Andrea Morandi

The Three-Dimensional Shapes of Galaxy Clusters

While clusters of galaxies are considered one of the most important cosmological probes, the standard spherical modelling of the dark matter and the intracluster medium is only a rough approximation. Indeed, it is well established both theoretically and observationally that galaxy clusters are much better approximated as triaxial objects. However, investigating the asphericity of galaxy clusters is still in its infancy. We review here this topic which is currently gathering a growing interest from the cluster community. We begin by introducing the triaxial geometry. Then we discuss the topic of deprojection and demonstrate the need for combining different probes of the cluster’s potential. We discuss the different works that have been addressing these issues. We present a general parametric framework intended to simultaneously fit complementary data sets (X-ray, Sunyaev Zel’dovich and lensing data). We discuss in details the case of Abell 1689 to show how different models/data sets lead to different haloe parameters. We present the results obtained from fitting a 3D NFW model to X-ray, SZ, and lensing data for 4 strong lensing clusters. We argue that a triaxial model generally allows to lower the inferred value of the concentration parameter compared to a spherical analysis. This may alleviate tensions regarding, e.g. the over-concentration problem. However, we stress that predictions from numerical simulations rely on a spherical analysis of triaxial halos. Given that triaxial analysis will have a growing importance in the observational side, we advocate the need for simulations to be analysed in the very same way, allowing reliable and meaningful comparisons. Besides, methods intended to derive the three dimensional shape of galaxy clusters should be extensively tested on simulated multi-wavelength observations.

X-ray, lensing and Sunyaev–Zel'dovich triaxial analysis of Abell 1835 out to R200

The measurement of the intrinsic shape and orientation of dark matter (DM) and intracluster (IC) gas in galaxy clusters is crucial for constraining their formation and evolution, and for enhancing the use of clusters as more precise cosmological probes. Extending our previous works, for the first time we present the results from a triaxial joint analysis of the galaxy cluster Abell 1835, using X-ray, strong lensing (SL) and Sunyaev–Zeldovich (SZ) data. We parametrically reconstruct the full three-dimensional structure (triaxial shape and principal axis orientation) of both the DM and the IC gas, and the level of non-thermal pressure of the IC gas. We find that the intermediate–major and minor–major axial ratios of the DM are 0.71 ± 0.08 and 0.59 ± 0.05, respectively, and that the major axis of the DM halo is inclined with respect to the line of sight at 18.3 ± 5.2 deg. We present the first observational measurement of the non-thermal pressure out to R200. This has been evaluated to be a few peru2009cent of the total energy budget in the internal regions, while it reaches approximately 20 peru2009cent in the outer volumes. We discuss the implications of our method for the viability of the cold dark matter (CDM) scenario, focusing on the concentration parameter C and the inner slope of the DM γ in order to test the CDM paradigm for structure formation. We measure γ = 1.01 ± 0.06 and C = 4.32 ± 0.44; these values are close to the predictions of the CDM model. The combination of X-ray/SL data at high spatial resolution, which are capable of resolving the cluster core, with the SZ data, which are more sensitive to the cluster outer volume, allows us to characterize the level and the gradient of the gas entropy distribution and non-thermal pressure out to R_200. Thus, we break the degeneracy among the physical models describing the thermal history of the intracluster medium.

Triaxiality and non-thermal gas pressure in Abell 1689

Clusters of galaxies are uniquely important cosmological probes of the evolution of the large-scale structure, whose diagnostic power depends quite significantly on the ability to reliably determine their masses. Clusters are typically modelled as spherical systems whose intracluster gas is in strict hydrostatic equilibrium (i.e. the equilibrium gas pressure is provided entirely by thermal pressure), with the gravitational field dominated by dark matter, assumptions that are only rough approximations. In fact, numerical simulations indicate that galaxy clusters are typically triaxial, rather than spherical, and that turbulent gas motions (induced during hierarchical merger events) provide an appreciable pressure component. Extending our previous work, we present results of a joint analysis of X-ray, weak-and strong-lensing measurements of Abell 1689. The quality of the data allows us to determine both the triaxial shape of the cluster and the level of non-thermal pressure that is required if the intracluster gas is in hydrostatic equilibrium. We find that the dark matter axial ratios are 1.24 +/- 0.13 and 2.02 +/- 0.01 on the plane of the sky and along the line of sight, respectively, and that about 20 per cent of the pressure is non-thermal. Our treatment demonstrates that the dynamical properties of clusters can be determined in a (mostly) bias-free way, enhancing the use of clusters as more precise cosmological probes.

Triaxiality, principal axis orientation and non-thermal pressure in Abell 383

While clusters of galaxies are regarded as one of the most important cosmological probes, the conventional spherical modelling of the intracluster medium and the dark matter (DM), and the assumption of strict hydrostatic equilibrium (i.e. the equilibrium gas pressure is provided entirely by thermal pressure) are very approximate at best. Extending our previous works, we developed further a method to reconstruct for the first time the full 3D structure (triaxial shape and principal-axis orientation) of both DM and intracluster (IC) gas, and the level of non-thermal pressure of the IC gas. We outline an application of our method to the galaxy cluster Abell 383, taken as part of the Cluster Lensing and Supernova Survey with Hubble (CLASH) multicycle treasury programme, presenting results of a joint analysis of X-ray and strong lensing measurements. We find that the intermediate–major and minor–major axis ratios of the DM are 0.71 ± 0.10 and 0.55 ± 0.06, respectively, and the major axis of the DM halo is inclined with respect to the line of sight of 211 ± 101. The level of non-thermal pressure has been evaluated to be about 10 per cent of the total energy budget. We discuss the implications of our method for the viability of the cold dark matter (CDM) scenario, focusing on the concentration parameter C and the inner slope of the DM, γ, since the cuspiness of DM density profiles in the central regions is one of the critical tests of the CDM paradigm for structure formation: we measure γ= 1.02 ± 0.06 on scales down to 25 Kpc, and C= 4.76 ± 0.51, values which are close to the predictions of the standard model, and providing further evidences that support the CDM scenario. Our method allows us to recover the 3D physical properties of clusters in a bias-free way, overcoming the limitations of the standard spherical modelling and enhancing the use of clusters as more precise cosmological probes.

Measuring the gas clumping in Abell 133

This paper continues a series in which we developed a non-parametric method to measure inhomogeneities in the gas distribution from X-ray observations of galaxy clusters. In this work, we apply our method to Chandra X-ray observations of Abell 133 and present the determination of the gas clumping factor from X-ray cluster data. We find that the gas clumping factor in Abell 133 increases with radius and reaches � 2 3 at 0.9R200. This is in good agreement with the predictions of hydrodynamical simulations and our previous determination. We then observe a general trend of steepening in the radial profiles of the clumping-corrected gas density beyond 0.3R200, with a logarithmic slope of � 2.6 at 0.9R200. The observed density profiles appear to be flatter compared to simulations, but in agreement with previous observational findings. In addition, we observe that the measured temperature decreases steadily with radius toward the outskirts of A133, while the entropy increases monotonically with radius, gently flattening in the outer volumes. With respect to theoretical predictions from pure gravitational collapse, the results presented here point to an entropy excess in the central regions, which extends out to large radii. These results suggest that gas inhomogeneities should be treated properly when interpreting X-ray measurements in the envelope of galaxy clusters. We finally discuss how the brightness distribution keeps a record of the large-scale structures formation scenario, providing a snapshot of the ’melting pot’ in the virialization region.

Studying cosmic reionization with observations of the global 21-cm signal

ABSTRACT We explore the ability of observations of the global brightness temperature of the21-cm signal to constrain the reionization history and the properties of the ionizingsources. In order to describe the reionization signal, we employ either a commonly-used toy model or a structure formation model that parameterizes the properties ofthe ionizing sources. If the structure formation model captures the actual evolutionof the reionization signal, then detecting the signal is somewhat easier than it wouldbe for the toy model; using the toy model in this case also leads to systematic errorsin reconstructing the reionization history, though a sufficiently sensitive experimentshould be able to distinguish between the two models. We show that under optimisticassumptions regardingsystematic noise and foregroundremoval, one-yearobservationsof the global 21-cm spectrum should be able to detect a wide range of realistic modelsand measure the main features of the reionization history while constraining the keyproperties of the ionizing sources.Key words: galaxies:high-redshift – cosmology:theory – galaxies:formation

Non-parametric method for measuring gas inhomogeneities from X-ray observations of galaxy clusters

We present a non-parametric method to measure inhomogeneities in the intracluster medium (ICM) from X-ray observations of galaxy clusters. Analysing mock Chandra X-ray observations of simulated clusters, we show that our new method enables the accurate recovery of the 3D gas density and gas clumping factor profiles out to large radii of galaxy clusters. We then apply this method to Chandra X-ray observations of Abell 1835 and present the first determination of the gas clumping factor from the X-ray cluster data. We find that the gas clumping factor in Abell 1835 increases with radius and reaches ~2-3 at r=R_{200}. This is in good agreement with the predictions of hydrodynamical simulations, but it is significantly below the values inferred from recent Suzaku observations. We further show that the radially increasing gas clumping factor causes flattening of the derived entropy profile of the ICM and affects physical interpretation of the cluster gas structure, especially at the large cluster-centric radii. Our new technique should be useful for improving our understanding of the cluster structure and to advance the use of galaxy clusters as cosmological probes, by helping to exploit rich data sets provided by Chandra and XMM-Newton X-ray space telescopes.

Cluster–cluster lensing and the case of Abell 383

Extensive surveys of galaxy clusters motivate us to assess the likelihood of cluster–cluster lensing (CCL), namely, gravitational lensing of a background cluster by a foreground cluster. We briefly describe the characteristics of CCLs in optical, X-ray and Sunyaev–Zeldovich effect measurements, and calculate their predicted numbers for Λ cold dark matter (ΛCDM) parameters and a viable range of cluster mass functions and their uncertainties. The predicted number of CCLs in the strong-lensing regime varies from several (<10) to as high as a few dozen, depending mainly on whether lensing triaxiality bias is accounted for, through theu2002c–Mu2002relation. A much larger number is predicted when taking into account also CCL in the weak-lensing regime. In addition to few previously suggested CCLs, we report a detection of a possible CCL in A383, where background candidate high-zu2002structures are magnified, as seen in deep Subaru observations.

Reconstructing three-dimensional parameters of galaxy clusters via multifrequency Sunyaev–Zeldovich observations

ABSTRACT The Sunyaev-Zeldovich (SZ) effect is a promising tool to study physical properties ofthe hot X-ray emitting intracluster medium (ICM) in galaxy clusters. To date, most SZobservations have been interpreted in combination with X-ray follow-up measurementsin order to determine the ICM temperature and estimate the cluster mass. Futurehigh-resolution, multifrequency SZ observations promise to enable detailed studies ofthe ICM structures, by measuring the ICM temperature through the temperature-dependent relativistic corrections. In this work we develop a non-parametric methodto derive three-dimensional physical quantities, including temperature, pressure, totalmass, and peculiar velocities, of galaxy clusters from SZ observations alone. We testthe performance of this method using hydrodynamical simulations of galaxy clusters,in order to assess systematic uncertainties in the reconstructed physical parameters.In particular, we analyze mock Cerro Chajnantor Atacama Telescope (CCAT) SZobservations,takinginto account varioussourcesofsystematic uncertainties associatedwith instrumental effects and astrophysical foregrounds. We show that our methodenables accurate reconstruction of the three-dimensional ICM profiles, while retainingfull informationabout the gasdistribution. We discuss the application ofthis techniquefor ongoing and future multifrequency SZ observations.Keywords: cosmology: observations – galaxies: clusters: general – X-rays: galaxies:clusters – cosmic microwave background

Are observations of the galaxy cluster A1689 consistent with a neutrino dark matter scenario

Recent weak and strong lensing data of the galaxy cluster A1689 are modelled by dark fermions that are quantum degenerate within some core. The gas density, deduced from X-ray observations up to 1u2009Mpc and obeying a cored power law, is taken as input, while the galaxy mass density is modelled. An additional dark matter tail may arise from cold or warm dark matter, axions or non-degenerate neutrinos. The fit yields that the fermions are degenerate within a 430-kpc radius. The fermion mass is a few eV and the best case involves three active plus three sterile neutrinos of equal mass, for which we deduce 1.51 ± 0.04 eV. The eV mass range will be tested in the KATRIN experiment.