Fraction Of Galaxy Clusters Research Articles

Constraining the mass and redshift evolution of the hydrostatic mass bias using the gas mass fraction in galaxy clusters

The gas mass fraction in galaxy clusters is a quantity which can be used as a robust cosmological probe. It is however subject to various effects from the baryonic physics inside galaxy clusters, which may bias the obtained cosmological constraints. Among different aspects of the baryonic physics, in these proceedings we focus on the impact of the hydrostatic equilibrium assumption. From X-ray measurements of cluster gas fraction in the Planck-ESZ sample, we analyse the hydrostatic mass bias B, constraining a possible mass and redshift evolution of this quantity and its impact on the cosmological constraints. We show a degeneracy between the redshift dependence of the bias and cosmological parameters. In particular we find a 3.8σ evidence for a redshift dependence of the bias when assuming a Planck prior on Ωm. On the other hand, assuming a constant mass bias would lead to the extreme large value of Ωm > 0.860. We however show these results to be depending on the mass and redshift selections inside the main sample. Nevertheless, in all the analyses we find a value for the amplitude of the bias that is consistent with B ∼ 0.8, as expected from hydrodynamical simulations and local measurements, but still in tension with the low value of B ∼ 0.6 derived from the combination of cosmic microwave background primary anisotropies with cluster number counts. We also discuss cosmological constraints obtained from gas fraction data, combined with other probes like cluster number counts.

Revisiting [formula omitted] cosmologies

We review the status of f(R,T) cosmological models, where T is the trace of the energy momentum tensor Tμν. We start focusing on the modified Friedmann equations for the minimally coupled gravitational Lagrangian of the type f(R,T)=R+αeβT+γnTn. We show that in such a minimally coupled case there exists a useful constraining relation between the effective fractionary total matter density with an arbitrary equation of state parameter and the modified gravity parameters. With this association the modified gravity sector can be independently constrained using estimations of the gas mass fraction in galaxy clusters. Using cosmological background cosmic chronometers data and demanding the universe is old enough to accommodate the existence of Galactic globular clusters with ages of at least ∼14 Gyrs we find a narrow range of the modified gravity free parameter space in which this class of theories remains viable for the late time cosmological evolution. This preferred parameter space region accommodates the ΛCDM limit of f(R,T) models. We also work out the non-minimally coupled case in the metric-affine formalism and find that there are no viable cosmologies in the latter situation. However, when analyzing the cosmological dynamics including a radiation component, we find that this energy density interacts with the matter field and it does not scale according to the typical behavior. We conclude stating that f(R,T) gravity is not able to provide a full cosmological scenario and should be ruled out as a modified gravity alternative to the dark energy phenomena.

Constraining the mass and redshift evolution of the hydrostatic mass bias using the gas mass fraction in galaxy clusters

The gas mass fraction in galaxy clusters is a convenient probe to use in cosmological studies, as it can help derive constraints on a range of cosmological parameters. This quantity is, however, subject to various effects from the baryonic physics inside galaxy clusters, which may bias the obtained cosmological constraints. Among different aspects of the baryonic physics at work, in this paper we focus on the impact of the hydrostatic equilibrium assumption. We analyzed the hydrostatic mass bias B, constraining a possible mass and redshift evolution for this quantity and its impact on the cosmological constraints. To that end, we considered cluster observations of the Planck-ESZ sample and evaluated the gas mass fraction using X-ray counterpart observations. We show a degeneracy between the redshift dependence of the bias and cosmological parameters. In particular we find evidence at 3.8σ for a redshift dependence of the bias when assuming a Planck prior on Ωm. On the other hand, assuming a constant mass bias would lead to the extremely large value of Ωm > 0.860. We show, however, that our results are entirely dependent on the cluster sample under consideration. In particular, the mass and redshift trends that we find for the lowest mass-redshift and highest mass-redshift clusters of our sample are not compatible. In addition, we show that assuming self-similarity in our study can impact the results on the evolution of the bias, especially with regard to the mass evolution. Nevertheless, in all the analyses, we find a value for the amplitude of the bias that is consistent with B ∼ 0.8, as expected from hydrodynamical simulations and local measurements. However, this result is still in tension with the low value of B ∼ 0.6 derived from the combination of cosmic microwave background primary anisotropies with cluster number counts.

Constraining hydrostatic mass bias and cosmological parameters with the gas mass fraction in galaxy clusters

The gas mass fraction in galaxy clusters is a convenient tool to use in the context of cosmological studies. Indeed this quantity allows to constrain the universal baryon fraction Ωb/Ωm, as well as other parameters like the matter density Ωm, the Hubble parameter h or the Equation of State of Dark Energy w. This gas mass fraction is also sensitive to baryonic effects that need to be taken into account, and that translate into nuisance parameters. Two of them are the depletion factor ϒ, and the hydrostatic mass bias B = (1 - b). The first one describes how baryons are depleted in clusters with respect to the universal baryon fraction, while the other encodes the bias coming from the fact that the mass is deduced from X-ray observations under the hypothesis of hydrostatic equilibrium. We will show preliminary results, obtained using the Planck-ESZ clusters observed by XMM-Newton, on both cosmological and cluster parameters. We will notably discuss the investigation on a possible redshift dependence of the mass bias, which is considered to be non-existent in hydrodynamic simulations based on Λ-CDM, and compare our results with other studies.

A model-independent test of the evolution of gas depletion factor for SPT-SZ and Planck ESZ clusters

The gas mass fraction in galaxy clusters has been widely used to determine cosmological parameters. This method assumes that the ratio of the cluster gas mass fraction to the cosmic baryon fraction (gamma (z)) is constant as a function of redshift. In this work, we look for a time evolution of gamma (z) at R_{500} by using both the SPT-SZ and Planck Early SZ (ESZ) cluster data, in a model-independent fashion without any explicit dependence on the underlying cosmology. For this calculation, we use a non-parametric functional form for the Hubble parameter obtained from Gaussian Process regression using cosmic chronometers. We parameterize gamma (z) as: gamma (z)= gamma _0(1+gamma _1 z) to constrain the redshift evolution. We find contradictory results between both the samples. For SPT-SZ, gamma (z) decreases as a function of redshift (at more than 5sigma ), whereas a positive trend with redshift is found for Planck ESZ data (at more than 4sigma ). We however find that the gamma _1 values for a subset of SPT-SZ and Planck ESZ clusters between the same redshift interval agree to within 1sigma . When we allow for a dependence on the halo mass in the evolution of the gas depletion factor, the 4-5sigma discrepancy reduces to 2sigma .

The gas depletion factor in galaxy clusters: implication from Atacama Cosmology Telescope Polarization experiment measurements

The gas depletion factor gamma (z), i.e., the average ratio of the gas mass fraction to the cosmic mean baryon fraction of galaxy clusters, plays a very important role in the cosmological application of the gas mass fraction measurements. In this paper, using the newest catalog of 182 galaxy clusters detected by the Atacama Cosmology Telescope (ACT) Polarization experiment, we investigate the possible redshift evolution of gamma (z) through a new cosmology-independent method. The method is based on non-parametric reconstruction using the measurements of Hubble parameters from cosmic chronometers. Unlike hydrodynamical simulations suggesting constant depletion factor, our results reveal the trend of gamma (z) decreasing with redshift. This result is supported by a parametric model fit as well as by calculations on the reduced ACTPol sample and on the alternative sample of 91 SZ clusters reported earlier in ACT compilation. Discussion of possible systematic effects leaves an open question about validity of the empirical relation M_{tot}–f_{gas} obtained on very close clusters. These results might pave the way to explore the hot gas fraction within large radii of galaxy clusters as well as its possible evolution with redshift, which should be studied further on larger galaxy cluster samples in the upcoming X-ray/SZ cluster surveys.

Is the non-isothermal double β-model incompatible with no time evolution of galaxy cluster gas mass fraction?

In this paper, we propose a new method to obtain the depletion factor γ(z), the ratio by which the measured baryon fraction in galaxy clusters is depleted with respect to the universal mean. We use exclusively galaxy cluster data, namely, X-ray gas mass fraction (fgas) and angular diameter distance measurements from Sunyaev–Zel’dovich effect plus X-ray observations. The galaxy clusters are the same in both data set and the non-isothermal spherical double β-model was used to describe their electron density and temperature profiles. In order to compare our results with those from recent cosmological hydrodynamical simulations, we suppose a possible time evolution for γ(z), such as, γ(z)=γ0(1+γ1z). As main conclusions we found that: the γ0 value is in full agreement with the simulations. On the other hand, although the γ1 value found in our analysis is compatible with γ1=0 within 2σ c.l., our results show a non-negligible time evolution for the depletion factor, unlike the results of the simulations. However, we also put constraints on γ(z) by using the fgas measurements and angular diameter distances obtained from the flat ΛCDM model (Planck results) and from a sample of galaxy clusters described by an elliptical profile. For these cases no significant time evolution for γ(z) was found. Then, if a constant depletion factor is an inherent characteristic of these structures, our results show that the spherical double β-model used to describe the galaxy clusters considered does not affect the quality of their fgas measurements.

Investigating cluster astrophysics and cosmology with cross-correlation of the thermal Sunyaev–Zel'dovich effect and weak lensing

Recent detections of the cross-correlation of the thermal Sunyaev–Zel'dovich (tSZ) effect and weak gravitational lensing (WL) enable unique studies of cluster astrophysics and cosmology. In this work, we present constraints on the amplitude of the non-thermal pressure fraction in galaxy clusters, α0, and the amplitude of the matter power spectrum, σ8, using measurements of the tSZ power spectrum from Planck, and the tSZ–WL cross-correlation from Planck and the Red Cluster Sequence Lensing Survey. We fit the data to a semi-analytic model with the covariance matrix using N-body simulations. We find that the tSZ power spectrum alone prefers σ8 ∼ 0.85 and a large fraction of non-thermal pressure (α0 ∼ 0.2–0.3). The tSZ–WL cross-correlation on the other hand prefers a significantly lower σ8 ∼ 0.6 and low α0 ∼ 0.05. We show that this tension can be mitigated by allowing for a steep slope in the stellar mass–halo mass relation, which would cause a reduction of the gas in low-mass haloes. In such a model, the combined data prefer σ8 ∼ 0.7 and α0 ∼ 0.2, consistent with predictions from hydrodynamical simulations.

Comparing Dark Energy Survey andHST–CLASH observations of the galaxy cluster RXC J2248.7−4431: implications for stellar mass versus dark matter

We derive the stellar mass fraction in the galaxy cluster RXC J2248.7-4431 observed with the Dark Energy Survey (DES) during the Science Verification period. We compare the stellar mass results from DES (five filters) with those from the Hubble Space Telescope Cluster Lensing And Supernova Survey (CLASH; 17 filters). When the cluster spectroscopic redshift is assumed, we show that stellar masses from DES can be estimated within 25% of CLASH values. We compute the stellar mass contribution coming from red and blue galaxies, and study the relation between stellar mass and the underlying dark matter using weak lensing studies with DES and CLASH. An analysis of the radial profiles of the DES total and stellar mass yields a stellar-to-total fraction of f*=(6.8+-1.7)x10^-3 within a radius of r_200c~2 Mpc. Our analysis also includes a comparison of photometric redshifts and star/galaxy separation efficiency for both data sets. We conclude that space-based small field imaging can be used to calibrate the galaxy properties in DES for the much wider field of view. The technique developed to derive the stellar mass fraction in galaxy clusters can be applied to the ~100 000 clusters that will be observed within this survey and yield important information about galaxy evolution.

The challenge of turbulent acceleration of relativistic particles in the intra-cluster medium

Acceleration of cosmic-ray electrons (CRe) in the intra-cluster medium (ICM) is probed by radio observations that detect diffuse, megaparsec-scale, synchrotron sources in a fraction of galaxy clusters. Giant radio halos are the most spectacular manifestations of non-thermal activity in the ICM and are currently explained assuming that turbulence, driven during massive cluster–cluster mergers, reaccelerates CRe at several giga-electron volts. This scenario implies a hierarchy of complex mechanisms in the ICM that drain energy from large scales into electromagnetic fluctuations in the plasma and collisionless mechanisms of particle acceleration at much smaller scales. In this paper we focus on the physics of acceleration by compressible turbulence. The spectrum and damping mechanisms of the electromagnetic fluctuations, and the mean free path (mfp) of CRe, are the most relevant ingredients that determine the efficiency of acceleration. These ingredients in the ICM are, however, poorly known, and we show that calculations of turbulent acceleration are also sensitive to these uncertainties. On the other hand this fact implies that the non-thermal properties of galaxy clusters probe the complex microphysics and the weakly collisional nature of the ICM.

Observational constraints on a phenomenological $$f\left( R,\partial R\right) $$ f R , ∂ R -model

This paper analyses the cosmological consequences of a modified theory of gravity whose action integral is built from a linear combination of the Ricci scalar $R$ and a quadratic term in the covariant derivative of $R$. The resulting Friedmann equations are of the fifth-order in the Hubble function. These equations are solved numerically for a flat space section geometry and pressureless matter. The cosmological parameters of the higher-order model are fit using SN Ia data and X-ray gas mass fraction in galaxy clusters. The best-fit present-day $t_{0}$ values for the deceleration parameter, jerk and snap are given. The coupling constant $\beta$\ of the model is not univocally determined by the data fit, but partially constrained by it. Density parameter $\Omega_{m0}$ is also determined and shows weak correlation with the other parameters. The model allows for two possible future scenarios: there may be either a premature Big Rip or a Rebouncing event depending on the set of values in the space of parameters. The analysis towards the past performed with the best-fit parameters shows that the model is not able to accommodate a matter-dominated stage required to the formation of structure.

A CHANDRA - SWIFT VIEW OF POINT SOURCES IN HICKSON COMPACT GROUPS: HIGH AGN FRACTION BUT A DEARTH OF STRONG AGNs

We present $Chandra$ X-ray point source catalogs for 9 Hickson Compact Groups (HCGs, 37 galaxies) at distances $34 - 89$ Mpc. We perform detailed X-ray point source detection and photometry, and interpret the point source population by means of simulated hardness ratios. We thus estimate X-ray luminosities ($L_X$) for all sources, most of which are too weak for reliable spectral fitting. For all sources, we provide catalogs with counts, count rates, power-law indices ($\Gamma$), hardness ratios, and $L_X$, in the full ($0.5-8.0$ keV), soft ($0.5-2.0$ keV) and hard ($2.0-8.0$ keV) bands. We use optical emission-line ratios from the literature to re-classify 24 galaxies as star-forming, accreting onto a supermassive black hole (AGNs), transition objects, or low-ionization nuclear emission regions (LINERs). Two-thirds of our galaxies have nuclear X-ray sources with $Swift$/UVOT counterparts. Two nuclei have $L_{X,{\rm 0.5-8.0 keV}}$~$ > 10^{42}$ erg s$^{-1}$, are strong multi-wavelength AGNs and follow the known $\alpha_{\rm OX}-\nu L_{\nu,\rm near UV}$ correlation for strong AGNs. Otherwise, most nuclei are X-ray faint, consistent with either a low-luminosity AGN or a nuclear X-ray binary population, and fall in the "non-AGN locus" in $\alpha_{\rm OX}-\nu L_{\nu,\rm near UV}$ space, which also hosts other, normal, galaxies. Our results suggest that HCG X-ray nuclei in high specific star formation rate spiral galaxies are likely dominated by star formation, while those with low specific star formation rates in earlier types likely harbor a weak AGN. The AGN fraction in HCG galaxies with $M_R \le -20$ and $L_{X,{\rm 0.5-8.0 keV}} \ge 10^{41}$ erg s$^{-1}$ is $0.08^{+0.35}_{-0.01}$, somewhat higher than the $\sim 5%$ fraction in galaxy clusters.

Baryons do trace dark matter 380,000 years after the big bang: Search for compensated isocurvature perturbations with WMAP 9-year data

Primordial isocurvature fluctuations between photons and either neutrinos or non-relativistic species such as baryons or dark matter are known to be sub-dominant to adiabatic fluctuations. Perturbations in the relative densities of baryons and dark matter (known as compensated isocurvature perturbations, or CIPs), however, are surprisingly poorly constrained. CIPs leave no imprint in the cosmic microwave background (CMB) on observable scales, at least at linear order in their amplitude and zeroth order in the amplitude of adiabatic perturbations. It is thus not yet empirically known if baryons trace dark matter at the surface of last scattering. If CIPs exist, they would spatially modulate the Silk damping scale and acoustic horizon, causing distinct fluctuations in the CMB temperature/polarization power spectra across the sky: this effect is first order in both the CIP and adiabatic mode amplitudes. Here, temperature data from the Wilkinson Microwave Anisotropy Probe (WMAP) are used to conduct the first CMB-based observational search for CIPs, using off-diagonal correlations and the CMB trispectrum. Reconstruction noise from weak lensing and point sources is shown to be negligible for this data set. No evidence for CIPs is observed, and a 95%-confidence upper limit of $1.1\times 10^{-2}$ is imposed to the amplitude of a scale-invariant CIP power spectrum. This limit agrees with CIP sensitivity forecasts for WMAP, and is competitive with smaller scale constraints from measurements of the baryon fraction in galaxy clusters. It is shown that the root-mean-squared CIP amplitude on 5-100 degree scales is smaller than 0.07-0.17 (depending on the scale) at the 95%-confidence level. Temperature data from the Planck satellite will provide an even more sensitive probe for the existence of CIPs, as will the upcoming ACTPol and SPTPol experiments on smaller angular scales.

Tully-Fisher analysis of the multiple cluster system Abell 901/902

Context.

The MUSIC of galaxy clusters – I. Baryon properties and scaling relations of the thermal Sunyaev–Zel'dovich effect

We introduce the Marenostrum-MultiDark SImulations of galaxy Clusters (MUSIC) data set. It constitutes one of the largest samples of hydrodynamically simulated galaxy clusters with more than 500 clusters and 2000 groups. The objects have been selected from two large N-body simulations and have been resimulated at high resolution using smoothed particle hydrodynamics (SPH) together with relevant physical processes that include cooling, UV photoionization, star formation and different feedback processes associated with supernovae explosions. In this first paper we focus on the analysis of the baryon content (gas and star) of clusters in the MUSIC data set as a function of both aperture radius and redshift. The results from our simulations are compared with a compilation of the most recent observational estimates of the gas fraction in galaxy clusters at different overdensity radii. We confirm, as in previous simulations, that the gas fraction is overestimated if radiative physics are not properly taken into account. On the other hand, when the effects of cooling and stellar feedbacks are included, the MUSIC clusters show a good agreement with the most recent observed gas fractions quoted in the literature. A clear dependence of the gas fractions with the total cluster mass is also evident. However, we do not find a significant evolution with redshift of the gas fractions at aperture radius corresponding to overdensities smaller than 1500 with respect to critical density. At smaller radii, the gas fraction does exhibit a decrease with redshift that is related to the gas depletion due to star formation in the central region of the clusters. The impact of the aperture radius choice, when comparing integrated quantities at different redshifts, is tested. The standard, widely used definition of radius at a fixed overdensity with respect to critical density is compared with a definition of aperture radius based on the redshift dependent overdensity with respect to background matter density: we show that the latter definition is more successful in probing the same fraction of the virial radius at different redshifts, providing a more reliable derivation of the time evolution of integrated quantities. We also present in this paper a detailed analysis of the scaling relations of the thermal Sunyaev–Zel'dovich (SZ) effect derived from MUSIC clusters. The integrated SZ brightness, Y, is related to the cluster total mass, M, as well as, the M − Y counterpart which is more suitable for observational applications. Both laws are consistent with predictions from the self-similar model, showing a very low scatter which is σlog Y ≃ 0.04 and even a smaller one (σlog M ≃ 0.03) for the inverse M–Y relation. The effects of the gas fraction on the Y–M scaling relation are also studied. At high overdensities, the dispersion of the gas fractions introduces non-negligible deviation from self-similarity, which is directly related to the fgas–M relation. The presence of a possible redshift dependence on the Y–M scaling relation is also explored. No significant evolution of the SZ relations is found at lower overdensities, regardless of the definition of overdensity used.

A matter-dominated cosmological model with variable G and Λ and its confrontation with observational data

In the framework of renormalization-group improved cosmologies, we analyze both theoretically and observationally the exact and general solution of the matter-dominated cosmological equations, using the expression of \Lambda = \Lambda(G) already determined by the integration method employed in a previous paper. A rough comparison between such a model and the concordance \LambdaCDM model as to the magnitude-redshift relationship has been already done, without showing any appreciable differences. We here perform a more refined study of how astrophysical data (Union2 set) on type-I supernovae, gamma ray bursts (in a sample calibrated in a model independent way with the SneIa dataset), and gas fraction in galaxy clusters (using a sample of Chandra measurements of the X-ray gas mass fraction) affect the model and constrain its parameters. We also apply a cosmographic approach to our cosmological model and estimate the cosmographic parameters by fitting both the supernovae and the gamma ray bursts datasets. We show that this matter-dominated cosmological model with variable Newton parameter and variable cosmological term is indeed compatible with the observations above (on type Ia supernovae, the gamma ray bursts Hubble diagram, and the gas mass fraction in X-ray luminous galaxy clusters). The cosmographic approach adopted confirms such conclusions. Finally, it seems possible to include radiation into the model, since numerical integration of the equations derived by the presence of both radiation and matter shows that, after inflation, the total density parameter is initially dominated by the radiation contribution and later by the matter one.

Holographic dark energy at the Ricci scale

We consider a holographic cosmological model in which the infrared cutoff is fixed by the Ricci's length and dark matter and dark energy do not evolve separately but interact non-gravitationally with one another. This substantially alleviates the cosmic coincidence problem as the ratio between both components remains finite throughout the expansion. We constrain the model with observational data from supernovae, cosmic background radiation, baryon acoustic oscillations, gas mass fraction in galaxy clusters, the history of the Hubble function, and the growth function. The model shows consistency with observation.

Model of interacting holographic dark energy at the Ricci scale

We study a holographic cosmological model in which the infrared cutoff is set by the Ricci's length and dark matter and dark energy do not evolve on their own but interact non-gravitationally with each other. This greatly alleviates the cosmic coincidence problem because the ratio between both components does not vanish at any time. We constrain the model with observational data from supernovae, cosmic background radiation, baryon acoustic oscillations, gas mass fraction in galaxy clusters, the history of the Hubble function, and the growth function.

Constraining Dark Matter-Dark Energy Interaction with Gas Mass Fraction in Galaxy Clusters

AbstractThe recent observational evidence for the current cosmic acceleration have stimulated renewed interest in alternative cosmologies, such as scenarios with interaction in the dark sector (dark matter and dark energy). In general, such models contain an unknown negative-pressure dark component coupled with the pressureless dark matter and/or with the baryons that results in an evolution for the Universe rather different from the one predicted by the standard ΛCDM model. In this work we test the observational viability of such scenarios by using the most recent galaxy cluster gas mass fraction versus redshift data (42 X-ray luminous, dynamically relaxed galaxy clusters spanning the redshift range 0.063 < z < 1.063), Allen et al. (2008), to place bounds on the parameter ε that characterizes the dark matter/dark energy coupling. The resulting are consistent with, and typically as constraining as, those derived from other cosmological data. Although a time-independent cosmological constant (ΛCDM model) is a good fit to these galaxy cluster data, an interacting energy component cannot yet be ruled out.

Forming Early-Type Galaxies in Groups Prior to Cluster Assembly

We study a unique proto-cluster of galaxies, the supergroup SG1120-1202. We quantify the degree to which morphological transformation of cluster galaxies occurs prior to cluster assembly in order to explain the observed early-type fractions in galaxy clusters at z=0. SG1120-1202 at z~0.37 is comprised of four gravitationally bound groups that are expected to coalesce into a single cluster by z=0. Using HST ACS observations, we compare the morphological fractions of the supergroup galaxies to those found in a range of environments. We find that the morphological fractions of early-type galaxies (~60 %) and the ratio of S0 to elliptical galaxies (0.5) in SG1120-1202 are very similar to clusters at comparable redshift, consistent with pre-processing in the group environment playing the dominant role in establishing the observed early-type fraction in galaxy clusters.