On the Discrepancy between Theoretical and X-Ray Concentration-Mass Relations for Galaxy Clusters

[Abridged] In the past 15 years, the concentration-mass relation has been investigated diffusely in theoretical studies. On the other hand, only recently has this relation been derived from X-ray observations. When that happened, the results caused a certain level of concern: the X-ray normalizations and slopes were found significantly dissimilar from those predicted by theory. We analyzed 52 objects, simulated each time with different physical recipes for the baryonic component, as well as 60 synthetic X-ray images, to determine if these discrepancies are real or artificial. In particular, we investigate how the simulated concentration-mass relation depends (1) on the radial range used to derive the concentration, (2) on the presence of baryons in the simulations, and on the prescription used to reproduce the gas. Finally, we evaluate (3) how the results differ when adopting an X-ray approach for the analysis and (4) how the selection functions based on X-ray luminosity can impact the results. All effects studied go in the direction of alleviating the discrepancy between observations and simulations, although with different significance: while the fitting radial range and the baryonic component play only a minor role, the X-ray approach and selection function have profound repercussion on the resulting concentration-mass relation.Comment: 15 pages, 11 figures, 3 tables, ApJ in press. Significant extension of the study of the selection-function influence and more attentive treatment of errors (results unchanged

A New Interpretation of the Mass-Temperature Relation and Mass Calibration of Galaxy Clusters Based on the Fundamental Plane

Observations and numerical simulations have shown that the relation between the mass scaled with the critical density of the universe and the X-ray temperature of galaxy clusters is approximately represented by $M_\Delta \propto T_X^{3/2}$ (e.g. $\Delta=500$). This relation is often interpreted as evidence that clusters are in virial equilibrium. However, the recently discovered fundamental plane (FP) of clusters indicates that the temperature of clusters primarily depends on a combination of the characteristic mass $M_s$ and radius $r_s$ of the Navarro-Frenk-White profile rather than $M_\Delta$. Moreover, the angle of the FP revealed that clusters are not in virial equilibrium because of continuous mass accretion from the surrounding matter. By considering both the FP and the mass dependence of the cluster concentration parameter, we show that this paradox can be solved and the relation $M_\Delta \propto T_X^{3/2}$ actually reflects the central structure of clusters. We also find that the intrinsic scatter in the halo concentration-mass relation can largely account for the spread of clusters on the FP. We also show that X-ray data alone form the FP and the angle and the position are consistent with those of the FP constructed from gravitational lensing data. We demonstrate that a possible shift between the two FPs can be used to calibrate cluster masses obtained via X-ray observations.Comment: Published on ApJ. Matched to published versio

On the coherent rotation of diffuse matter in numerical simulations of galaxy clusters

We present a study on the coherent rotation of the intracluster medium and dark matter components of simulated galaxy clusters extracted from a volume-limited sample of the MUSIC project. The set is re-simulated with three different recipes for the gas physics: $(i)$ non-radiative, $(ii)$ radiative without AGN feedback, and $(iii)$ radiative with AGN feedback. Our analysis is based on the 146 most massive clusters identified as relaxed, 57 per cent of the total sample. We classify these objects as rotating and non-rotating according to the gas spin parameter, a quantity that can be related to cluster observations. We find that 4 per cent of the relaxed sample is rotating according to our criterion. By looking at the radial profiles of their specific angular momentum vector, we find that the solid body model is not a suitable description of rotational motions. The radial profiles of the velocity of the dark matter show a prevalence of the random velocity dispersion. Instead, the intracluster medium profiles are characterized by a comparable contribution from the tangential velocity and the dispersion. In general, the dark matter component dominates the dynamics of the clusters, as suggested by the correlation between its angular momentum and the gas one, and by the lack of relevant differences among the three sets of simulations.Comment: 12 pages, updated to match the MNRAS versio

Discovery of a new fundamental plane dictating galaxy cluster evolution from gravitational lensing

In cold dark matter (CDM) cosmology, objects in the Universe have grown under the effect of gravity of dark matter. The intracluster gas in a galaxy cluster was heated when the dark-matter halo formed through gravitational collapse. The potential energy of the gas was converted to thermal energy through this process. However, this process and the thermodynamic history of the gas have not been clearly characterized in connection with with the formation and evolution of the internal structure of dark-matter halos. Here, we show that observational CLASH data of high-mass galaxy clusters lie on a plane in the three-dimensional logarithmic space of their characteristic radius $r_s$, mass $M_s$, and X-ray temperature $T_X$ with a very small orthogonal scatter. The tight correlation indicates that the gas temperature was determined at a specific cluster formation time, which is encoded in $r_s$ and $M_s$. The plane is tilted with respect to $T_X \propto M_s/r_s$, which is the plane expected in case of simplified virial equilibrium. We show that this tilt can be explained by a similarity solution, which indicates that clusters are not isolated but continuously growing through matter accretion from their outer environments. Numerical simulations reproduce the observed plane and its angle. This result holds independently of the gas physics implemented in the code, revealing the fundamental origin of this plane.Comment: Replaced with a revised version to match the ApJ accepted versio

Thermodynamic Profiles of Galaxy Clusters from a Joint X-ray/SZ Analysis

We jointly analyze Bolocam Sunyaev-Zeldovich (SZ) effect and Chandra X-ray data for a set of 45 clusters to derive gas density and temperature profiles without using spectroscopic information. The sample spans the mass and redshift range $3 \times 10^{14} M_{\odot} \le M_{500} \le 25 \times 10^{14} M_{\odot}$ and $0.15\le z \le 0.89$. We define cool-core (CC) and non-cool core (NCC) subsamples based on the central X-ray luminosity, and 17/45 clusters are classified as CC. In general, the profiles derived from our analysis are found to be in good agreement with previous analyses, and profile constraints beyond $r_{500}$ are obtained for 34/45 clusters. In approximately 30% of the CC clusters our analysis shows a central temperature drop with a statistical significance of $>3\sigma$; this modest detection fraction is due mainly to a combination of coarse angular resolution and modest S/N in the SZ data. Most clusters are consistent with an isothermal profile at the largest radii near $r_{500}$, although 9/45 show a significant temperature decrease with increasing radius. The sample mean density profile is in good agreement with previous studies, and shows a minimum intrinsic scatter of approximately 10% near $0.5 \times r_{500}$. The sample mean temperature profile is consistent with isothermal, and has an intrinsic scatter of approximately 50% independent of radius. This scatter is significantly higher compared to earlier X-ray-only studies, which find intrinsic scatters near 10%, likely due to a combination of unaccounted for non-idealities in the SZ noise, projection effects, and sample selection.Comment: 42 pages, 52 figure

Halo Concentrations and the Fundamental Plane of Galaxy Clusters

According to the standard cold dark matter (CDM) cosmology, the structure of dark halos including those of galaxy clusters reflects their mass accretion history. Older clusters tend to be more concentrated than younger clusters. Their structure, represented by the characteristic radius $r_s$ and mass $M_s$ of the Navarro--Frenk--White (NFW) density profile, is related to their formation time. In~this study, we showed that $r_s$, $M_s$, and the X-ray temperature of the intracluster medium (ICM), $T_X$, form a thin plane in the space of $(\log r_s, \log M_s, \log T_X)$. This tight correlation indicates that the ICM temperature is also determined by the formation time of individual clusters. Numerical simulations showed that clusters move along the fundamental plane as they evolve. The plane and the cluster evolution within the plane could be explained by a similarity solution of structure formation of the universe. The angle of the plane shows that clusters have not achieved "virial equilibrium" in the sense that mass/size growth and pressure at the boundaries cannot be ignored. The distribution of clusters on the plane was related to the intrinsic scatter in the halo concentration--mass relation, which originated from the variety of cluster ages. The well-known mass--temperature relation of clusters ($M_\Delta\propto T_X^{3/2}$) can be explained by the fundamental plane and the mass dependence of the halo concentration without the assumption of virial equilibrium. The fundamental plane could also be used for calibration of cluster masses.Comment: Invited review article, to be published in "From Dark Haloes to Visible Galaxies", special issue of Galaxie

Scaling Rrelation in two situations of extreme mergers

Clusters of galaxies are known to be dynamically active systems, yet X-ray studies of the low redshift population exhibit tight scaling laws. In this work, we extend previous studies of this apparent paradox using numerical simulations of two extreme merger cases, one is a high Mach number (above 2.5) satellite merger similar to the "bullet cluster" and the other a merger of nearly equal mass progenitors. Creating X-ray images densely sampled in time, we construct TX, Mgas, and YX measures within R500 and compare to the calibrations of Kravtsov et al. (2006). We find that these extreme merger cases respect the scaling relations, for both intrinsic measures and for measures derived from appropriately masked, synthetic Chandra X-ray images. The masking procedure plays a critical role in the X-ray temperature calculation while it is irrelevant in the X-ray gas mass derivation. Mis-centering up to 100 kpc does not influence the result. The observationally determined radius R500 might conduce to systematic shifts in Mgas, and YX which increase the total mass scatter.Comment: 9 pages, 7 figures, accepted in Ap

Morphological estimators on Sunyaev-Zel'dovich maps of MUSIC clusters of galaxies

The determination of the morphology of galaxy clusters has important repercussions for cosmological and astrophysical studies of them. In this paper, we address the morphological characterization of synthetic maps of the Sunyaev-Zel'dovich (SZ) effect for a sample of 258 massive clusters (Mvir> 5×1014h-1M⊙at z=0), extracted from theMUSIC hydrodynamical simulations. Specifically, we use five known morphological parameters (which are already used in X-ray) and two newly introduced ones, and we combine them in a single parameter. We analyse two sets of simulations obtained with different prescriptions of the gas physics (non-radiative and with cooling, star formation and stellar feedback) at four red shifts between 0.43 and 0.82. For each parameter, we test its stability and efficiency in discriminating the true cluster dynamical state, measured by theoretical indicators. The combined parameter is more efficient at discriminating between relaxed and disturbed clusters. This parameter had a mild correlation with the hydrostatic mass (~0.3) and a strong correlation (~0.8) with the offset between the SZ centroid and the cluster centre of mass. The latter quantity is, thus, the most accessible and efficient indicator of the dynamical state for SZ studiesThis work has been partially supported by funding from Sapienza University of Rome - Progetti di Ricerca Anno 2015 prot. C26A15LXNR. GY and FS acknowledge financial support from MINECO/FEDER under research grant AYA2015-63810-P. ER acknowledge financial contribution from the agreement ASI-INAF n 2017-14-H.