On the Dynamical Origin of Bias in Clusters of Galaxies

We study the effect of the dynamical friction induced by the presence of substructure on the statistics of the collapse of density peaks. Applying the results of a former paper we show that within high density environments, like rich clusters of galaxies, the collapse of smaller peaks is strongly delayed until very late epochs. A bias of dynamical nature thus naturally arises because high density peaks preferentially collapse For a standard CDM model we find that this dynamical bias can account for a substantial part of the total bias required by observations on cluster scales.Comment: 14 pages, postscript, 3 postscript figures available by anonymous ftp on convex.ct.astro.it in /nbody/fig#.ps. Submitted to Astrophysical Journal Letter

Studying the leptonic structure of galaxy cluster atmospheres from the spectral properties of the SZ effect

We study the energetics of galaxy cluster atmospheres by analyzing the SZ effect spectra around the crossover frequency. We calculated analytically the expressions of both the crossover frequency and the spectral slope of the SZE around the crossover frequency in various cases: a thermal electron population; a power-law, non-thermal electron population; and a population of electrons experiencing a stochastic acceleration. We find that the value of the crossover frequency X_0 of the SZE depends significantly on the cluster peculiar velocity V_r (which determines the amplitude of the kinematic SZE), while the value of the slope of the SZE does not depend on the kinematic SZE spectrum in the optimal frequency range around the crossover frequency of the thermal SZE, i.e. in the (a-dimensional) frequency range x = 3.5 - 4.5. Thus, while the amplitude of the kinematic SZE produces a systematic bias in the position of the crossover frequency, it does not affect significantly the spectral slope of the SZE. We therefore propose to use measurements of the spectral slope of the SZE to obtain unbiased information about the specific properties of various electron distributions in galaxy clusters as well as in other cosmic structures in which a SZE can be produced.Comment: 11 pages, 8 figures, published in A&

Properties of galaxy halos in Clusters and Voids

We use the results of a high resolution N-body simulation to investigate the role of the environment on the formation and evolution of galaxy-sized halos. Starting from a set of constrained initial conditions, we have produced a final configuration hosting a double cluster in one octant and a large void extending over two octants of the simulation box. We present results for two statistics: the relationship between 1-D velocity dispersion and mass and the probability distribution of the spin parameter $P(\lambda)$. The \svm relationship is well reproduced by the Truncated Isothermal Sphere (TIS) model introduced by Shapiro et al. (1999), although the slope is different from the original prediction. A series of \svm relationships for different values of the anisotropy parameter $\beta$, obtained using the theoretical predictions by Lokas and Mamon (2001) for NFW density profiles are found to be only marginally consistent with the data. Using some properties of the equilibrium TIS models, we construct subsamples of {\em fiducial} equilibrium TIS halos from each of the three subregions, and we study their properties. For these halos, we do find an environmental dependence of their properties, in particular of the spin parameter distribution $P(\lambda)$. We study in more detail the TIS model, and we find new relationships between the truncation radius and other structural parameters. No gravitationally bound halo is found having a radius larger than the critical value for gravithermal instability for TIS halos (\rt $\ge 34.2 r_{0}$, where $r_{0}$ is the core radius of the TIS solution). We do however find a dependence of this relationship on the environment, like for the $P(\lambda)$ statistics. These facts hint at a possible r\^{o}le of tidal fields at determining the statistical properties of halos.Comment: 12 pages, 14 figures. Accepted by MNRAS. Adopted an improved algorithm for halo finding and added a comparison with NFW model

An analysis of electron distributions in galaxy clusters by means of the flux ratio of iron lines FeXXV and XXVI

The interpretation of hard X-ray emission from galaxy clusters is still ambiguous and different models proposed can be probed using various observational methods. Here we explore a new method based on Fe line observations. Spectral line emissivities have usually been calculated for a Maxwellian electron distribution. In this paper a generalized approach to calculate the iron line flux for a modified Maxwellian distribution is considered. We have calculated the flux ratio of iron lines for the various possible populations of electrons that have been proposed to account for measurements of hard X-ray excess emission from the clusters A2199 and Coma. We found that the influence of the suprathermal electron population on the flux ratio is more prominent in low temperature clusters (as Abell 2199) than in high temperature clusters (as Coma).Comment: 6 pages, 3 figures, accepted for publication in A&

Comptonization of the cosmic microwave background by high energy particles residing in AGN cocoons

X-ray cavities and extended radio sources (`cocoons') surrounding active galactic nuclei (AGN) have been detected by the Chandra X-ray mission and radio interferometers. A joint analysis of X-ray and radio maps suggests that pressure values of non-thermal radio-emitting particles derived from the radio maps are not sufficient to inflate the X-ray cavities. We propose using the Sunyaev-Zel'dovich (SZ) effect, whose intensity strongly depends on the pressure, to find the hitherto undetected, dynamically-dominant component in the radio cocoons. We demonstrate that the spectral function at a frequency of 217 GHz has an absolute maximum at a temperature higher than $10^9$ K, therefore the measurement of the SZ effect at this frequency is a powerful tool for potentially revealing the dynamically-dominant component inside AGN jet-driven radio cocoons. A new method is proposed for excluding the contribution from the low energy, non-relativistic electrons to the SZ effect by means of observations at two frequencies. We show how one may correct for a possible contribution from the kinematic SZ effect. The intensity maps of the SZ effect are calculated for the self-similar Sedov solution, and application of a predicted ring-like structure on the SZ map at a frequency of 217 GHz is proposed to determine the energy released during the active jet stage. The SZ intensity map for an AGN cocoon in a distant elliptical is calculated using a 2-D numerical simulation and including relativistic corrections to the SZ effect. We show the intensity spectrum of the SZ effect is flat at high frequencies if gas temperature is as high as $k_\mathrm{b} T_{\mathrm{e}}=500$ keV.Comment: 12 pages, 15 figures, accepted for publication in Astronomy and Astrophysic

An X-ray and optical study of the cluster A33

We report the first detailed X-ray and optical observations of the medium-distant cluster A33 obtained with the Beppo-SAX satellite and with the UH 2.2m and Keck II telescopes at Mauna Kea. The information deduced from X-ray and optical imaging and spectroscopic data allowed us to identify the X-ray source 1SAXJ0027.2-1930 as the X-ray counterpart of the A33 cluster. The faint, F_{2-10 keV} \approx 2.4 \times 10^{-13} \ergscm2, X-ray source 1SAXJ0027.2-1930, $\sim 2$ arcmin away from the optical position of the cluster as given in the Abell catalogue, is identified with the central region of A33. Based on six cluster galaxy redshifts, we determine the redshift of A33, $z=0.2409$; this is lower than the value derived by Leir and Van Den Bergh (1977). The source X-ray luminosity, L_{2-10 keV} = 7.7 \times 10^{43} \ergs, and intracluster gas temperature, $T = 2.9$ keV, make this cluster interesting for cosmological studies of the cluster $L_X-T$ relation at intermediate redshifts. Two other X-ray sources in the A33 field are identified. An AGN at z$=$0.2274, and an M-type star, whose emission are blended to form an extended X-ray emission $\sim 4$ arcmin north of the A33 cluster. A third possibly point-like X-ray source detected $\sim 3$ arcmin north-west of A33 lies close to a spiral galaxy at z$=$0.2863 and to an elliptical galaxy at the same redshift as the cluster.Comment: 9 pages, 6 Figures, Latex (using psfig,l-aa), to appear in Astronomy and Astrophysics S. (To get better quality copies of Figs.1-3 send an email to: [email protected]). A&AS, in pres

Unravelling the origin of large-scale magnetic fields in galaxy clusters and beyond through Faraday Rotation Measures with the SKA

We investigate the possibility for the SKA to detect and study the magnetic fields in galaxy clusters and in the less dense environments surrounding them using Faraday Rotation Measures. To this end, we produce 3-dimensional magnetic field models for galaxy clusters of different masses and in different stages of their evolution, and derive mock rotation measure observations of background radiogalaxies. According to our results, already in phase I, we will be able to infer the magnetic field properties in galaxy clusters as a function of the cluster mass, down to $10^{13}$ solar-masses. Moreover, using cosmological simulations to model the gas density, we have computed the expected rotation measure through shock-fronts that occur in the intra-cluster medium during cluster mergers. The enhancement in the rotation measure due to the density jump will permit to constraint the magnetic field strength and structure after the shock passage. SKA observations of polarised sources located behind galaxy clusters will answer several questions about the magnetic field strength and structure in galaxy clusters, and its evolution with cosmic time.Comment: 9 pages, 4 Figures, to appear as part of 'Cosmic Magnetism' in Proceedings 'Advancing Astrophysics with the SKA (AASKA14)', PoS(AASKA14

Comparison of the Efficacy of Brolucizumab with Natural Disease Progression in Wet AMD Using Clinical Data from the Phase III HAWK and HARRIER Trials and Modelled Placebo Data

Aim: To compare the treatment effect of brolucizumab, a novel anti-vascular endothelial growth factor therapeutic, with a putative placebo in patients with wet age-related macular degeneration. Materials and Methods: Clinical treatment-effect data from patients receiving brolucizumab 6 mg in the HAWK and HARRIER studies were compared with modelled placebo data using a previously developed and validated indirect response, non-linear, mixed effects model describing the natural visual acuity decline in wet age-related macular degeneration. The placebo model incorporated patient-level data from the sham injection arms of the MARINA and PIER studies, corrected for baseline best corrected visual acuity and age difference between these studies and the HAWK and HARRIER studies. Results: Compared with a modelled placebo, brolucizumab treatment was associated with an overall best corrected visual acuity gain of approximately 22 Early Treatment Diabetic Retinopathy Study letters at Week 48 and 28 letters at Week 96. Conclusions: As anti-vascular endothelial growth factor therapy is now a standard of care for wet age-related macular degeneration, it is not feasible to conduct placebo-controlled trials for new wet age-related macular degeneration treatments. By allowing comparison with the natural decline in visual acuity without treatment, this analysis conveys the clinical importance of brolucizumab for the treatment of wet age-related macular degeneration

Cluster magnetic fields through the study of polarized radio halos in the SKA era

Galaxy clusters are unique laboratories to investigate turbulent fluid motions and large scale magnetic fields. Synchrotron radio halos at the center of merging galaxy clusters provide the most spectacular and direct evidence of the presence of relativistic particles and magnetic fields associated with the intracluster medium. The study of polarized emission from radio halos is extremely important to constrain the properties of intracluster magnetic fields and the physics of the acceleration and transport of the relativistic particles. However, detecting this polarized signal is a very hard task with the current radio facilities.We use cosmological magneto-hydrodynamical simulations to predict the expected polarized surface brightness of radio halos at 1.4 GHz. We compare these expectations with the sensitivity and the resolution reachable with the SKA1. This allows us to evaluate the potential for studying intracluster magnetic fields in the surveys planned for SKA1.Comment: 11 pages, 4 figures; to appear as part of 'Cosmic Magnetism' in Proceedings 'Advancing Astrophysics with the SKA (AASKA14)', PoS(AASKA14)10