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We show that ς, the radial location of the minimum in the differential radial mass profile M′(r) of a galaxy cluster, can probe the theory of gravity. We derived M′(r) of the dark matter halos of galaxy clusters from N-body cosmological simulations that implement two different theories of gravity: standard gravity in the ΛCDM model, and f(R). We extracted 49 169 dark matter halos in 11 redshift bins in the range 0 ≤ z ≤ 1 and in three different mass bins in the range 0.9 < M200c/1014 h−1 M⊙ < 11. We investigated the correlation of ς with the redshift and the mass accretion rate (MAR) of the halos. We show that ς decreases from ∼3R200c to ∼2R200c when z increases from 0 to 1 in the ΛCDM model. At z ∼ 0.1, ς decreases from 2.8R200c to ∼2.5R200c when the MAR increases from ∼104 h−1 M⊙ yr−1 to ∼2 × 105 h−1 M⊙ yr−1. In the f(R) model, ς is ∼15% larger than in ΛCDM. The median test shows that for samples of ≳400 dark matter halos at z ≤ 0.8, ς is able to distinguish between the two theories of gravity with a p-value ≲10−5. Upcoming advanced spectroscopic and photometric programs will allow a robust estimation of the mass profile of enormous samples of clusters up to large clustercentric distances. These samples will allow us to statistically exploit ς as probe of the theory of gravity, which complements other large-scale probes. [ABSTRACT FROM AUTHOR]

We measure the central stellar velocity dispersion function for quiescent galaxies in a set of nine northern clusters in the redshift range 0.18 < z < 0.29 and with strong lensing arcs in Hubble Space Telescope images. The velocity dispersion function links galaxies directly to their dark matter halos. From dense SDSS and MMT/Hectospec spectroscopy, we identify 222–463 spectroscopic members in each cluster. We derive physical properties of cluster members including redshift, , and central stellar velocity dispersion and we include a table of these measurements for 3419 cluster members. We construct the velocity dispersion functions for quiescent galaxies with > 1.5 and within R200. The cluster velocity dispersion functions all show excesses at σ ≳ 250 km s−1 compared to the field velocity dispersion function. The velocity dispersion function slope at large velocity dispersion (σ > 160 km s−1) is steeper for more massive clusters, consistent with the trend observed for cluster luminosity functions. The spatial distribution of galaxies with large velocity dispersion at radii larger than R200 further underscores the probable major role of dry mergers in the growth of massive cluster galaxies during cluster assembly. [ABSTRACT FROM AUTHOR]

The Andromeda galaxy is observed to have a system of two large dwarf ellipticals and ~13 smaller satellite galaxies that are currently corotating in a thin plane, in addition to 2 counterrotating satellite galaxies. We explored the consistency of those observations with a scenario where the majority of the corotating satellite galaxies originated from a subhalo group, where NGC 205 was the host and the satellite galaxies occupied dark matter sub-subhaloes. We ran N-body simulations of a close encounter between NGC 205 and M31. In the simulations, NGC 205 was surrounded by massless particles to statistically sample the distribution of the subsubhaloes expected in a subhalo that has a mass similar to NGC 205. We made Monte Carlo samplings and found that, using a set of reference parameters, the probability of producing a thinner distribution of sub-subhaloes than the observed NGC 205 + 15 smaller satellites (thus including the two counter-rotators, but excluding M32) increased from <10-8 for the initial distribution to ~10-2 at pericentre. The probability of the simulated sub-subhaloes occupying the locations of the observed corotating satellites in the line-of-sight velocity versus projected on-sky distance plane is at most 2 x 10-3 for 11 out of 13 satellites. Increasing the mass of M31 and the extent of the initial distribution of sub-subhaloes gives a maximum probability of 4 x 10-3 for all 13 corotating satellites, but the probability of producing the thinness would drop to ~10-3. [ABSTRACT FROM AUTHOR]

Abell 383 is a famous rich cluster (z = 0.1887) imaged extensively as a basis for intensive strong- and weak-lensing studies. Nonetheless, there are few spectroscopic observations. We enable dynamical analyses by measuring 2360 new redshifts for galaxies with rPetro ⩽ 20.5 and within 50′ of the Brightest Cluster Galaxy (BCG; R.A.2000 = 42.°014125, decl.2000 = –03.°529228). We apply the caustic technique to identify 275 cluster members within 7 h–1 Mpc of the hierarchical cluster center. The BCG lies within –11 ± 110 km s–1 and 21 ± 56 h–1 kpc of the hierarchical cluster center; the velocity dispersion profile of the BCG appears to be an extension of the velocity dispersion profile based on cluster members. The distribution of cluster members on the sky corresponds impressively with the weak-lensing contours of Okabe et al. especially when the impact of foreground and background structure is included. The values of R200 = 1.22 ± 0.01 h–1 Mpc and M200 = (5.07 ± 0.09) × 1014h–1M☼ obtained by application of the caustic technique agree well with recent completely independent lensing measures. The caustic estimate extends direct measurement of the cluster mass profile to a radius of ∼5 h–1 Mpc. [ABSTRACT FROM AUTHOR]

The caustic technique measures themass of galaxy clusters in both their virial and infall regions and, as a byproduct, yields the list of cluster galaxy members. Here we use 100 galaxy clusters with mass M200 ⩾ 1014 h -1M☉ extracted from a cosmological N-body simulation of a ⋀CDM universe to test the ability of the caustic technique to identify the cluster galaxy members.We identify the true three-dimensional members as the gravitationally bound galaxies. The caustic technique uses the caustic location in the redshift diagram to separate the cluster members from the interlopers. We apply the technique to mock catalogs containing 1000 galaxies in the field of view of 12 h -1 Mpc on a side at the cluster location. On average, this sample size roughly corresponds to 180 real galaxy members within 3r200, similar to recent redshift surveys of cluster regions. The caustic technique yields a completeness, the fraction of identified true members, fc = 0.95 ± 0.03, within 3r200. The contamination, the fraction of interlopers in the observed catalog of members, increases from fi = 0.020+0.046 -0.015 at r200 to fi = 0.08+0.11 -0.05 at 3r200. No other technique for the identification of the members of a galaxy cluster provides such large completeness and small contamination at these large radii. The caustic technique assumes spherical symmetry and the asphericity of the cluster is responsible for most of the spread of the completeness and the contamination. By applying the technique to an approximately spherical system obtained by stacking the individual clusters, the spreads decrease by at least a factor of two. We finally estimate the cluster mass within 3r200 after removing the interlopers: for individual clusters, the mass estimated with the virial theorem is unbiased and within 30% of the actual mass; this spread decreases to less than 10% for the spherically symmetric stacked cluster. [ABSTRACT FROM AUTHOR]

ABSTRACT We present a new particle mesh cosmological N-body code for accurately solving the modified Poisson equation of the quasi-linear formulation of modified Newtonian dynamics (MOND). We generate initial conditions for the cosmological model, which is identical to Λ cold dark matter (ΛCDM) except that the CDM is switched for a single species of thermal sterile neutrinos. We set the initial conditions at z= 250 for a (512 Mpc h−1)3 box with 2563 particles, and we evolve them down to z= 0. We clearly demonstrate the ability of MOND to develop the large-scale structure in a hot dark matter cosmology and contradict the naive expectation that MOND cannot form galaxy clusters. We find that the correct order of magnitude of X-ray clusters (with TX > 4.5 keV) can be formed, but that we overpredict the number of very rich clusters and seriously underpredict the number of lower mass clusters. We present evidence that suggests the density profiles of our simulated clusters are compatible with those of the observed X-ray clusters in MOND. As a last test, we computed the relative velocity between pairs of haloes within 10 Mpc and find that pairs with velocities larger than 3000 km s−1, like the bullet cluster, can form without difficulty. [ABSTRACT FROM AUTHOR]

We run adiabatic N-body/hydrodynamical simulations of isolated self-gravitating gas clouds to test whether conformal gravity, an alternative theory to general relativity, is able to explain the properties of X-ray galaxy clusters without resorting to dark matter. We show that the gas clouds rapidly reach equilibrium with a density profile which is well fitted by a β-model whose normalization and slope are in approximate agreement with observations. However, conformal gravity fails to yield the observed thermal properties of the gas cloud: (i) the mean temperature is at least an order of magnitude larger than the observed and (ii) the temperature profiles increase with the square of the distance from the cluster centre, in clear disagreement with real X-ray clusters. These results depend on a gravitational potential whose parameters reproduce the velocity rotation curves of spiral galaxies. However, this parametrization stands on an arbitrarily chosen conformal factor. It remains to be seen whether a different conformal factor, specified by a spontaneous breaking of the conformal symmetry, can reconcile this theory with observations. [ABSTRACT FROM AUTHOR]

We compare the properties of galaxy groups extracted from the Updated Zwicky Catalogue (UZC) with those of groups extracted from N-body simulations of the local Universe, in a Λ cold dark matter (CDM) and a τCDM cosmology. In the simulations, the initial conditions of the dark matter density field are set to reproduce the present time distribution of the galaxies within from the Milky Way. These initial conditions minimize the uncertainty originated by cosmic variance, which has affected previous analyses of this small volume of the Universe. The simulations also model the evolution of the photometric properties of the galaxy population with semi-analytic prescriptions. The models yield a galaxy luminosity function sensibly different from that of the UZC and are unable to reproduce the distribution of groups and their luminosity content. The discrepancy between the model and the UZC reduces substantially, if we redistribute the luminosity among the galaxies in the simulation according to the UZC luminosity function while preserving the galaxy luminosity rank. The modified ΛCDM model provides the best match to the UZC: the abundances of groups by harmonic radius, velocity dispersion, mass and luminosity are consistent with observations. We find that this model also reproduces the halo occupation number of groups and clusters. However, the large-scale distribution of groups is marginally consistent with the UZC and the redshift-space correlation function of galaxies on scales larger than is still more than 3σ smaller than observed. We conclude that reproducing the properties of the observed groups certainly requires a more sophisticated treatment of galaxy formation, and possibly an improvement of the dark matter model. [ABSTRACT FROM AUTHOR]

We study the formation and evolution of voids in the dark matter distribution using various simulations of the popularΛ cold dark matter cosmogony. We identify voids by requiring them to be regions of space with a mean overdensity of−0.8 or less– roughly the equivalent of using a spherical overdensity group finder for haloes. Each of the simulations contains thousands of voids. The distribution of void sizes in the different simulations shows good agreement when differences in particle and grid resolution are accounted for. Voids very clearly correspond to minima in the smoothed initial density field. Apart from a very weak dependence on the mass resolution, the rescaled mass profiles of voids in the different simulations agree remarkably well. We find a universal void mass profile of the form, wherereff is the effective radius of a void and. The mass function of haloes in voids is steeper than that of haloes that populate denser regions. In addition, the abundances of void haloes seem to evolve somewhat more strongly between redshifts∼1 and 0 than the global abundances of haloes. [ABSTRACT FROM AUTHOR]

We precisely constrain the inner mass profile of A2261 (z = 0.225) for the first time and determine that this cluster is not “overconcentrated” as found previously, implying a formation time in agreement with ΛCDM expectations. These results are based on multiple strong-lensing analyses of new 16-band Hubble Space Telescope imaging obtained as part of the Cluster Lensing and Supernova survey with Hubble. Combining this with revised weak-lensing analyses of Subaru wide-field imaging with five-band Subaru + KPNO photometry, we place tight new constraints on the halo virial mass Mvir = (2.2 ± 0.2) × 1015M☼h–170 (within rvir ≈ 3 Mpc h–170) and concentration cvir = 6.2 ± 0.3 when assuming a spherical halo. This agrees broadly with average c(M, z) predictions from recent ΛCDM simulations, which span 5 ≲ 〈 c〉 ≲ 8. Our most significant systematic uncertainty is halo elongation along the line of sight (LOS). To estimate this, we also derive a mass profile based on archival Chandra X-ray observations and find it to be ∼35% lower than our lensing-derived profile at r2500 ∼ 600 kpc. Agreement can be achieved by a halo elongated with a ∼2:1 axis ratio along our LOS. For this elongated halo model, we find Mvir = (1.7 ± 0.2) × 1015M☼h–170 and cvir = 4.6 ± 0.2, placing rough lower limits on these values. The need for halo elongation can be partially obviated by non-thermal pressure support and, perhaps entirely, by systematic errors in the X-ray mass measurements. We estimate the effect of background structures based on MMT/Hectospec spectroscopic redshifts and find that these tend to lower Mvir further by ∼7% and increase cvir by ∼5%. [ABSTRACT FROM AUTHOR]