distances to the clusters at {approx}99.99% confidence, with {Omega}{sub DE}=0.86{+-}0.21 for a non-flat LCDM model. The detection of dark energy is comparable in significance to recent SNIa studies and represents strong, independent evidence for cosmic acceleration. Systematic scatter remains undetected in the f{sub gas} data, despite a weighted mean statistical scatter in the distance measurements of only {approx}5%. For a flat cosmology with constant w, we measure {Omega}{sub m}=0.28{+-}0.06 and w=-1.14{+-}0.31. Combining the fgas data with independent constraints from CMB and SNIa studies removes the need for priors on {Omega}{sub b}h{sup 2} and H{sub 0} and leads to tighter constraints: {Omega}{sub m}=0.253{+-}0.021 and w=-0.98{+-}0.07 for the same constant-w model. More general analyses in which we relax the assumption of flatness and/or allow evolution in w remain consistent with the cosmological constant paradigm. Our analysis includes conservative allowances for systematic uncertainties. The small systematic scatter and tight constraints bode well for future dark energy studies using the f{sub gas} method.

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.

Cosmological Parameters from Observations of Galaxy Clusters

Studies of galaxy clusters have proved crucial in helping to establish the standard model of cosmology, with a Universe dominated by dark matter and dark energy. A theoretical basis that describes clusters as massive, multicomponent, quasi-equilibrium systems is growing in its capability to interpret multiwavelength observations of expanding scope and sensitivity. We review current cosmological results, including contributions to fundamental physics, obtained from observations of galaxy clusters. These results are consistent with and complementary to those from other methods. We highlight several areas of opportunity for the next few years, and emphasize the need for accurate modeling of survey selection and sources of systematic error. Capitalizing on these opportunities will require a multiwavelength approach and the application of rigorous statistical frameworks, utilizing the combined strengths of observers, simulators, and theorists.

Chandra imaging of the complex X-ray core of the Perseus cluster

ABSTRACT We report subarcsec-resolution X-ray imaging of the core ofthe Perseus cluster around thegalaxy NGC1275 with the Chandra X-ray Observatory. The ROSAT-discovered holes asso-ciated with the radio lobes have X-ray bright rims which are cooler than the surrounding gasand not due to shocks. The holes themselves may contain some hotter gas. We map strongphotoelectric absorption across the Northern lobe and rim due to a small infalling irregulargalaxy, known as the high velocity system. Two outer holes, one of which was previouslyknown, are identified with recently found spurs of low-frequency radio emission. The spiralappearance of the X-ray cooler gas and the outer optical parts of NGC1275 may be due toangular momentum in the cooling flow.Key words: galaxies: individual: Perseus – cooling flows – galaxies: in dividual: NGC1275– X-rays: galaxies 1 INTRODUCTIONThe Perseus cluster, Abell426, at a redshift z = 0.0183 or dis-tance about 100 Mpc is the brightest cluster in the sky in X-rays. Ithosts the nearest large cooling flow (e.g. Fabian et al 1981; A llenet al 1990; Fabian et al 1994). X-ray analysis of ASCA spectrain-dicates that the mass deposition rate is about 300M

A very deep Chandra observation of the Perseus cluster: shocks, ripples and conduction

We present the first results from a very deep Chandra X-ray observation of the core of the Perseus cluster of galaxies. A pressure map reveals a clear thick band of high pressure around the inner radio bubbles. The gas in the band must be expanding outward and the sharp front to it is identified as a shock front, yet we see no temperature jump across it; indeed there is more soft emission behind the shock than in front of it. We conclude that in this inner region either thermal conduction operates efficiently or the co-existing relativistic plasma seen as the radio mini-halo is mediating the shock. If common, isothermal shocks in cluster cores mean that we cannot diagnose the expansion speed of radio bubbles from temperature measurements alone. They can at times expand more rapidly than currently assumed without producing significant regions of hot gas. Bubbles may also be significantly more energetic. The pressure ripples found in earlier images are identified as isothermal sound waves. A simple estimate based on their amplitude confirms that they can be an effective distributed heat source able to balance radiative cooling.We see multiphase gas with about 10{sup 9}M{sub {circle_dot}} at a temperature of about 0.5 keV. Much, but not all, of this cooler gas is spatially associated with the optical filamentary nebula around the central galaxy, NGC1275. A residual cooling flow of about 50M{sub {circle_dot}} yr{sup -1} may be taking place. A channel is found in the pressure map along the path of the bubbles, with indications found of outer bubbles. The channel connects in the S with a curious cold front.

A deep Chandra observation of the Perseus cluster: shocks and ripples

We present preliminary results from a deep observation lasting almost 200 ks of the centre of the Perseus cluster of galaxies around NGC 1275. The X-ray surface brightness of the intracluster gas beyond the inner 20 kpc, which contains the inner radio bubbles, is very smooth apart from some low-amplitude quasi-periodic ripples. A clear density jump at a radius of 24 kpc to the north-east, about 10 kpc out from the bubble rim, appears to be due to a weak shock driven by the northern radio bubble. A similar front may exist around both inner bubbles but is masked elsewhere by rim emission from bright cooler gas. The continuous blowing of bubbles by the central radio source, leading to the propagation of weak shocks and viscously dissipating sound waves seen as the observed fronts and ripples, gives a rate of working which balances the radiative cooling within the inner 50 kpc of the cluster core.

The relation between accretion rate and jet power in X-ray luminous elliptical galaxies

Using Chandra X-ray observations of nine nearby, X-ray luminous elliptical galaxies with good optical velocity dispersion measurements, we show that a tight correlation exists between the Bondi accretion rates calculated from the observed gas temperature and density profiles and estimated black hole masses, and the power emerging from these systems in relativistic jets. The jet powers, which are inferred from the energies and timescales required to inflate cavities observed in the surrounding X-ray emitting gas, can be related to the accretion rates using a power law model of the form log(PBondi/10 43 ergs 1 ) = A + B log(Pjet/10 43 ergs 1 ), with A = 0.65± 0.16 and B = 0.77± 0.20. Our results show that a significant fraction of the energy associated with the rest mass of material entering the Bondi accretion radius (2.2 +1.0 0.7 per cent, for Pjet = 10 43 ergs 1 ) eventually emerges in the relativistic jets. The data also hint that this fraction may rise slightly with increasing jet power. Our results have significant implications for studies of accretion, jet formation and galaxy formation. The observed tight correlation suggests that the Bondi formulae provide a reasonable description of the accretion process in these systems, despite the likely presence of magnetic pressure and angular momentum in the accreting gas. The similarity of the PBondi and Pjet values argues that a significant fraction of the matter entering the accretion radius flows down to regions close to the black holes, where the jets are presumably formed. The tight correlation between PBondi and Pjet also suggests that the accretion flows are approximately stable over timescales of a few million years. Our results show that the black hole ‘engines’ at the hearts of large elliptical galaxies and groups can feed back sufficient energy to stem cooling and s formation, leading naturally to the observed exponential cut off at the bright end of the galaxy luminosity function.

A ROSAT study of the cores of clusters of galaxies - I. Cooling flows in an X-ray flux-limited sample

ABSTRA C T This is the first part of a study of the detailed X-ray properties of the cores of nearby clusters. We have used the flux-limited sample of 55 clusters listed by Edge et al., and archival and proprietary data from the ROSAT observatory. In this paper an X-ray spatial analysis based on the surface-brightness-deprojection technique is applied to the clusters in the sample with the aim of studying their cooling flow properties. We determine the fraction of cooling flows in this sample to be 70‐90 per cent, and estimate the contribution of the flow region to the cluster Xray luminosity. We show that the luminosity within a strong cooling flow can account for up to 70 per cent of a cluster X-ray bolometric luminosity. Our analysis indicates that about 40 per cent of the clusters in the sample have flows depositing more than 100 M( yr π1 throughout the cooling region, and that these possibly have been undisturbed for many Gyr, confirming that cooling flows are the natural state of cluster cores. New cooling flows in the sample are presented, and previously ambiguous ones are clarified. We have constructed a catalogue of some intracluster medium properties for the clusters in this sample. The profiles of the mass deposited from cooling flows are analysed, and evidence is presented for the existence of breaks in some of the profiles. Comparison is made to recent optical and radio data. We crosscorrelate our sample with the Green Bank, NVSS and FIRST surveys, and with the volumelimited sample of brightest cluster galaxies presented by Lauer & Postman. Although weak trends exist, no strong correlation between optical magnitude or radio power of the brightest cluster galaxy and the strength of the flow is found.

The ROSAT Brightest Cluster Sample – III. Optical spectra of the central cluster galaxies

We present new spectra of dominant galaxies in X-ray selected clusters of galaxies, which combine with our previously published spectra to form a sample of 256 dominant galaxies in 215 clusters. 177 of the clusters are members of the ROSAT Brightest Cluster Sample (BCS; Ebeling et al. 1998), and 18 have no previous measured redshift. This is the first paper in a series correlating the properties of brightest cluster galaxies and their host clusters in the radio, optical and X-ray wavebands. 27 per cent of the central dominant galaxies have emission-line spectra, all but five with line intensity ratios typical of cooling flow nebulae. A further 6 per cent show only (NII)��6548,6584 with Hin absorption. We find no evidence for an increase in the frequency of line emission with X-ray luminosity. Purely X-ray-selected clusters at low redshift have a higher probability of containing line emission. The projected separation between the optical position of the dominant galaxy and its host cluster X- ray centroid is less for the line-emitting galaxies than for those without line emission, consistent with a closer association of the central galaxy and the gravitational centre in cooling flow clusters. The more H�-luminous galaxies have larger emission-line regions and show a higher ratio of Balmer to forbidden line emission, although there is a continuous trend of ionization behaviour across four decades in Hluminosity. Galaxies with the more luminous line emission (L(H�)> 10 41 ergs 1 ) show a significantly bluer continuum, whereas lower-luminosity and (NII)-only line emitters have continua that differ little from those of non-line emitting dominant galaxies. Values of the Balmer decrement in the more luminous systems commonly imply intrinsic reddening of E(B-V)� 0.3, and when this is corrected for, the excess blue light can be characterized by a population of massive young stars. Several of the galaxies require a large population of O stars, which also provide sufficient photoionization to produce theobserved Hluminosity. The large number of lower-mass stars relative to the O star population suggests that this anomalous population is due to a series of starbursts in the central galaxy. The lower H�-luminosity systems show a higher ionization state and few massive stars, requiring instead the introduction of a harder source of photoionization, such as turbulent mixing layers, or low-level nuclear activity. The line emission from the systems showing only (NII) is very similar to low-level LINER activity commonly found in many normal elliptical galaxies.

The observed growth of massive galaxy clusters – I. Statistical methods and cosmological constraints

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 Ω m = 0.23 ± 0.04, σ 8 = 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 Ω m , = 0.27 ± 0.02, σ 8 = 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 w 0 = -0.88 ± 0.21 and w et = -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.