154 research outputs found

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

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 $z\le
0.05$ in the mass range of [0.3-1]$\times10^{14}\,h^{-1}M_{\odot}$ 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

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

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

### On the Universality of the Bound-Zone Peculiar Velocity Profile

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

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

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

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

We study the mean tidal coherence of galaxy environments as a function of
intrinsic luminosity determined by the absolute $r$-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 $2$
and $30\,h^{-1}$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?

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

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

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

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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