Mass-Richness relations for X-ray and SZE-selected clusters at 0.4<z<2.00.4 < z <2.0 as seen by SpitzerSpitzer at 4.5μ\mum

We study the mass-richness relation of 116 spectroscopically-confirmed massive clusters at $0.4 < z < 2$ by mining the $Spitzer$ archive. We homogeneously measure the richness at 4.5$\mu$m for our cluster sample within a fixed aperture of $2^{\prime}$ radius and above a fixed brightness threshold, making appropriate corrections for both background galaxies and foreground stars. We have two subsamples, those which have a) literature X-ray luminosities and b) literature Sunyaev-Zeldovich effect masses. For the X-ray subsample we re-derive masses adopting the most recent calibrations. We then calibrate an empirical mass-richness relation for the combined sample spanning more than one decade in cluster mass and find the associated uncertainties in mass at fixed richness to be $\pm 0.25$ dex. We study the dependance of the scatter of this relation with galaxy concentration, defined as the ratio between richness measured within an aperture radius of 1 and 2 arcminutes. We find that at fixed aperture radius the scatter increases for clusters with higher concentrations. We study the dependance of our richness estimates with depth of the [4.5]$\mu$m imaging data and find that reaching a depth of at least [4.5]= 21 AB mag is sufficient to derive reasonable mass estimates. We discuss the possible extension of our method to the mid-infrared $WISE$ all-sky survey data, and the application of our results to the $Euclid$ mission. This technique makes richness-based cluster mass estimates available for large samples of clusters at very low observational cost.Comment: Submitted to ApJ on Aug 31 2016, Revised version resubmitted on Apr 11th 201

A deep Chandra observation of the cluster environment of the z=1.786 radio galaxy 3C294

We report the results from a 200 ks Chandra observation of the z=1.786 radio galaxy 3C294 and its cluster environment, increasing by tenfold our earlier observation. The diffuse emission, extending about 100 kpc around the nucleus, has a roughly hourglass shape in the N-S direction with surprisingly sharp edges to the N and S. The spectrum of the diffuse emission is well fitted by either a thermal model of temperature 3.5 keV and abundance <0.9 solar (2-sigma), or a power-law with photon index 2.3. If the emission is due to hot gas then the sharp edges mean that it is probably not in hydrostatic equilibrium. Much of the emission is plausibly due to inverse Compton scattering of the Cosmic Microwave Background (CMB) by nonthermal electrons produced earlier by the radio source. The required relativistic electrons would be of much lower energy and older than those responsible for the present radio lobes. This could account for the lack of detailed spatial correspondence between the X-rays and the radio emission, the axis of which is at a position angle of about 45 deg. Hot gas would still be required to confine the relativistic plasma; the situation could parallel that of the radio bubbles seen as holes in nearby clusters, except that in 3C294 the bubbles are bright in X-rays owing to the extreme power in the source and the sixty fold increase in the energy density of the CMB. The X-ray spectrum of the radio nucleus is hard, showing a reflection spectrum and iron line. The source is therefore an obscured radio-loud quasar.Comment: In press (MNRAS), 10 pages, 12 figures (2 colour

On the evolution of cooling cores in X-ray galaxy clusters

(Abridged) To define a framework for the formation and evolution of the cooling cores in X-ray galaxy clusters, we study how the physical properties change as function of the cosmic time in the inner regions of a 4 keV and 8 keV galaxy cluster under the action of radiative cooling and gravity only. The cooling radius, R_cool, defined as the radius at which the cooling time equals the Universe age at given redshift, evolves from ~0.01 R200 at z>2, where the structures begin their evolution, to ~0.05 R200 at z=0. The values measured at 0.01 R200 show an increase of about 15-20 per cent per Gyr in the gas density and surface brightness and a decrease with a mean rate of 10 per cent per Gyr in the gas temperature. The emission-weighted temperature diminishes by about 25 per cent and the bolometric X-ray luminosity rises by a factor ~2 after 10 Gyrs when all the cluster emission is considered in the computation. On the contrary, when the core region within 0.15 R500 is excluded, the gas temperature value does not change and the X-ray luminosity varies by 10-20 per cent only. The cooling time and gas entropy radial profiles are well represented by power-law functions. The behaviour of the inner slopes of the gas temperature and density profiles are the most sensitive and unambiguous tracers of an evolving cooling core. Their values after 10 Gyrs of radiative losses, T_gas ~ r^0.4 and n_gas ~ r^(-1.2) for the hot (cool) object, are remarkably in agreement with the observational constraints available for nearby X-ray luminous cooling core clusters. Because our simulations do not consider any AGN heating, they imply that the feedback process does not greatly alter the gas density and temperature profiles as generated by radiative cooling alone.Comment: 8 pages. MNRAS in pres

Seeing Galaxies Through Thick & Thin. III. HST Imaging of the Dust in Backlit Spiral Galaxies

We present analysis of WFPC2 imaging of two spiral galaxies partially backlit by E/S0 systems in the pairs AM1316-241 and AM0500-620, and the spiral foreground system in NGC 1275. Images in B and I are used to determine the reddening curve of in these systems. The spiral component of AM1316-241 shows dust strongly concentrated in discrete arms, with a reddening law very close to the Milky Way mean. The dust distribution is scale-free between about 100 pc and the arm scale. The spiral in AM0500-620 shows dust concentrated in arms and interarm spurs, with measurable interarm extinction as well. Although its dust properties are less well-determined, we find evidence for a steeper extinction law here. The shape of the reddening law suggests that, at least in AM1316-241, we have resolved most of the dust structure. In AM0500-620, the slope of the fractal perimeter-scale relation steepens systematically from low to high extinction. In AM1316-241, we cannot determine a unique fractal dimension from the defining area-perimeter relation, so the projected dust distribution is best defined as fractal-like. In neither galaxy do we see regions even on single-pixel scales in spiral arms with AB > 2.5. The measurements in NGC 1275 are compromised by our lack of independent knowledge of the foreground system's light distribution, but masked sampling of the absorption suggests an effective reddening curve much flatter than the Milky Way mean (perhaps indicating that the foreground system has been affected by immersion in the hot intracluster gas).Comment: Astronomical Journal, in press. 13 figures. Full-size PostScript figures available at http://www.astr.ua.edu/preprints/kee

Weighing simulated galaxy clusters using lensing and X-ray

We aim at investigating potential biases in lensing and X-ray methods to measure the cluster mass profiles. We do so by performing realistic simulations of lensing and X-ray observations that are subsequently analyzed using observational techniques. The resulting mass estimates are compared among them and with the input models. Three clusters obtained from state-of-the-art hydrodynamical simulations, each of which has been projected along three independent lines-of-sight, are used for this analysis. We find that strong lensing models can be trusted over a limited region around the cluster core. Extrapolating the strong lensing mass models to outside the Einstein ring can lead to significant biases in the mass estimates, if the BCG is not modeled properly for example. Weak lensing mass measurements can be largely affected by substructures, depending on the method implemented to convert the shear into a mass estimate. Using non-parametric methods which combine weak and strong lensing data, the projected masses within R200 can be constrained with a precision of ~10%. De-projection of lensing masses increases the scatter around the true masses by more than a factor of two due to cluster triaxiality. X-ray mass measurements have much smaller scatter (about a factor of two smaller than the lensing masses) but they are generally biased low by 5-20%. This bias is ascribable to bulk motions in the gas of our simulated clusters. Using the lensing and the X-ray masses as proxies for the true and the hydrostatic equilibrium masses of the simulated clusters and averaging over the cluster sample we are able to measure the lack of hydrostatic equilibrium in the systems we have investigated.Comment: 27 pages, 21 figures, accepted for publication on A&A. Version with full resolution images can be found at http://pico.bo.astro.it/~massimo/Public/Papers/massComp.pd

On a novel approach using massive clusters at high redshifts as cosmological probe

In this work we propose a novel method for testing the validity of the fiducial LCDM cosmology by measuring the cumulative distribution function of the most massive haloes in a sample of subvolumes of identical size tiled on the sky at a fixed redshift. The fact that the most massive clusters probe the high-mass tail of the mass function, where the difference between LCDM and alternative cosmological models is strongest, makes our method particularly interesting as a cosmological probe. We utilise general extreme value statistics (GEV) to obtain a cumulative distribution function of the most massive objects in a given volume. We sample this distribution function according to the number of patches covered by the survey area for a range of different "test cosmologies" and for differently accurate mass estimations of the haloes. By fitting this sample with the GEV distribution function, we can study which parameters are the most sensitive with respect to the test cosmologies. We find that the peak of the probability distribution function of the most massive halo is well suited to test the validity of the fiducial LCDM model, once we are able to establish a sufficiently complete large-area survey with M_lim=10^14.5 M_sun/h (M_lim=10^14 M_sun/h) at redshifts above z=1 (z=1.5). Being of cumulative nature the proposed measure is robust and an accuracy of 20-30% in the cluster masses would be sufficient to test for alternative models. Since one only needs the most massive system in each angular patch, this method would be ideally suited as a first fast consistency check before going into a more complex statistical analysis of the observed halo sample.Comment: 11 pages, 13 figures, 1 Table, MNRAS accepted versio

Sunyaev-Zel'dovich effect in the Virgo cluster from WMAP and ROSAT data

WMAP observations at mm wavelengths are sensitive to the Sunyaev-Zel'dovich effect in galaxy clusters. Among all the objects in the sky, the Virgo cluster is expected to provide the largest integrated signal. Based on models compatible with the X-ray emission observed in the ROSAT All Sky Survey, we predict a two-sigma detection of the SZ effect from Virgo in the WMAP 3-year data. Our analysis reveals a 3-sigma signal on scales of 5 degrees, although the frequency dependence deviates from the theoretical expectation for the SZ effect. The main sources of uncertainty are instrumental noise, and most importantly, possible contamination from point sources and diffuse back/foregrounds. In particular, a population of unresolved extragalactic sources in Virgo would explain the observed intensity and frequency dependence. In order to resolve this question one needs to wait for experiments like Planck to achieve the required accuracy.Comment: 11 pages. 10 figures. Submitted to MNRA

Enrichment of the hot intracluster medium: observations

Four decades ago, the firm detection of an Fe-K emission feature in the X-ray spectrum of the Perseus cluster revealed the presence of iron in its hot intracluster medium (ICM). With more advanced missions successfully launched over the last 20 years, this discovery has been extended to many other metals and to the hot atmospheres of many other galaxy clusters, groups, and giant elliptical galaxies, as evidence that the elemental bricks of life - synthesized by stars and supernovae - are also found at the largest scales of the Universe. Because the ICM, emitting in X-rays, is in collisional ionisation equilibrium, its elemental abundances can in principle be accurately measured. These abundance measurements, in turn, are valuable to constrain the physics and environmental conditions of the Type Ia and core-collapse supernovae that exploded and enriched the ICM over the entire cluster volume. On the other hand, the spatial distribution of metals across the ICM constitutes a remarkable signature of the chemical history and evolution of clusters, groups, and ellipticals. Here, we summarise the most significant achievements in measuring elemental abundances in the ICM, from the very first attempts up to the era of XMM-Newton, Chandra, and Suzaku and the unprecedented results obtained by Hitomi. We also discuss the current systematic limitations of these measurements and how the future missions XRISM and Athena will further improve our current knowledge of the ICM enrichment.Comment: 49 pages. Review paper. Accepted for publication on Space Science Reviews. This is the companion review of "Enrichment of the hot intracluster medium: numerical simulations

Compact Lyman-alpha Emitting Candidates at z~2.4 in Deep Medium-band HST WFPC2 Images

Medium-band imaging with HST/WFPC2 in the F410M filter has previously revealed a population of compact Lyman-alpha emission objects around the radio galaxy 53W002 at z~2.4. We report detections of similar objects at z~2.4 in random, high-latitude HST parallel observations of three additional fields, lending support to the idea that they constitute a widespread population at these redshifts. The three new fields contain 18 Lyman-alpha candidates, in contrast to the 17 detected in the deeper exposure of the single WFPC2 field around 53W002. We find substantial differences in the number of candidates from field to field, suggesting that significant large-scale structure is already present in the galaxy distribution at this cosmic epoch. The likely existence of z~2.4 sub-galactic clumps in several random fields shows that these objects may have been common in the early universe and strengthens the argument that such objects may be responsible for the formation of a fraction of the luminous present-day galaxies through hierarchical merging.Comment: Uses slightly modified AASTeX preprint style file (included). Contains 22 pages, including 5 figures and 2 tables. Accepted for the December issue of the Astronomical Journa

X-ray total mass estimate for the nearby relaxed cluster A3571

We constrain the total mass distribution in the cluster A3571, combining spatially resolved ASCA temperature data with ROSAT imaging data with the assumption that the cluster is in hydrostatic equilibrium. The total mass within r_500 (1.7/h_50 Mpc) is M_500 = 7.8[+1.4,-2.2] 10^14/ h_50 Msun at 90% confidence, 1.1 times smaller than the isothermal estimate. The Navarro, Frenk & White ``universal profile'' is a good description of the dark matter density distribution in A3571. The gas density profile is shallower than the dark matter profile, scaling as r^{-2.1} at large radii, leading to a monotonically increasing gas mass fraction with radius. Within r_500 the gas mass fraction reaches a value of f_gas = 0.19[+0.06,-0.03] h_50^{-3/2} (90% confidence errors). Assuming that this value of f_gas is a lower limit for the the universal value of the baryon fraction, we estimate the 90% confidence upper limit of the cosmological matter density to be Omega_m < 0.4.Comment: 10 pages, 4 figures, accepted by Ap