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This document is an erratum to a previously published paper on weak lensing of cluster satellite galaxies. The erratum explains that there was a bug in the codes used for data analysis, resulting in underestimation of errorbars in some figures. The affected figures and tables are presented in the erratum, along with the updated values. The authors state that these changes do not impact their conclusions and qualitative inferences. The document includes tables presenting data on various parameters for different ranges, such as R_sat, M_clu, M_sat, M_bary, sigma_mc, and f_orp. The tables also compare constraints on the orphan fraction using different covariance estimation techniques, showing consistent results within errors. [Extracted from the article]

While collisionless cold dark matter models have been largely successful in explaining a wide range of observational data, some tensions still exist, and it remains possible that dark matter possesses a non-negligible level of self-interactions. In this paper, we investigate a possible observable consequence of self-interacting dark matter: offsets between the central galaxy and the centre of mass of its parent halo. We examine 23 relaxed galaxy clusters in a redshift range of 0.1–0.3 drawn from clusters in the Dark Energy Survey and the Sloan Digital Sky Survey which have archival Chandra X-ray data of sufficient depth for centre and relaxation determination. We find that most clusters in our sample show non-zero offsets between the X-ray centre, taken to be the centroid within the cluster core, and the central galaxy position. All of the measured offsets are larger, typically by an order of magnitude, than the uncertainty in the X-ray position due to Poisson noise. In all but six clusters, the measured offsets are also larger than the estimated, combined astrometric uncertainties in the X-ray and optical positions. A more conservative cut on concentration to select relaxed clusters marginally reduces but does not eliminate the observed offset. With our more conservative sample, we find an estimated median X-ray to central galaxy offset of |$\mu = 6.0 ^{+ 1.4}_{- 1.5}$| kpc. Comparing to recent simulations, this distribution of offsets is consistent with some level of dark matter self-interaction, though further simulation work is needed to place constraints. [ABSTRACT FROM AUTHOR]

This paper reports our discovery of the most massive supercluster, termed the King Ghidorah Supercluster (KGSc), at z = 0.50–0.64 in the Third Public Data Release of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP PDR3) over 690 deg2, as well as an initial result for a galaxy and dark matter mapping. The primary structure of the KGSc comprises triple broad weak-lensing (WL) peaks over 70 comoving Mpc. Such extensive WL detection at z > 0.5 can only currently be achieved using the wide-field high-quality images produced by the HSC-SSP. The structure is also contiguous with multiple large-scale structures across a ∼400 comoving Mpc scale. The entire field has a notable overdensity (δ = 14.7 ± 4.5) of red-sequence clusters. Additionally, large-scale underdensities can be found in the foreground along the line of sight. We confirmed the overdensities in stellar mass and dark matter distributions to be tightly coupled and estimated the total mass of the main structure to be 1 × 1016 solar masses, according to the mock data analyses based on large-volume cosmological simulations. Further, upcoming wide-field multi-object spectrographs such as the Subaru Prime Focus Spectrograph may aid in providing additional insights into distant superclusters beyond the 100 Mpc scale. [ABSTRACT FROM AUTHOR]

The masses of galaxy clusters can be measured using data obtained exclusively from wide photometric surveys in one of two ways: directly from the amplitude of the weak lensing signal or, indirectly, through the use of scaling relations calibrated using binned lensing measurements. In this paper, we build on a recently proposed idea and implement an alternative method based on the radial profile of the satellite distribution. This technique relies on splashback, a feature associated with the apocentre of recently accreted galaxies that offers a clear window into the phase-space structure of clusters without the use of velocity information. We carry out this dynamical measurement using the stacked satellite distribution around a sample of luminous red galaxies in the fourth data release of the Kilo-Degree Survey and validate our results using abundance-matching and lensing masses. To illustrate the power of this measurement, we combine dynamical and lensing mass estimates to robustly constrain scalar–tensor theories of gravity at cluster scales. Our results exclude departures from General Relativity of the order of unity. We conclude the paper by discussing the implications for future data sets. Because splashback mass measurements scale only with the survey volume, stage-IV photometric surveys are well-positioned to use splashback to provide high-redshift cluster masses. [ABSTRACT FROM AUTHOR]

In this paper, we study the applicability of a set of supervised machine learning (ML) models specifically trained to infer observed related properties of the baryonic component (stars and gas) from a set of features of dark matter (DM)-only cluster-size haloes. The training set is built from the three hundred project that consists of a series of zoomed hydrodynamical simulations of cluster-size regions extracted from the 1 Gpc volume MultiDark DM-only simulation (MDPL2). We use as target variables a set of baryonic properties for the intracluster gas and stars derived from the hydrodynamical simulations and correlate them with the properties of the DM haloes from the MDPL2  N -body simulation. The different ML models are trained from this data base and subsequently used to infer the same baryonic properties for the whole range of cluster-size haloes identified in the MDPL2. We also test the robustness of the predictions of the models against mass resolution of the DM haloes and conclude that their inferred baryonic properties are rather insensitive to their DM properties that are resolved with almost an order of magnitude smaller number of particles. We conclude that the ML models presented in this paper can be used as an accurate and computationally efficient tool for populating cluster-size haloes with observational related baryonic properties in large volume N -body simulations making them more valuable for comparison with full sky galaxy cluster surveys at different wavelengths. We make the best ML trained model publicly available. [ABSTRACT FROM AUTHOR]

The paper presents the analysis of optically selected GAMA groups and clusters in the SRG/eROSITA X-ray map of eROSITA Final Equatorial Depth Survey, in the halo mass range 1013−5 × 1014 M⊙ and at z < 0.2. All X-ray detections have a clear GAMA counterpart, but most of the GAMA groups in the halo mass range 1013−1014 M⊙ remain undetected. We compare the X-ray surface brightness profiles of the eROSITA detected groups with the mean stacked profile of the undetected low-mass haloes at fixed halo mass. Overall, we find that the undetected groups exhibit less concentrated X-ray surface brightness, dark matter, and galaxy distributions with respect to the X-ray-detected haloes. The mean gas mass fraction profiles are consistent in the two samples within 1.5σ, indicating that the gas follows the dark matter profile. The low-mass concentration and the magnitude gap indicate that these systems are young. They reside with a higher probability in filaments while X-ray-detected groups favour the nodes of the Cosmic Web. Because of the lower central emission, the undetected systems tend to be X-ray underluminous at fixed halo mass and to lie below the LX − M halo relation. Interestingly, the X-ray-detected systems inhabiting the nodes scatter the less around the relation, while those in filaments tend to lie below it. We do not observe any strong relationship between the system X-ray appearance and the active galactic nucleus (AGN) activity. We cannot exclude the role of the past AGN feedback in affecting the gas distribution over the halo lifetime. However, the data suggests that the observed differences might be related to the halo assembly bias. [ABSTRACT FROM AUTHOR]

We present the measurement of total and stellar/dark matter decomposed mass density profile around a sample of galaxy groups and clusters with dynamical masses derived from integral-field stellar kinematics from the MaNGA survey in Paper I and weak lensing derived from the DECaLS imaging survey. Combining the two data sets enables accurate measurement of the radial density distribution from several kpc to Mpc scales. Intriguingly, we find that the excess surface density derived from stellar kinematics in the inner region cannot be explained by simply adding an NFW dark matter halo extrapolated from lensing measurement at a larger scale to a stellar mass component derived from the NASA-Sloan Atlas (NSA) catalogue. We find that a good fit to both data sets requires a stellar mass normalization about three times higher than that derived from the NSA catalogue, which would require an unrealistically too-heavy initial mass function for stellar mass estimation. If we keep the stellar mass normalization to that of the NSA catalogue but allow a varying inner dark matter density profile, we obtain an asymptotic slope of γgnfw =  |$1.82_{-0.25}^{+0.15}$| and γgnfw =  |$1.48_{-0.41}^{+0.20}$| for the group bin and the cluster bin, respectively, significantly steeper than the NFW case. We also compare the total mass inner density slopes with those from TNG300 and find that the values from the simulation are lower than the observation by about 2σ level. [ABSTRACT FROM AUTHOR]

The article titled "Correction to: Redshift drift and strong gravitational lensing" acknowledges a mistake made in a recent paper by the author. The mistake relates to the measurement of the Hubble constant using the difference in redshift between images of a gravitational-lens system. The author clarifies that the redshift difference can still be used to measure other cosmological parameters without knowledge of the Hubble constant. The rest of the paper remains unaffected, discussing various aspects of standard cosmology and gravitational lensing. No new data is associated with the article. [Extracted from the article]

The abundance of galaxy clusters is a sensitive probe to the amplitude of matter density fluctuations, the total amount of matter in the Universe as well as its expansion history. Inferring correct values and accurate uncertainties of cosmological parameters requires accurate knowledge of cluster abundance statistics, encoded in the likelihood function. In this paper, we test the accuracy of cluster abundance likelihoods used in the literature, namely the Poisson and Gaussian likelihoods as well as the more complete description of the Gauss–Poisson Compound likelihood. This is repeated for a variety of binning choices and analysis setups. In order to evaluate the accuracy of a given likelihood, this work compares individual posterior covariances to the covariance of estimators over the 1000 simulated dark matter halo catalogues obtained from PINOCCHIO algorithm. We find that for Rubin/ LSST and Euclid -like surveys the Gaussian likelihood gives robust constraints over a large range of binning choices. The Poisson likelihood, that does not account for sample covariance, always underestimates the errors on the parameters, even when the sample volume is reduced or only high-mass clusters are considered. We find no benefit in using the more complex Gauss–Poisson Compound likelihood as it gives essentially the same results as the Gaussian likelihood, but at a greater computational cost. Finally, in this ideal setup, we note only a small gain on the parameter error bars when using a large number of bins in the mass–redshift plane. [ABSTRACT FROM AUTHOR]

This paper explores the role of small-scale environment (<1 Mpc) in modulating accretion events on to supermassive black holes by studying the incidence of active galactic nuclei (AGNs) in massive clusters of galaxies. A flexible, data-driven semi-empirical model is developed based on a minimal set of parameters and under the zero-order assumption that the incidence of AGNs in galaxies is independent of environment. This is used to predict how the fraction of X-ray selected AGN among galaxies in massive dark matter haloes (⁠|$\gtrsim 3\times 10^{14}\, \mathrm{M}_{\odot }$|⁠) evolves with redshift and reveal tensions with observations. At high redshift, z ∼ 1.2, the model underpredicts AGN fractions, particularly at high X-ray luminosities, |$L_X(\rm 2\rm{-}10\, keV) \gtrsim 10^{44}\, erg \, s^{-1}$|⁠. At low redshift, z ∼ 0.2, the model estimates fractions of moderate luminosity AGN (⁠|$L_X(\rm 2\rm{-}10\, keV) \gtrsim 10^{43}\, erg \, s^{-1}$|⁠) that are a factor of 2–3 higher than the observations. These findings reject the zero-order assumption on which the semi-empirical model hinges and point to a strong and redshift-dependent influence of the small-scale environment on the growth of black holes. Cluster of galaxies appear to promote AGN activity relative to the model expectation at z ∼ 1.2 and suppress it close to the present day. These trends could be explained by the increasing gas content of galaxies towards higher redshift combined with an efficient triggering of AGNs at earlier times in galaxies that fall on to clusters. [ABSTRACT FROM AUTHOR]

As established in previous papers of this series, observables in highly distorted and magnified multiple images caused by the strong gravitational lensing effect can be used to constrain the distorting properties of the gravitational lens at the image positions. If the background source is extended and contains substructure, like star forming regions, which is resolved in multiple images, all substructure that can be matched across a minimum of three multiple images can be used to infer the local distorting properties of the lens. In this work, we replace the manual feature selection by an automated feature extraction based on SExtractor for Python and show its superior performance. Despite its aimed development to improve our lens reconstruction, it can be employed in any other approach, as well. Valuable insights on the definition of an 'image position' in the presence of noise are gained from our calibration tests. Applying it to observations of a five-image configuration in galaxy cluster CL0024 and the triple-image configuration containing Hamilton's object, we determine local lens properties for multiple wavebands separately. Within current confidence bounds, all of them are consistent with each other, corroborating the wavelength-independence of strong lensing and offering a tool to detect deviations caused by micro-lensing and dust in further examples. [ABSTRACT FROM AUTHOR]

In this paper, we explore the use of spatial clustering algorithms as a new computational approach for modelling the cosmic web. We demonstrate that such algorithms are efficient in terms of computing time needed. We explore three distinct spatial methods which we suitably adjust for (i) detecting the topology of the cosmic web and (ii) categorizing various cosmic structures as voids, walls, clusters, and superclusters based on a variety of topological and physical criteria such as the physical distance between objects, their masses, and local densities. The methods explored are (1) a new spatial method called Gravity Lattice ; (2) a modified version of another spatial clustering algorithm, the abacus ; and (3) the well known spatial clustering algorithm hdbscan. We utilize hdbscan in order to detect cosmic structures and categorize them using their overdensity. We demonstrate that the abacus method can be combined with the classic dtfe method to obtain similar results in terms of the achieved accuracy with about an order of magnitude less computation time. To further solidify our claims, we draw insights from the computer science domain and compare the quality of the results with and without the application of our method. Finally, we further extend our experiments and verify their effectiveness by showing their ability to scale well with different cosmic web structures that formed at different redshifts. [ABSTRACT FROM AUTHOR]

In this paper, we study the effect of reacceleration provided by turbulences on electrons produced by dark matter (DM) annihilation in the Coma cluster. We use a simplified phenomenological model to describe the effect of the turbulences, and explore a limited subset of three possible DM models for neutralino particles with different mass and annihilation channel. We find that, for values of the annihilation cross-section of the order of the upper limits found with Fermi –LAT measurements in astrophysical objects, and for conservative values of the boosting factor due to DM substructures, the reacceleration due to turbulences can enhance the radio emission produced by DM-originated electrons up to the level of the observed flux of the radio halo in Coma, for moderate reacceleration intensity in relatively short times. Therefore, we conclude that, even if it is not possible to distinguish between the fits obtained in this paper because of the scattering present in the radio flux data, the electrons produced by DM annihilation can be possible seed electrons for the reacceleration, as well as secondary electrons of hadronic origin. A possible discriminant between these two classes of models is the flux produced in the gamma ray band, which in the case of DM-originated electrons should be more than two orders of magnitude smaller than the present Fermi –LAT upper limits, whereas in the hadronic case the expected gamma ray flux should be close to the value of present upper limits. [ABSTRACT FROM AUTHOR]

We demonstrate how the total luminosity in satellite galaxies is a powerful probe of dark matter haloes around central galaxies. The method cross-correlates central galaxies in spectroscopic galaxy samples with fainter galaxies detected in photometric surveys. Using models, we show that the total galaxy luminosity, L sat, scales linearly with host halo mass, making L sat an excellent proxy for M h. L sat is also sensitive to the formation time of the halo. We demonstrate that probes of galaxy large-scale environment can break this degeneracy. Although this is an indirect probe of the halo, it yields a high signal-to-noise ratio measurement for galaxies expected to occupy haloes at <1012 M⊙, where other methods suffer from larger errors. In this paper, we focus on observational and theoretical systematics in the L sat method. We test the robustness of our method of finding central galaxies and our methods of estimating the number of background galaxies. We implement this method on galaxies in the Sloan Digital Sky Survey (SDSS) data, with satellites identified in fainter imaging data. We find excellent agreement between our theoretical predictions and the observational measurements. Finally, we compare our L sat measurements to weak lensing estimates of M h for red and blue subsamples. In the stellar mass range where the measurements overlap, we find consistent results, where red galaxies live in larger haloes. However, the L sat approach allows us to probe significantly lower mass galaxies. At these masses, the L sat values are equivalent. This example shows the potential of L sat as a probe of dark haloes. [ABSTRACT FROM AUTHOR]

This is the fifth in a series of papers studying the astrophysics and cosmology of massive, dynamically relaxed galaxy clusters. Our sample comprises 40 clusters identified as being dynamically relaxed and hot in Papers I and II of this series. Here we use constraints on cluster mass profiles from X-ray data to test some of the basic predictions of cosmological structure formation in the cold dark matter (CDM) paradigm. We present constraints on the concentration-mass relation for massive clusters, finding a power-law mass dependence with a slope of κm = −0.16 ± 0.07, in agreement with CDM predictions. For this relaxed sample, the relation is consistent with a constant as a function of redshift (power-law slope with 1 + z of κζ =−0.17 ± 0.26), with an intrinsic scatter of σln c = 0.16 ± 0.03. We investigate the shape of cluster mass profiles over the radial range probed by the data (typically ~50 kpc-1 Mpc), and test for departures from the simple Navarro-Frenk-White (NFW) form, for which the logarithmic slope of the density profile tends to −1 at small radii. Specifically, we consider as alternatives the generalized NFW (GNFW) and Einasto parametrizations. For the GNFW model, we find an average value of (minus) the logarithmic inner slope of β = 1.02 ± 0.08, with an intrinsic scatter of σβ = 0.22 ± 0.07, while in the Einasto case we constrain the average shape parameter to be α = 0.29 ± 0.04 with an intrinsic scatter of σα = 0.12 ± 0.04. Our results are thus consistent with the simple NFW model on average, but we clearly detect the presence of intrinsic, cluster-to-cluster scatter about the average. [ABSTRACT FROM AUTHOR]

The phase space structure of dark matter haloes can be used to measure the mass of the halo, infer mass accretion rates, and probe the effects of modified gravity. Previous studies showed that the splashback radius can be measured in position space using a sharp drop in the density profile. Using N -body simulations, we model the distribution of the kinematically distinct infalling and orbiting populations of subhaloes and haloes. We show that the two are mixed spatially all the way to r edge, which extends past the splashback radius defined by the drop in the spherically averaged density profile. This edge radius can be interpreted as a radius that contains a fixed fraction of the apocentres of dark matter particles. Our results highlight the possibility of measuring the outer boundary of a dark matter halo using its phase space structure and provide a firm theoretical foundation to the satellite galaxy model adopted in the companion paper, where we analysed the phase space distribution of Sloan Digital Sky Survey redMaPPer clusters. [ABSTRACT FROM AUTHOR]

This paper continues a series in which we intend to show how all observables of galaxy clusters can be combined to recover the two-dimensional, projected gravitational potential of individual clusters. Our goal is to develop a non-parametric algorithm for joint cluster reconstruction taking all cluster observables into account. For this reason we focus on the line-of-sight projected gravitational potential, proportional to the lensing potential, in order to extend existing reconstruction algorithms. In this paper, we begin with the relation between the Compton-y parameter and the Newtonian gravitational potential, assuming hydrostatic equilibrium and a polytropic stratification of the intracluster gas. Extending our first publication we now consider a spheroidal rather than a spherical cluster symmetry. We show how a Richardson-Lucy deconvolution can be used to convert the intensity change of the CMB due to the thermal Sunyaev-Zel'dovich effect into an estimate for the two-dimensional gravitational potential. We apply our reconstruction method to a cluster based on an N-body/hydrodynamical simulation processed with the characteristics (resolution and noise) of the ALMA interferometer for which we achieve a relative error of ≲20 per cent for a large fraction of the virial radius. We further apply our method to an observation of the galaxy cluster RXJ1347 for which we can reconstruct the potential with a relative error of ≲20 per cent for the observable cluster range. [ABSTRACT FROM AUTHOR]

The top-hat spherical collapse model (TSC) is one of the most fundamental analytical frameworks to describe the non-linear growth of cosmic structure. TSC has motivated, and been widely applied in, various investigations even in the current era of precision cosmology. While numerous studies exist to examine its validity against numerical simulations in a statistical fashion, there are few analyses which compare the TSC dynamics in an individual object-wise basis, which is what we attempt in the present paper. We extract 100 halos at z = 0 from a cosmological N-body simulation according to the conventional TSC criterion for the spherical over-density. Then we trace back their spherical counterparts at earlier epochs. Just prior to the turn-around epoch of the halos, their dynamics are well approximated by TSC, but their turn-around epochs are systematically delayed and the virial radii are larger by ~20% on average relative to the TSC predictions. We find that this systematic deviation can mainly be ascribed to the non-uniformity/inhomogeneity of dark matter density profiles and the non-zero velocity dispersions, both of which are neglected in TSC. In particular, the inside-out collapse and shell-crossing of dark matter halos play an important role in generating the significant velocity dispersion. The implications of the present result are briefly discussed. [ABSTRACT FROM AUTHOR]

The shape of dark matter (DM) haloes plays a critical role in constraining cosmology with upcoming large-scale structure surveys. In this paper, we study the correlations between the triaxial shapes and formation histories in DM haloes in the MultiDark Planck 2 N -body cosmological simulation. We find that halo ellipticity is strongly correlated with halo properties that serve as proxies of halo formation history, such as halo concentration and the normalized peak-centroid offset. These correlations are nearly independent of the halo density peak height. We present a simple model for the correlation between halo ellipticity and concentration using conditional abundance matching, and provide fitting formulae for the multidimensional distributions of triaxial halo shape as a function of halo peak height. We apply our halo shape model to gauge the effects of halo ellipticity and orientation bias on the excess surface mass density profiles in cluster-sized haloes. Our model should be useful for exploring the impact of triaxial halo shape on cosmological constraints in upcoming weak lensing surveys of galaxy clusters. [ABSTRACT FROM AUTHOR]

We study the distribution of line-of-sight velocities of galaxies in the vicinity of Sloan Digital Sky Survey (SDSS) red-sequence Matched-filter Probabilistic Percolation (redMaPPer) galaxy clusters. Based on their velocities, galaxies can be split into two categories: galaxies that are dynamically associated with the cluster, and random line-of-sight projections. Both the fraction of galaxies associated with the galaxy clusters, and the velocity dispersion of the same, exhibit a sharp feature as a function of radius. The feature occurs at a radial scale R edge ≈ 2.2 R λ, where R λ is the cluster radius assigned by redMaPPer. We refer to R edge as the 'edge radius'. These results are naturally explained by a model that further splits the galaxies dynamically associated with a galaxy cluster into a component of galaxies orbiting the halo and an infalling galaxy component. The edge radius R edge constitutes a true 'cluster edge', in the sense that no orbiting structures exist past this radius. A companion paper tests whether the 'halo edge' hypothesis holds when investigating the full three-dimensional phase-space distribution of dark matter substructures in numerical simulations, and demonstrates that this radius coincides with a suitably defined splashback radius. [ABSTRACT FROM AUTHOR]

We introduce the Marenostrum-MultiDark SImulations of galaxy Clusters (MUSIC) data set. It constitutes one of the largest samples of hydrodynamically simulated galaxy clusters with more than 500 clusters and 2000 groups. The objects have been selected from two large N-body simulations and have been resimulated at high resolution using smoothed particle hydrodynamics (SPH) together with relevant physical processes that include cooling, UV photoionization, star formation and different feedback processes associated with supernovae explosions.In this first paper we focus on the analysis of the baryon content (gas and star) of clusters in the MUSIC data set as a function of both aperture radius and redshift. The results from our simulations are compared with a compilation of the most recent observational estimates of the gas fraction in galaxy clusters at different overdensity radii. We confirm, as in previous simulations, that the gas fraction is overestimated if radiative physics are not properly taken into account. On the other hand, when the effects of cooling and stellar feedbacks are included, the MUSIC clusters show a good agreement with the most recent observed gas fractions quoted in the literature. A clear dependence of the gas fractions with the total cluster mass is also evident. However, we do not find a significant evolution with redshift of the gas fractions at aperture radius corresponding to overdensities smaller than 1500 with respect to critical density. At smaller radii, the gas fraction does exhibit a decrease with redshift that is related to the gas depletion due to star formation in the central region of the clusters. The impact of the aperture radius choice, when comparing integrated quantities at different redshifts, is tested. The standard, widely used definition of radius at a fixed overdensity with respect to critical density is compared with a definition of aperture radius based on the redshift dependent overdensity with respect to background matter density: we show that the latter definition is more successful in probing the same fraction of the virial radius at different redshifts, providing a more reliable derivation of the time evolution of integrated quantities. We also present in this paper a detailed analysis of the scaling relations of the thermal Sunyaev–Zel'dovich (SZ) effect derived from MUSIC clusters. The integrated SZ brightness, Y, is related to the cluster total mass, M, as well as, the M − Y counterpart which is more suitable for observational applications. Both laws are consistent with predictions from the self-similar model, showing a very low scatter which is σlog Y ≃ 0.04 and even a smaller one (σlog M ≃ 0.03) for the inverse M–Y relation. The effects of the gas fraction on the Y–M scaling relation are also studied. At high overdensities, the dispersion of the gas fractions introduces non-negligible deviation from self-similarity, which is directly related to the fgas–M relation. The presence of a possible redshift dependence on the Y–M scaling relation is also explored. No significant evolution of the SZ relations is found at lower overdensities, regardless of the definition of overdensity used. [ABSTRACT FROM AUTHOR]

ABSTRACT The Canadian Cluster Comparison Project is a comprehensive multi-wavelength survey targeting 50 massive X-ray selected clusters of galaxies to examine baryonic tracers of cluster mass and to probe the cluster-to-cluster variation in the thermal properties of the hot intra-cluster medium. In this paper we present the weak lensing masses, based on the analysis of deep wide field imaging data obtained using the Canada-France-Hawaii Telescope. The final sample includes two additional clusters that were located in the field-of-view. We take these masses as our reference for the comparison of cluster properties at other wavelengths. In this paper we limit the comparison to published measurements of the Sunyaev-Zel'dovich effect. We find that this signal correlates well with the projected lensing mass, with an intrinsic scatter of 12 ± 5 per cent at ∼ r2500, demonstrating it is an excellent proxy for cluster mass. [ABSTRACT FROM AUTHOR]

A520 is a hot and luminous galaxy cluster, where gravitational lensing and X-ray measures reveal a different spatial distribution of baryonic and dark matter. This cluster hosts a radio halo, whose map shows a separation between the north-east and the south-west part of the cluster, similarly to what is observed in gravitational lensing maps. In this paper we study the possibility that the diffuse radio emission in this cluster is produced by dark matter annihilation. We find that in the whole cluster the radio emission should be dominated by baryonic phenomena; if a contribution from dark matter is present, it should be searched in a region in the NE part of the cluster, where a peak of the radio emission is located close to a dark matter subhalo, in a region where the X-ray emission is not very strong. By estimating the radio spectrum integrated in this region using data from publicly available surveys, we find that this spectrum can be reproduced by a dark matter model for a neutralino with mass 43 GeV and annihilation final state |$b \bar{b}$| for a magnetic field of 5 |$\mu$| G. [ABSTRACT FROM AUTHOR]

Galaxies acting as gravitational lenses are surrounded by, at most, a handful of images. This apparent paucity of information forces one to make the best possible use of what information is available to invert the lens system. In this paper, we explore the use of a genetic algorithm to invert in a non-parametric way strong lensing systems containing only a small number of images. Perhaps the most important conclusion of this paper is that it is possible to infer the mass distribution of such gravitational lens systems using a non-parametric technique. We show that including information about the null space (i.e. the region where no images are found) is prerequisite to avoid the prediction of a large number of spurious images, and to reliably reconstruct the lens mass density. While the total mass of the lens is usually constrained within a few per cent, the fidelity of the reconstruction of the lens mass distribution depends on the number and position of the images. The technique employed to include null space information can be extended in a straightforward way to add additional constraints, such as weak-lensing data or time-delay information. [ABSTRACT FROM AUTHOR]

In a previous paper, we described a new method for including detailed information about substructure in semi-analytic models of halo formation based on merger trees. In this paper, we compare the predictions of our model with results from self-consistent numerical simulations. We find that in general the two methods agree extremely well, particularly once numerical effects and selection effects in the choice of haloes are taken into account. As expected from the original analyses of the simulations, we see some evidence for artificial overmerging in the innermost regions of the simulated haloes, either because substructure is being disrupted artificially or because the group-finding algorithms used to identify substructure are not detecting all the bound clumps in the highest-density regions. Our analytic results suggest that greater mass and force resolution may be required before numerical overmerging becomes negligible in all current applications. We discuss the implications of this result for observational and experimental tests of halo substructure, such as the analysis of discrepant magnification ratios in strongly lensed systems, terrestrial experiments to detect dark matter particles directly or indirect detection experiments searching for positrons, gamma-rays, neutrinos or other dark matter decay or annihilation products. [ABSTRACT FROM AUTHOR]

In a previous paper, we described a new method for including detailed information about substructure in semi-analytic models of dark matter halo formation based on merger trees. In this paper, we present the basic predictions of our full model of halo formation. We first describe the overall properties of substructure in galaxy, group or cluster haloes at the present day. We then discuss the evolution of substructure, and the effect of the mass-accretion history of an individual halo on the mass function and orbital grouping of its subhalo population. We show, in particular, that the shape of the subhalo mass function is strongly correlated with the formation epoch of the halo. In a third paper in this series, we will compare the results of our semi-analytic method with the results of self-consistent numerical simulations of halo formation. [ABSTRACT FROM AUTHOR]

The spatial distribution of satellite galaxies encodes rich information of the structure and assembly history of galaxy clusters. In this paper, we select a red-sequence Matched-filter Probabilistic Percolation cluster sample in SDSS Stripe 82 region with 0.1 ≤ z ≤ 0.33, 20 < λ < 100, and Pcen > 0.7. Using the high-quality weak lensing data from CS82 Survey, we constrain the mass profile of this sample. Then we compare directly themass density profile with the satellite number density profile.We find that the total mass and number density profiles have the same shape, both well fitted by an NFW profile. The scale radii agree with each other within a 1σ error (rs,gal = 0.34+0.04 -0.03 Mpc versus rs = 0.37+0.15 -0.10 Mpc). [ABSTRACT FROM AUTHOR]

We consider the dynamics in and near galaxy clusters. Gas, dark matter and galaxies are presently falling into the clusters between approximately 1 and 5 virial radii. At very large distances, beyond 10 virial radii, all matter follows the Hubble flow, and inside the virial radius the matter particles have on average zero radial velocity. The cosmological parameters are imprinted on the infall profile of the gas; however, no method exists which allows a measurement of it. We consider the results of two cosmological simulations (using the numerical codes RAMSES and GADGET) and find that the gas and dark matter (DM) radial velocities are very similar. We derive the relevant dynamical equations, in particular the generalized hydrostatic equilibrium equation, including both the expansion of the Universe and the cosmological background. This generalized gas equation is the main new contribution of this paper. We combine these generalized equations with the results of the numerical simulations to estimate the contribution to the measured clustermasses from the radial velocity: inside the virial radius it is negligible, and inside two virial radii the effect is below 40 per cent, in agreement the earlier analyses for DM. We point out how the infall velocity in principle may be observable, by measuring the gas properties to distance of about two virial radii; however, this is practically not possible today. [ABSTRACT FROM AUTHOR]

Gravitational lensing by clusters of galaxies offers a powerful probe of their structure and mass distribution. Several research groups have developed techniques independently to achieve this goal. While these methods have all provided remarkably high-precision mass maps, particularly with exquisite imaging data from the Hubble Space Telescope (HST), the reconstructions themselves have never been directly compared. In this paper, we present for the first time a detailed comparison of methodologies for fidelity, accuracy and precision. For this collaborative exercise, the lens modelling community was provided simulated cluster images that mimic the depth and resolution of the ongoing HST Frontier Fields. The results of the submitted reconstructions with the un-blinded true mass profile of these two clusters are presented here. Parametric, free-form and hybrid techniques have been deployed by the participating groups and we detail the strengths and trade-offs in accuracy and systematics that arise for each methodology. We note in conclusion that several properties of the lensing clusters are recovered equally well by most of the lensing techniques compared in this study. For example, the reconstruction of azimuthally averaged density and mass profiles by both parametric and freeform methods matches the input models at the level of ~10 per cent. Parametric techniques are generally better at recovering the 2D maps of the convergence and of the magnification. For the best-performing algorithms, the accuracy in the magnification estimate is ~10 per cent at μtrue = 3 and it degrades to ~30 per cent at μtrue ~ 10. [ABSTRACT FROM AUTHOR]

While the Virgo cluster is the nearest galaxy cluster and therefore the best observed one, little is known about its formation history. In this paper, a set of cosmological simulations that resemble the Local Universe is used to shed the first light on this mystery. The initial conditions for these simulations are constrained with galaxy peculiar velocities of the second catalogue of the Cosmicflows project using algorithms developed within the Constrained Local UniversE Simulation project. Boxes of 500 h-1 Mpc on a side are set to run a series of dark matter only constrained simulations. In each simulation, a unique dark matter halo can be reliably identified as Virgo's counterpart. The properties of these Virgo haloes are in agreement at a 10- 20 per cent level with the global properties of the observed Virgo cluster. Their zero-velocity masses agree at 1σ with the observational mass estimate. In all the simulations, the matter falls on to the Virgo objects along a preferential direction that corresponds to the observational filament and the slowest direction of collapse. A study of the mass accretion history of the Virgo candidates reveals the most likely formation history of the Virgo cluster, namely a quiet accretion over the last 7 Gyr. [ABSTRACT FROM AUTHOR]

We study the characteristics of the galaxy cluster samples expected from the European Space Agency's Euclid satellite and forecast constraints on parameters describing a variety of cosmological models. In this paper we use the same method of analysis already adopted in the Euclid Red Book, which is based on the Fisher matrix approach. Based on our analytical estimate of the cluster selection function in the photometric Euclid survey, we forecast the constraints on cosmological parameters corresponding to different extensions of the standard Λ cold dark matter model. Using only Euclid clusters, we find that the amplitude of the matter power spectrum will be constrained to Δσ8 = 0.0014 and the mass density parameter to ΔΩm = 0.0011. The dynamical evolution of dark energy will be constrained to Δw0 = 0.03 and Δwa = 0.2 with free curvature Ωk, resulting in a (w0, wa) figure of merit (FoM) of 291. In combination with Planck cosmic microwave background (CMB) constraints, the amplitude of primordial non-Gaussianity will be constrained to ΔfNL ≃ 6.6 for the local shape scenario. The growth factor parameter γ, which signals deviations from general relativity, will be constrained to Δγ = 0.02, and the neutrino density parameter to ΔΩν = 0.0013 (or ΔΣmν = 0.01). Including the Planck CMB covariance matrix improves dark energy constraints to Δw0 = 0.02, Δwa = 0.07, and a FoM = 802. Knowledge of the observable-cluster mass scaling relation is crucial to reach these accuracies. Imaging and spectroscopic capabilities of Euclid will enable internal mass calibration from weak lensing and the dynamics of cluster galaxies, supported by external cluster surveys. [ABSTRACT FROM AUTHOR]

We examine subhaloes and galaxies residing in a simulated Λ cold dark matter galaxy cluster (M crit 200 =1.1 1015 h-1 M⊙) produced by hydrodynamical codes ranging from classic smooth particle hydrodynamics (SPH), newer SPH codes, adaptive and moving mesh codes. These codes use subgrid models to capture galaxy formation physics. We compare how well these codes reproduce the same subhaloes/galaxies in gravity-only, non-radiative hydrodynamics and full feedback physics runs by looking at the overall subhalo/galaxy distribution and on an individual object basis. We find that the subhalo population is reproduced to within ≲10 percent for both dark matter only and non-radiative runs, with individual objects showing code-to-code scatter of ≲0.1 dex, although the gas in non-radiative simulations shows significant scatter. Including feedback physics significantly increases the diversity. Subhalo mass and Vmax distributions vary by ≈20 percent. The galaxy populations also show striking code-to-code variations. Although the Tully-Fisher relation is similar in almost all codes, the number of galaxies with 109 h-1M⊙ ≲ M* ≲ 1012 h-1M⊙ can differ by a factor of 4. Individual galaxies show code-to-code scatter of ~0.5 dex in stellar mass. Moreover, systematic differences exist, with some codes producing galaxies 70 percent smaller than others. The diversity partially arises from the inclusion/absence of active galactic nucleus feedback. Our results combined with our companion papers demonstrate that subgrid physics is not just subject to fine-tuning, but the complexity of building galaxies in all environments remains a challenge. We argue that even basic galaxy properties, such as stellar mass to halo mass, should be treated with errors bars of ~0.2-0.4 dex. [ABSTRACT FROM AUTHOR]

Merging galaxy systems provide observational evidence of the existence of dark matter and constraints on its properties. Therefore, statistically uniform samples of merging systems would be a powerful tool for several studies. In this paper, we present a new methodology for the identification of merging systems and the results of its application to galaxy redshift surveys. We use as a starting point a mock catalogue of galaxy systems, identified using friends-of-friends algorithms, that have experienced a major merger, as indicated by its merger tree. By applying machine learning techniques in this training sample, and using several features computed from the observable properties of galaxy members, it is possible to select galaxy groups that have a high probability of having experienced a major merger. Next, we apply a mixture of Gaussian techniques on galaxy members in order to reconstruct the properties of the haloes involved in such mergers. This methodology provides a highly reliable sample of merging systems with low contamination and precisely recovered properties. We apply our techniques to samples of galaxy systems obtained from the Sloan Digital Sky Survey Data Release 7, the Wide-Field Nearby Galaxy-Cluster Survey (WINGS) and the Hectospec Cluster Survey (HeCS). Our results recover previously known merging systems and provide several new candidates. We present their measured properties and discuss future analysis on current and forthcoming samples. [ABSTRACT FROM AUTHOR]

For the last 30 yr many observational and theoretical evidences have shown that galaxy clusters are not spherical objects, and that their shape is much better described by a triaxial geometry. With the advent of multiwavelength data of increasing quality, triaxial investigations of galaxy clusters is gathering a growing interest from the community, especially in the time of 'precision cosmology'. In this work, we aim to provide the first statistically significant predictions in the unexplored mass range above 3 × 1014 M⊙h-1, using haloes from two redshift snapshots (z = 0 and z = 1) of the Millennium XXL simulation. The size of this cosmological dark matter-only simulation (4.1 Gpc) allows the formation of a statistically significant number of massive cluster scale haloes (500 with M > 2 × 1015 M⊙h-1, and 780 000 with M > 1014 M⊙h-1). Besides, we aim to extend this investigation to lower masses in order to look for universal predictions across nearly six orders of magnitude in mass, from 1010 to almost 1016 M⊙h-1. For this purpose we use the SBARBINE simulations, allowing us to model haloes of masses starting from 1010 M⊙h-1. We use an elliptical overdensity method to select haloes and compute the shapes of the unimodal ones (approximately 50 per cent), while we discard the more unrelaxed. The minor to major and intermediate to major axis ratio distributions are found to be well described by simple universal functional forms that do not depend on cosmology or redshift. Our results extend the findings of Jing & Suto to a higher precision and a wider range of mass. This 'recipe' is made available to the community in this paper and in a dedicated web page. [ABSTRACT FROM AUTHOR]

Estimating the ellipticity of dark-matter haloes at the galaxy or galaxy–cluster scale can provide important constraints on the formation of galaxies or clusters, as well as on the nature of dark matter. We show in this paper that the spin-3 gravitational flexion can add useful information on the ellipticity of lensing haloes. We introduce a general formalism to decompose fields with arbitrary spin into radial and tangential components. The ratio of the tangential and radial flexion components directly estimates the lens ellipticity. We point out that any centroid offset will significantly bias our estimate, which on the other hand can be used to determine the centre of the lens halo. [ABSTRACT FROM PUBLISHER]

ABSTRACT The X-ray properties of groups and clusters of galaxies obey scaling relations that provide insight into the physics of their formation and evolution. In this paper, we constrain gas and dark matter parameters of these systems, by comparing the observed relations to theoretical expectations, obtained assuming that the gas is in hydrostatic equilibrium with the dark matter and follows a polytropic relation. In this exercise, we vary four parameters: the gas polytropic index Γ, its temperature at large radii, the dark matter logarithmic slope at large radii ζ and its concentration. When comparing the model to the observed mass-temperature relation of local high-mass systems, we find our results to be independent of both the gas temperature at large radii and of the dark matter concentration. We thus obtain constraints on Γ, by fixing the dark matter profile, and on ζ, by fixing the gas profile. For a Navarro-Frenk-White dark matter profile, we find that Γ must lie between 6/5 and 13/10. This value is consistent with numerical simulations and observations of individual clusters. Taking 6/5 ≲Γ≲ 13/10 allows the dark matter profile to be slightly steeper than the Navarro-Frenk-White profile at large radii. Upon including local low-mass systems, we obtain constraints on the mass dependence of Γ and on the value of the gas temperature at large radii. Interestingly, by fixing Γ= 6/5 and ζ=−3, we reproduce the observed steepening/breaking of the mass-temperature relation at low masses if the temperature of the intercluster medium is between 106 and 107 K, consistent with numerical simulations and observations of the warm-hot intergalactic medium. When extrapolated to high redshift, the model with a constant Γ reproduces the expected self-similar behaviour. Given our formulation, we can also naturally account for the observed, non-self-similar relations provided by some high-redshift clusters, as they simply provide constraints on the evolution of Γ. In addition, comparing our model to the observed luminosity-temperature relation, we are able to discriminate between different mass-concentration relations and find that a weak dependence of concentration on mass is currently preferred by data. In summary, this simple theoretical model can still account for much of the complexity of recent, improved X-ray scaling relations, provided that we allow for a mild dependence of the polytropic index on mass or for a gas temperature at large radii consistent with intercluster values. [ABSTRACT FROM AUTHOR]

The final data release of the Sloan Digital Sky Survey (SDSS) provides reliable photometry and spectroscopy for about half a million galaxies with median redshift 0.09. Here, we use these data to estimate projected autocorrelation functions for the light of galaxies in the five SDSS photometric bands. Comparison with the analogous stellar mass autocorrelation, estimated in a previous paper, shows that stellar luminosity is less strongly clustered than stellar mass in all bands and on all scales. Over the full non-linear range our autocorrelation estimates are extremely well represented by power laws. The parameters of the corresponding spatial functions vary systematically from and for the bluest band (the u band) to and for the reddest one (the z band). These may be compared with and for the stellar mass. Ratios of between two given wavebands are proportional to the mean colour of correlated stars at projected distance from a randomly chosen star. The ratio of the stellar mass to luminosity autocorrelations measures an analogous mean stellar mass-to-light ratio ( ). All colours get redder and all mass-to-light ratios get larger with decreasing , with the amplitude of the effects decreasing strongly to redder passbands. Even for the u band the effects are quite modest, with maximum shifts of about 0.1 in and about 25 per cent in . These trends provide a precise characterization of the well-known dependence of stellar populations on environment. [ABSTRACT FROM AUTHOR]

In a recent publication, an aperture mass statistic for gravitational flexion was derived and shown to be effective, at least with simulated data, in detecting massive structures and substructures within clusters of galaxies. Further, it was suggested that the radius at which the flexion aperture mass signal falls to zero might allow for estimation of the mass or density profile of the structures detected. In this paper, we more fully explore this possibility, considering the behaviour both of the peak signal and the zero-signal contours for two mass models – the singular isothermal sphere and Navarro–Frenk–White profiles – under varying aperture size, filter shape and mass concentration parameter. We demonstrate the effectiveness of the flexion aperture mass statistic in discriminating between mass profiles and concentration parameters, and in providing an accurate estimate of the mass of the lens, to within a factor of 1.5 or better. In addition, we compare the aperture mass method to a direct non-parametric reconstruction of the convergence from flexion measurements. We demonstrate that the aperture mass technique is much better able to constrain the shape of the central density profile, obtains much finer angular resolution in reconstructions, and does not suffer from ambiguity in the normalization of the signal, in contrast to the direct method. [ABSTRACT FROM AUTHOR]

Recent advances in N-body simulations of dark matter haloes have shown that three-parameter models, in particular the Einasto profile characterized by with a shape parameter , are able to produce better fits to the 3D spatial density profiles than two-parameter models like the Navarro, Frenk and White and Moore et al. profiles. In this paper, we present for the first time an analytically motivated form for the 2D surface mass density of the Einasto family of dark matter haloes, in terms of the 3D spatial density parameters for a wide range of the shape parameter . Our model describes a projected (2D) Einasto remarkably well between 0 and , with errors less than 0.3 per cent for and less than 2 per cent for α as large as 1. This model (in 2D) can thus be used to fit strong and weak lensing observations of galaxies and clusters whose total spatial (3D) density distributions are believed to be Einasto-like. Further, given the dependence of our model on the 3D parameters, one can reliably estimate structural parameters of the spatial (3D) density from 2D observations. We also consider a Sersic-like parametrization for the above family of projected Einasto and observe that fits with a Sersic model are sensitive to whether one fits the projected density in linear scale or logarithmic scale and yield widely varying results. Structural parameters of Einasto-like systems, inferred from fits with a Sersic model, should be used with caution. [ABSTRACT FROM AUTHOR]

The problem of detecting dark matter filaments in the cosmic web is considered. Weak lensing is an ideal probe of dark matter, and therefore forms the basis of particularly promising detection methods. We consider and develop a number of weak lensing techniques that could be used to detect filaments in individual or stacked cluster fields, and apply them to synthetic lensing data sets in the fields of clusters from the Millennium Simulation. These techniques are multipole moments of the shear and convergence, mass reconstruction and parametrized fits to filament mass profiles using a Markov chain Monte Carlo approach. In particular, two new filament detection techniques are explored (multipole shear filters and Markov chain Monte Carlo mass profile fits), and we outline the quality of data required to be able to identify and quantify filament profiles. We also consider the effects of large-scale structure on filament detection. We conclude that using these techniques, there will be realistic prospects of detecting filaments in data from future space-based missions. The methods presented in this paper will be of great use in the identification of dark matter filaments in future surveys. [ABSTRACT FROM AUTHOR]

We investigate the effect of dust on the scaling properties of galaxy clusters based on hydrodynamic N-body simulations of structure formation. We have simulated five dust models plus radiative cooling and adiabatic models using the same initial conditions for all runs. The numerical implementation of dust was based on the analytical computations of Montier & Giard. We set up dust simulations to cover different combinations of dust parameters that make evident the effects of size and abundance of dust grains. Comparing our radiative plus dust cooling runs with a purely radiative cooling simulation, we find that dust has an impact on cluster scaling relations. It mainly affects the normalization of the scalings (and their evolution), whereas it introduces no significant differences in their slopes. The strength of the effect critically depends on the dust abundance and grain size parameters as well as on the cluster scaling. Indeed, cooling due to dust is effective in the cluster regime and has a stronger effect on the ‘baryon driven’ statistical properties of clusters such as scaling relations. Major differences, relative to the radiative cooling model, are as high as 25 per cent for the normalization, and about 10 per cent for the Y– M and S– M normalizations at redshift zero. On the other hand, we find that dust has almost no impact on the ‘dark matter driven’ scaling relation. The effects are found to be dependent in equal parts on both dust abundances and grain size distributions for the scalings investigated in this paper. Higher dust abundances and smaller grain sizes cause larger departures from the radiative cooling (i.e. with no dust) model. [ABSTRACT FROM AUTHOR]

Gravitational flexion has been introduced as a technique by which one can map out and study substructure in clusters of galaxies. Previous analyses involving flexion have measured the individual galaxy–galaxy flexion signal, or used either parametric techniques or a Kaiser, Squires and Broadhurst (KSB)-type inversion to reconstruct the mass distribution in Abell 1689. In this paper, we present an aperture mass statistic for flexion, and apply it to the lensed images of background galaxies obtained by ray-tracing simulations through a simple analytic mass distribution and through a galaxy cluster from the Millennium Simulation. We show that this method is effective at detecting and accurately tracing structure within clusters of galaxies on subarcminute scales with high signal to noise even using a moderate background source number density and image resolution. In addition, the method provides much more information about both the overall shape and the small-scale structure of a cluster of galaxies than can be achieved through a weak lensing mass reconstruction using gravitational shear data. Lastly, we discuss how the zero-points of the aperture mass might be used to infer the masses of structures identified using this method. [ABSTRACT FROM AUTHOR]

The diffuse plasma that fills galaxy groups and clusters (the intracluster medium) is a by-product of galaxy formation. The present thermal state of this gas results from a competition between gas cooling and heating. The heating comes from two distinct sources: gravitational heating associated with the collapse of the dark matter halo and additional thermal input from the formation of galaxies and their black holes. A long-term goal of this research is to decode the observed temperature, density and entropy profiles of clusters and to understand the relative roles of these processes. However, a long-standing problem has been that cosmological simulations based on smoothed particle hydrodynamics (SPH) and Eulerian mesh-based codes predict different results even when cooling and galaxy/black hole heating are switched off. Clusters formed in SPH simulations show near power-law entropy profiles, while those formed in Eulerian simulations develop a core and do not allow gas to reach such low entropies. Since the cooling rate is closely connected to the minimum entropy of the gas distribution, the differences are of potentially key importance. In this paper, we investigate the origin of this discrepancy. By comparing simulations run using theGADGET-2 SPH code and theFLASH adaptive Eulerian mesh code, we show that the discrepancy arises during the idealized merger of two clusters and that the differences are not the result of the lower effective resolution of Eulerian cosmological simulations. The difference is not sensitive to the minimum mesh size (in Eulerian codes) or the number of particles used (in SPH codes). We investigate whether the difference is the result of the different gravity solvers, the Galilean non-invariance of the mesh code or an effect of unsuitable artificial viscosity in the SPH code. Instead, we find that the difference is inherent to the treatment of vortices in the two codes. Particles in the SPH simulations retain a close connection to their initial entropy, while this connection is much weaker in the mesh simulations. The origin of this difference lies in the treatment of eddies and fluid instabilities. These are suppressed in the SPH simulations, while the cluster mergers generate strong vortices in the Eulerian simulations that very efficiently mix the fluid and erase the low-entropy gas. We discuss the potentially profound implications of these results. [ABSTRACT FROM AUTHOR]

In this paper, we study the variations of group galaxy properties according to the assembly history in Sloan Digital Sky Survey Data Release 6 (SDSS-DR6) selected groups. Using mock SDSS group catalogues, we find two suitable indicators of group formation time: (i) the isolation of the group, defined as the distance to the nearest neighbour in terms of its virial radius and (ii) the concentration, measured as the group inner density calculated using the fifth nearest bright galaxy to the group centre. Groups within narrow ranges of mass in the mock catalogue show increasing group age with isolation and concentration. However, in the observational data the stellar age, as indicated by the spectral type, only shows a correlation with concentration. We study groups of similar mass and different assembly history, finding important differences in their galaxy population. Particularly, in high-mass SDSS groups, the number of members, mass-to-light ratios, red galaxy fractions and the magnitude difference between the brightest and second-brightest group galaxies, show different trends as a function of isolation and concentration, even when it is expected that the latter two quantities correlate with group age. Conversely, low-mass SDSS groups appear to be less sensitive to their assembly history. The correlations detected in the SDSS are not consistent with the trends measured in the mock catalogues. However, discrepancies can be explained in terms of the disagreement found in the age-isolation trends, suggesting that the model might be overestimating the effects of environment. We discuss how the modelling of the cold gas in satellite galaxies could be responsible for this problem. These results can be used to improve our understanding of the evolution of galaxies in high-density environments. [ABSTRACT FROM AUTHOR]

This continuing study of intragroup light in compact groups of galaxies aims to establish new constraints to models of formation and evolution of galaxy groups, specially of compact groups, which are a key part in the evolution of larger structures, such as clusters. In this paper we present three additional groups (HCG 15, 35 and 51) using deep wide-field B- and R-band images observed with the LAICA camera at the 3.5-m telescope at the Calar Alto observatory (CAHA). This instrument provides us with very stable flat-fielding, a mandatory condition for reliably measuring intragroup diffuse light. The images were analysed with theov_wav package, a wavelet technique that allows us to uncover the intragroup component in an unprecedented way. We have detected that 19, 15 and 26 per cent of the total light of HCG 15, 35 and 51, respectively, are in the diffuse component, with colours that are compatible with old stellar populations and with mean surface brightness that can be as low as 28.4 B mag arcsec−2. Dynamical masses, crossing times and mass-to-light ratios were recalculated using the new group parameters. Also tidal features were analysed using the wavelet technique. [ABSTRACT FROM AUTHOR]

According to the current paradigm, galaxies initially form as disc galaxies at the centres of their own dark matter haloes. During their subsequent evolution, they may undergo a transformation to a red, early-type galaxy, thus giving rise to the build-up of the red sequence. Two important, outstanding questions are (i) which transformation mechanisms are most important and (ii) in what environment do they occur. In this paper, we study the impact of transformation mechanisms that operate only on satellite galaxies, such as strangulation, ram-pressure stripping and galaxy harassment. Using a large galaxy group catalogue constructed from the Sloan Digital Sky Survey, we compare the colours and concentrations of satellites galaxies to those of central galaxies of the same stellar mass, adopting the hypothesis that the latter are the progenitors of the former. On average, satellite galaxies are redder and more concentrated than central galaxies of the same stellar mass, indicating that satellite-specific transformation processes do indeed operate. Central-satellite pairs that are matched in both stellar mass and colour, however, show no average concentration difference, indicating that the transformation mechanisms operating on satellites affect colour more than morphology. We also find that the colour and concentration differences of matched central-satellite pairs are completely independent of the mass of the host halo (not to be confused with the subhalo) of the satellite galaxy, indicating that satellite-specific transformation mechanisms are equally efficient in host haloes of all masses. This strongly rules against mechanisms that are thought to operate only in very massive haloes, such as ram-pressure stripping or harassment. Instead, we argue that strangulation is the main transformation mechanism for satellite galaxies. Finally, we determine the relative importance of satellite quenching for the build-up of the red sequence. We find that roughly 70 per cent of red-sequence satellite galaxies with had their star formation quenched as satellites. This drops rapidly with increasing stellar mass, reaching virtually zero at . Therefore, a very significant fraction of red satellite galaxies were already quenched before they became a satellite. [ABSTRACT FROM AUTHOR]

Analytic models based on spherical and ellipsoidal gravitational collapse have been used to derive the mass functions of dark matter haloes and their progenitors (the conditional mass function). The ellipsoidal model generally provides a better match to simulation results, but there has been no simple analytic expression in this model for the conditional mass function that is accurate for small time-steps, a limit that is important for generating halo merger trees and computing halo merger rates. We remedy the situation by deriving accurate analytic formulae for the first-crossing distribution, the conditional mass function and the halo merger rate in the ellipsoidal collapse model in the limit of small look-back times. We show that our formulae provide a closer match to the Millennium simulation results than those in the spherical collapse model and the ellipsoidal model of Sheth and Tormen. [ABSTRACT FROM AUTHOR]

Ram-pressure stripping can remove significant amounts of gas from galaxies that orbit in clusters and massive groups, and thus has a large impact on the evolution of cluster galaxies. In this paper, we reconstruct the present-day distribution of ram pressure and the ram-pressure histories of cluster galaxies. To this aim, we combine the Millennium Simulation and an associated semi-analytic model of galaxy evolution with analytic models for the gas distribution in clusters. We find that about one quarter of galaxies in massive clusters are subject to strong ram pressures that are likely to cause an expedient loss of all gas. Strong ram pressures occur predominantly in the inner core of the cluster, where both the gas density and the galaxy velocity are higher. Since their accretion on to a massive system, more than 64 per cent of galaxies that reside in a cluster today have experienced strong ram pressures of which most likely led to a substantial loss of the gas. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the possibility of detecting lensing signals in high-resolution and high-sensitivity cosmic microwave background (CMB) experiments. At scales below 1 arcmin, the CMB background is dominated by the Sunyaev–Zel'dovich effect in clusters and by Ostriker–Vishniac effect distortions elsewhere. This uniform signal can be used as a background for lensing studies potentially allowing the detection of high-redshift clusters with none or little Sunyaev–Zel'dovich effect signal. We focus on the Ostriker–Vishniac effect and study the possibility of the detection of the lensing signatures by high-redshift clusters. We pay special attention to contaminants, such as instrumental noise and point sources. After designing an optimal filter for this particular lensing signal, we explore the signal-to-noise ratio for different scenarios varying the resolution of the experiment, its sensitivity and the level of contamination due to point sources. Our results show that the next generation of experiments should be able to introduce new and exciting science through the study of the Ostriker–Vishniac effect and its lensing effect. [ABSTRACT FROM AUTHOR]

We have performed a series of N-body/hydrodynamical (treesph) simulations of clusters and groups of galaxies, selected from a cosmological volume within a Lambda cold dark matter (ΛCDM) framework: these objects have been resimulated at higher resolution to , in order to follow also the dynamical, thermal and chemical input on to the intracluster medium (ICM) from stellar populations within galaxies. The simulations include metallicity-dependent radiative cooling, star formation according to different initial mass functions (IMFs), energy feedback as strong starburst-driven galactic superwinds, chemical evolution with non-instantaneous recycling of gas and heavy elements, effects of a metagalactic ultraviolet (UV) field and thermal conduction in the ICM. In this paper, the first in a series of three, we derive results, mainly at , on the temperature and entropy profiles of the ICM, its X-ray luminosity, the cluster cold components [cold fraction as well as mass-to-light ratio (MLR)] and the metal distribution between ICM and stars. In general, models with efficient superwinds (produced by the action of supernovae and, in some simulations, of active galactic nuclei (AGNs), along with a top-heavy stellar IMF, are able to reproduce fairly well the observed relation, the entropy profiles and the cold fraction: both features are found to be needed in order to remove high-density and low-entropy cold gas at core scales, although additional alternative feedback mechanisms would still be required to prevent late-time central cooling flows, and subsequent overproduction of stars and heavy elements at the centre. Observed radial ICM temperature profiles can be matched, except for the gradual decline in temperature inside . Metal enrichment of the ICM gives rise to somewhat steep inner iron gradients; yet, the global level of enrichment compares well to observational estimates when a top-heavy IMF is adopted, and after correcting for the stars formed at late times at the base of the cooling flows, the metal partition between stars and ICM gets into good agreement with observations. The overall abundance and profile of iron in the ICM is found essentially unchanged from to present time. Finally, the of the gas is found to increase steadily with radius, decreasing over time. [ABSTRACT FROM AUTHOR]