Mauro Sereno

Measuring the Three-dimensional Structure of Galaxy Clusters. I. Application to a Sample of 25 Clusters

We discuss a method to constrain the intrinsic shapes of galaxy clusters by combining X-ray and SunyaevZel’dovich observations. The method is applied to a sample of 25 X-ray–selected clusters, with measured SunyaevZel’dovich temperature decrements. The sample turns out to be slightly biased, with strongly elongated clusters preferentially aligned along the line of sight. This result demonstrates that X-ray–selected cluster samples may be affected by morphological and orientation effects, even if a relatively high threshold signal-to-noise ratio is used to select the sample. A large majority of the clusters in our sample exhibit a marked triaxial structure; the spherical hypothesis is strongly rejected for most sample members. Cooling-flow clusters do not show preferentially regular morphologies. We also show that identification of multiple gravitationally lensed images, together with measurementsoftheSunyaev-Zel’dovicheffectandX-raysurfacebrightness,canprovideasimultaneousdeterminationofthe three-dimensional structure of a cluster, of the Hubble constant, and of the cosmological energy density parameters. Subject headingg cosmic microwave background — cosmology: observations — distance scale — galaxies: clusters: general — gravitational lensing — X-rays: galaxies: clusters

The XXL Survey I. Scientific motivations - XMM-Newton observing plan - Follow-up observations and simulation programme

Context. The quest for the cosmological parameters that describe our universe continues to motivate the scientific community to undertake very large survey initiatives across the electromagnetic spectrum. Over the past two decades, the Chandra and XMM-Newton observatories have supported numerous studies of X-ray-selected clusters of galaxies, active galactic nuclei (AGNs), and the X-ray background. The present paper is the first in a series reporting results of the XXL-XMM survey; it comes at a time when the Planck mission results are being finalised. Aims. We present the XXL Survey, the largest XMM programme totaling some 6.9 Ms to date and involving an international consortium of roughly 100 members. The XXL Survey covers two extragalactic areas of 25 deg(2) each at a point-source sensitivity of similar to 5 x 10(-15) erg s(-1) cm(-2) in the [0.5-2] keV band (completeness limit). The surveys main goals are to provide constraints on the dark energy equation of state from the space-time distribution of clusters of galaxies and to serve as a pathfinder for future, wide-area X-ray missions. We review science objectives, including cluster studies, AGN evolution, and large-scale structure, that are being conducted with the support of approximately 30 follow-up programmes. Methods. We describe the 542 XMM observations along with the associated multi-lambda and numerical simulation programmes. We give a detailed account of the X-ray processing steps and describe innovative tools being developed for the cosmological analysis. Results. The paper provides a thorough evaluation of the X-ray data, including quality controls, photon statistics, exposure and background maps, and sky coverage. Source catalogue construction and multi-lambda associations are briefly described. This material will be the basis for the calculation of the cluster and AGN selection functions, critical elements of the cosmological and science analyses. Conclusions. The XXL multi-lambda data set will have a unique lasting legacy value for cosmological and extragalactic studies and will serve as a calibration resource for future dark energy studies with clusters and other X-ray selected sources. With the present article, we release the XMM XXL photon and smoothed images along with the corresponding exposure maps.

A comparison between star formation rate diagnostics and rate of core collapse supernovae within 11 Mpc

Aims. The core collapse supernova rate provides a strong lower limit for the star formation rate (SFR). Progress in using it as a cosmic SFR tracer requires some confidence that it is consistent with more conventional SFR diagnostics in the nearby Universe. This paper compares standard SFR measurements based on Hα, far ultraviolet (FUV) and total infrared (TIR) galaxy luminosities with the observed core collapse supernova rate in the same galaxy sample. The comparison can be viewed from two perspectives. Firstly, by adopting an estimate of the minimum stellar mass to produce a core collapse supernova one can determine a SFR from supernova numbers. Secondly, the radiative SFR can be assumed to be robust and then the supernova statistics provide a constrain on the minimum stellar mass for core collapse supernova progenitors. Methods. The novel aspect of this study is that Hα, FUV and TIR luminosities are now available for a complete galaxy sample within the local 11 Mpc volume and the number of discovered supernovae in this sample within the last 13 years is high enough to perform a meaningful statistical comparison. We exploit the multi-wavelength dataset from 11 HUGS, a volume-limited survey designed to provide a census of SFR in the Local Volume. There are 14 supernovae discovered in this sample of galaxies within the last 13 years. Although one could argue that this may not be complete, it is certainly a robust lower limit. Results. Assuming a lower limit for core collapse of 8 M� (as proposed by direct detections of SN progenitor stars and white dwarf progenitors), the core-collapse supernova rate matches the SFR from the FUV luminosity. However, the SFR based on Hα luminosity is lower than these two estimates by a factor of nearly 2. If we assume that the FUV or Hα based luminosities are a true reflection of

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.

Measuring the Three-dimensional Structure of Galaxy Clusters. II. Are Clusters of Galaxies Oblate or Prolate?

The intrinsic shape of galaxy clusters can be obtained through a combination of X-ray and Sunyaev-Zeldovich effect observations once cosmological parameters are assumed to be known. In this paper we discuss the feasibility of modeling galaxy clusters as either prolate or oblate ellipsoids. We analyze the intracluster medium distribution for a sample of 25 X-ray-selected clusters, with measured Sunyaev-Zeldovich temperature decrements. A mixed population of prolate and oblate ellipsoids of revolution fits the data well, with prolate shapes preferred by ~60%-76%. We observe an excess of clusters nearly aligned along the line of sight, with respect to what is expected from a randomly oriented cluster population, which might imply the presence of a selection bias in our sample. We also find signs that a more general triaxial morphology might better describe the morphology of galaxy clusters. Additional constraints from gravitational lensing could disentangle the degeneracy between an ellipsoidal and a triaxial morphology, and could also allow an unbiased determination of the Hubble constant.

Triaxial strong-lensing analysis of the z > 0.5 MACS clusters: the mass–concentration relation

The high concentrations derived for several strong-lensing clusters present a major inconsistency between theoretical Λ cold dark matter (ΛCDM) expectations and measurements. Triaxiality and orientation biases might be at the origin of this disagreement, as clusters elongated along the line of sight would have a relatively higher projected mass density, boosting the resulting lensing properties. Analyses of statistical samples can probe further these effects and crucially reduce biases. In this work we perform a fully triaxial strong-lensing analysis of the 12 Massive Cluster Survey (MACS) clusters at z > 0.5, a complete X-ray-selected sample, and fully account for the impact of the intrinsic 3D shapes on their strong-lensing properties. We first construct strong-lensing mass models for each cluster based on multiple images, and fit projected ellipsoidal Navarro–Frenk–White haloes with arbitrary orientations to each mass distribution. We then invert the measured surface mass densities using Bayesian statistics. Although the Einstein radii of this sample are significantly larger than those predicted by ΛCDM, here we find that the mass–concentration relation is in full agreement with results from N-body simulations. The z > 0.5 MACS clusters suffer from a moderate form of the orientation bias as may be expected for X-ray-selected samples. Being mostly unrelaxed, at a relatively high redshift, with high X-ray luminosity and notable substructures, these clusters may lie outside the standard concentration–Einstein radius relation. Our results remark the importance of triaxiality and properly selected samples for understanding galaxy clusters properties and suggest that for higher-z, unrelaxed low-concentration clusters form a different class of prominent strong gravitational lenses. Arc redshift confirmation and weak-lensing data in the outer region are needed to further refine our analysis.

Microlensing search towards M31

We present the first results of the analysis of data collected during the 1998-99 observational campaign at the 1.3 meter McGraw-Hill Telescope, towards the Andromeda galaxy (M 31), aimed to detect gravitational microlensing effects as a probe for the presence of dark matter in our Galaxy and in the M 31 halo. The analysis is performed using the pixel lensing technique, which consists of the study of flux variations of unresolved sources and has been proposed and implemented by the AGAPE collaboration. We carry out a shape analysis by demanding that the detected flux variations be achromatic and compatible with a Paczynski light curve. We apply the Durbin-Watson hypothesis test to the residuals. Furthermore, we consider the background of variables sources. Finally five candidate microlensing events emerge from our selection. Comparing with the predictions of a Monte Carlo simulation, assuming a standard spherical model for the M 31 and Galactic haloes, and typical values for the MACHO mass, we find that our events are only marginally consistent with the distribution of observable parameters predicted by the simulation.

MEASUREMENT OF THE HALO BIAS FROM STACKED SHEAR PROFILES OF GALAXY CLUSTERS

We present observational evidence of the two-halo term in the stacked shear profile of a sample of ~1200 optically selected galaxy clusters based on imaging data and the public shear catalog from the CFHTLenS. We find that the halo bias, a measure of the correlated distribution of matter around galaxy clusters, has amplitude and correlation with galaxy cluster mass in very good agreement with the predictions based on the LCDM standard cosmological model. The mass-concentration relation is flat but higher than theoretical predictions. We also confirm the close scaling relation between the optical richness of galaxy clusters and their mass.

THREE-DIMENSIONAL MULTI-PROBE ANALYSIS OF THE GALAXY CLUSTER A1689*

The work is partially supported by the Ministry of Science and Technology of Taiwan under the grant MOST 103-2112-M-001-030-MY3. M. S. acknowledges financial contributions from contracts ASI/INAF I/023/ 12/0, by the PRIN MIUR 2010–2011 “The dark universe and the cosmic evolution of baryons: from current surveys to Euclid” and by the PRIN INAF 2012 “The universe in the box: multiscale simulations of cosmic structure.” M. N. acknowledges financial support from PRIN INAF 2014. J. M. D. acknowledges support of the consolider project CSD2010-00064 and AYA2012-39475-C02-01 funded by the Ministerio de Economia y Competitividad. N. O. is supported by a Grant-in-Aid from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (26800097). This work was partially supported by “World Premier International Research Center Initiative (WPI Initiative)” and the Funds for the Development of Human Resources in Science and Technology under MEXT, Japan. This research was performed while T. M. held a National Research Council Research Associateship Award at the Naval Research Laboratory (NRL). We thank John Carlstrom, Megan Gralla, Marshall Joy, Dan Marrone, and the entire SZA and OVRO/BIMA teams for providing the SZA and OVRO/BIMA data used in this study. Support for the SZA observations presented in this work was provided by NSF through award AST-0838187 and PHY-0114422 at the University of Chicago. The OVRO and BIMA observations presented here were supported by National Science Foundation grants AST 99-81546 and 02-28963.

Modelling the Milky Way through adiabatic compression of cold dark matter haloes

We use the adiabatic compression theory to build a physically well-motivated Milky Way mass model in agree- ment with observational data. The visible mass of the Galaxy is distributed in a spheroidal bulge and a multi-component disc parametrized by three galactic parameters, the Sun distance to the galactic centre, R0, the total bulge mass, Mbulge ,a nd the local disc surface density, Σ� . To model the dark matter component, we adiabatically compress a Navarro, Frenk and White (NFW) halo (with concentration c and total mass Mvir) for fixed values of the spin parameter λ, the fraction of the mass in baryons mb, and the thin disc contribution to total angular momentum jd. An iterative selection procedure is used to explore in detail the wide space of parameters only selecting those combinations of R0, Mbulge, Σ� ,λ ,mb, jd, c, Mvir that give rise to a Milky Way model in agreement with observational constraints. This analysis leads us to conclude that only models with R0 = 8.5 kpc, 0.8 × 10 10 M� < Mbulge < 1.6 × 10 10 Mand 49 Mpc −2 ≤ Σ� ≤ 56 Mpc −2 can be reconciled with the set of observational constraints. As regards the parameters entering the adiabatic compression, we find 0.03 ≤ λ ≤ 0.10 and 0.04 ≤ mb ≤ 0.10, while final estimates of the parameters describing the initial halo profile turn out to be 5 < c < 12 and 7 × 10 11 M� < Mvir < 17 × 10 11 M� (all at 95.7% CL).