154 research outputs found

    Estimating the Turn-Around Radii of Six Isolated Galaxy Groups in the Local Universe

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    The estimates of the turn-around radii of six isolated galaxy groups in the nearby universe are presented. From the Tenth Data Release of the Sloan Digital Sky Survey, we first select those isolated galaxy groups at redshifts z≀0.05z\le 0.05 in the mass range of [0.3-1]Γ—1014 hβˆ’1MβŠ™\times10^{14}\,h^{-1}M_{\odot} whose nearest neighbor groups are located at distances larger than fiften times their virial radii. Then, we search for a gravitationally interacting web-like structure around each isolated group, which appears as an inclined streak pattern in the anisotropic spatial distribution of the neighbor field galaxies . Out of 59 isolated groups, only seven are found to possess such web-like structures in their neighbor zones, but one of them turns out to be NGC 5353/4, whose turn-around radius was already measured in the previous work and thus excluded from our analysis. Applying the Turn-around Radius Estimator algorithm devised by Lee et al. to the identified web-like structures of the remaining six target groups, we determine their turn-around radii and show that three out of the six targets have larger turn-around radii than the spherical bound limit predicted by the Planck cosmology. We discuss possible sources of the apparent violations of the three groups, including the underestimated spherical bound-limit due to the approximation of the turn-around mass by the virial mass.Comment: accepted for publication in ApJ, 9 figures, 3 tables, the TRE algorithm refined, significantly improved estimates of the turn-around radii and their error

    Detection of the Vorticity Effect on the Disk Orientations

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    We present an observational evidence for the vorticity effect on the nonlinear evolution of the galaxy angular momentum. We first calculate the vorticity as the curl of the peculiar velocity field reconstructed from the 2MASS redshift survey. Then, measuring the alignments between the vorticity and the tidal shear fields, we study how the alignment strength and tendency depends on the cosmic web environment. It is found that in the knot and filament regions the vorticity vectors are anti-aligned with the directions of the maximal volume compression while in the void and sheet regions they are anti-aligned with the directions of the minimal compression. Determining the spin axes of the nearby large face-on and edge-on disk galaxies from the Seventh Data Release of the Sloan Digital Sky Survey and measuring their correlations with the vorticity vectors at the galaxy positions, we finally detect a clear signal of the alignments between the galaxy spin and the local vorticity fields. The null hypothesis that there is no alignment between them is rejected at the 99.999% confidence level. Our result supports observationally the recently proposed scenario that although the galaxy angular momentum originates from the initial tidal interaction in the linear regime its subsequent evolution is driven primarily by the cosmic vorticity field.Comment: submitted for publication in ApJ, 18 pages, 7 figures, comments welcom

    Normalization of the Matter Power Spectrum via the Ellipticity Function of Giant Galaxy Voids from SDSS DR5

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    The ellipticity function of cosmic voids exhibits strong dependence on the amplitude of the linear matter power spectrum. Analyzing the most recent void catalogs constructed by Foster and Nelson from the fifth data release of the Sloan Digital Sky Survey, we measure observationally the ellipticity function of giant galaxy voids. Then, we incorporate the redshift distortion and galaxy bias effect into the analytic model of the void ellipticity function and fit it to the observational result by adjusting the value of the power-spectrum normalization with the help of the generalized chi^{2}-minimization method. The best-fit normalization of the linear power spectrum is found to be sigma_{8}=0.90+/-0.04. Our result is higher than the WMAP sigma_{8}-value but consistent with that from the recent work of Liu and Li who have constructed a new improved CMB map independently.Comment: submitted for publication in ApJL, 4 pages, 3 figure

    The Effect of Primordial Anti-Biasing on the Local Measurement of the Key Cosmological Parameters

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    The best-fit values of the density parameter and the amplitude of the linear density power spectrum obtained from the Cosmic Microwave Background (CMB) temperature field scanned by the Planck satellite are found to notably disagree with those estimated from the abundance of galaxy clusters observed in the local universe. Basically, the observed cluster counts are significantly lower than the prediction of the standard flat LambdaCDM model with the key cosmological parameters set at the Planck best-fit values. We show that this inconsistency between the local and the early universe can be well resolved without failing the currently favored flat LambdaCDM cosmology if the local universe corresponds to a region embedded in a crest of the primordial gravitational potential field. Incorporating the condition of positive primordial potential into the theoretical prediction for the mass function of cluster halos, we show that the observed lower number densities of the galaxy clusters are in fact fully consistent with the Planck universe.Comment: MNRAS in press, accepted version, physical meaning of the characteristic scale of the primordial potential clarified, improved discussion, figure 3 revised, typos and mistakes correcte

    On the Universality of the Bound-Zone Peculiar Velocity Profile

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    A numerical evidence for the universality of the bound-zone peculiar velocity profile in a LambdaCDM universe is presented. Analyzing the dark matter halo catalogs from the Millennium-II simulation, we determine the average peculiar velocity profiles of the objects located in the bound zones around massive group-size halos at various redshifts and compare them to an analytic formula characterized by two parameters, the amplitude and slope of the profile. The best-fit values of the two parameters are found to be robust against the changes of the mass scales and the key cosmological parameters. It is also found that the amplitude and slope parameters of the bound-zone peculiar velocity profile are constant but only in the limited ranges of redshifts. In the dark matter dominated epoch corresponding to z> 0.6 the two parameters have constant values. In the transition period corresponding to 0.2=< z=< 0.6 when the density of Lambda begins to exceed that of dark matter the two parameters grow almost linearly with redshifts. At later epochs z<0.2 when the Lambda-domination prevails the two parameters regain constancy settling upon higher constant values. Noting that the length of the transition period depends on the amount of Lambda and speculating that the linear evolution of the profile with redshifts in the transition period is a unique feature of the Lambda-dominated universe, we suggest that the redshift evolution of the bound-zone peculiar velocity profile should be a powerful local discriminator of dark energy candidates.Comment: accepted for publication in ApJ, a mistake in the calculation of the bound-zone peculiar velocity profile correcte

    A New Perspective on the Large-Scale Tidal Effect on the Galaxy Luminosity and Morphology

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    We study the mean tidal coherence of galaxy environments as a function of intrinsic luminosity determined by the absolute rr-band magnitude. The tidal coherence of a galaxy environment is estimated as the cosine of the angle between two minor eigenvectors of the tidal field smoothed at the scales of 22 and 30 hβˆ’130\,h^{-1}Mpc centered on each of the local galaxies from the Sloan Digital Sky Data Release 10. Creating four luminosity-selected samples of the Sloan galaxies, we control them to have identical density distributions, in order to nullify the dominant effect of the local density. It is found that the samples containing more luminous wall and field galaxies yield lower mean values of the tidal coherence, which trend turns out to be robust against the variation of the smoothing scales. At fixed morphology, the same trend is found for the late-type spiral and lenticular galaxies in both of the field and wall environments. The early-type spiral field galaxies show no significant dependence on the tidal coherence, while both of the least and most luminous elliptical wall galaxies are found to dwell in the regions with highest tidal coherence.Comment: accepted for publication in ApJ, 8 figures, 1 table, revised version, minor change

    The Misalignments between Matter and Galaxy Distributions in Triaxial Clusters: A Signature of a Possible Fifth Force?

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    The standard structure formation model based on a LCDM cosmology predicts that the galaxy clusters have triaxial shapes and that the cluster galaxies have a strong tendency to be located preferentially along the major axes of host cluster's dark matter distributions due to the gravitational tidal effect. The predicted correlations between dark matter and galaxy distributions in triaxial clusters are insensitive to the initial cosmological parameters and to the galaxy bias, and thus can provide a unique test-bed for the nonlinear structure formation of the LCDM cosmology. Recently, Oguri et al. determined robustly the dark matter distributions in the galaxy clusters using the two dimensional weak lensing shear fitting and showed that the orientations of the cluster galaxy distributions are only very weakly correlated with those of the underlying dark matter distributions determined robustly, which is in contrast to with the LCDM-based prediction. We reanalyze and compare quantitatively the observational result with the LCDM-based prediction from the Millennium Run simulation with the help of the bootstrap resampling and generalized chi^{2}-statistics. The hypothesis that the observational result is consistent with the LCDM-based prediction is ruled out at the 99% confidence level. A local fifth force induced by a non-minimal coupling between dark energy and dark matter might be responsible for the observed misalignments between dark matter and galaxy distributions in triaxial clusters.Comment: submitted to ApJL, 14 pages, 3 figures, comments welcom

    Reconstructing the Intrinsic Triaxial Shape of the Virgo Cluster

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    To use galaxy clusters as a cosmological probe, it is important to account for their triaxiality. Assuming that the triaxial shapes of galaxy clusters are induced by the tidal interaction with the surrounding matter, Lee and Kang recently developed a reconstruction algorithm for the measurement of the axial ratio of a triaxial cluster. We examine the validity of this reconstruction algorithm by performing an observational test of it with the Virgo cluster as a target. We first modify the LK06 algorithm by incorporating the two dimensional projection effect. Then, we analyze the 1275 member galaxies from the Virgo Cluster Catalogue and find the projected direction of the Virgo cluster major axis by measuring the anisotropy in the spatial distribution of the member galaxies in the two dimensional projected plane. Applying the modified reconstruction algorithm to the analyzed data, we find that the axial ratio of the triaxial Virgo cluster is (1: 0.54 : 0.73). This result is consistent with the recent observational report from the Virgo Cluster Survey, proving the robustness of the reconstruction algorithm. It is also found that at the inner radii the shape tends to be more like prolate. We discuss the possible effect of the Virgo cluster triaxiality on the mass estimation.Comment: submitted to ApJ, 16 page, 3 figure

    The Variation of Galaxy Morphological Type with the Shear of Environment

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    Recent N-body simulations have shown that the assembly history of galactic halos depend on the density of large-scale environment. It implies that the galaxy properties like age and size of bulge may also vary with the surrounding large-scale structures, which are characterized by the tidal shear as well as the density. By using a sample of 15,882 well-resolved nearby galaxies from the Tully Catalog and the real space tidal field reconstructed from the 2Mass Redshift Survey (2MRS), we investigate the dependence of galaxy morphological type on the shear of large-scale environment where the galaxies are embedded. We first calculate the large scale dimensionless overdensities (d) and the large-scale ellipticities (e) of the regions where the Tully galaxies are located and classify the Tully galaxies according to their morphological types and create subsamples selected at similar value of d but span different ranges in e. We calculate the mean ellipticity, , averaged over each subsample and find a signal of variation of with galaxy morphological type: For the case of 0.5<=d<=1.0, the ellipticals are found to be preferentially located in the regions with low ellipticity. For the case of -0.3<=d<=0.1, the latest-type spirals are found to be preferentially located in the regions with high ellipticity. The null hypothesis that the mean ellipticities of the regions where the ellipticals and the latest type spirals are located are same as the global mean ellipticity averaged over all types is rejected at 3 sigma level when -0.3<=d<=0.1. Yet, no signal of galaxy-shear correlation is found in the highly overdense/underdense regions. The observed trend suggests that the formation epochs of galactic halos might be a function not only of halo mass and large-scale density but also of large-scale shear.Comment: ApJ in press, accepted version, analysis improved, new discussion adde

    Bullet Cluster: A Challenge to LCDM Cosmology

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    To quantify how rare the bullet-cluster-like high-velocity merging systems are in the standard LCDM cosmology, we use a large-volume 27 (Gpc/h)^3 MICE simulation to calculate the distribution of infall velocities of subclusters around massive main clusters. The infall-velocity distribution is given at (1-3)R_{200} of the main cluster (where R_{200} is similar to the virial radius), and thus it gives the distribution of realistic initial velocities of subclusters just before collision. These velocities can be compared with the initial velocities used by the non-cosmological hydrodynamical simulations of 1E0657-56 in the literature. The latest parameter search carried out recently by Mastropietro and Burkert showed that the initial velocity of 3000 km/s at about 2R_{200} is required to explain the observed shock velocity, X-ray brightness ratio of the main and subcluster, and displacement of the X-ray peaks from the mass peaks. We show that such a high infall velocity at 2R_{200} is incompatible with the prediction of a LCDM model: the probability of finding 3000 km/s in (2-3)R_{200} is between 3.3X10^{-11} and 3.6X10^{-9}. It is concluded that the existence of 1E0657-56 is incompatible with the prediction of a LCDM model, unless a lower infall velocity solution for 1E0657-56 with < 1800 km/s at 2R_{200} is found.Comment: accepted for publication in ApJ, light-cone effect discussed, minor typos corrected, 22 pages, 6 figures, 3 table
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