371 research outputs found
Detection of Non-Random Galaxy Orientations in X-ray Subclusters of the Coma Cluster
This study on the Coma cluster suggests that there are deviations from a
completely random galaxy orientation on small scales. Since we found a
significant coincidence of hot-gas features identified in the latest X-ray
observations of Coma with these local anisotropies, they may indicate regions
of recent mutual interaction of member galaxies within subclusters which are
currently falling in on the main cluster.Comment: 4 pages, 4 figures, 3 tables v2: Rewritten introduction, amendments
to the 'Interpretation' sectio
Weak lensing surveys and the intrinsic correlation of galaxy ellipticities
We explore the possibility that an intrinsic correlation between galaxy
ellipticities arising during the galaxy formation process may account for part
of the shear signal recently reported by several groups engaged in weak lensing
surveys. Using high resolution N-body simulations we measure the projected
ellipticities of dark matter halos and their correlations as a function of pair
separation. With this simplifying, but not necessarily realistic assumption
(halo shapes as a proxy for galaxy shapes), we find a positive detection of
correlations up to scales of at least 20 h^-1mpc (limited by the box size). The
signal is not strongly affected by variations in the halo finding technique, or
by the resolution of the simulations. We translate our 3d results into angular
measurements of ellipticity correlation functions and shear variance which can
be directly compared to observations. We also measure similar results from
simulated angular surveys made by projecting our simulation boxes onto the
plane of the sky and applying a radial selection function. Interestingly, the
shear variance we measure is a small, but not entirely negligible fraction
(from ~10-20 %) of that seen by the observational groups, and the ellipticity
correlation functions approximately mimic the functional form expected to be
caused by weak lensing. The amplitude depends on the width in redshift of the
galaxy distribution. If photometric redshifts are used to pick out a screen of
background galaxies with a small width, then the intrinsic correlation may
become comparable to the weak lensing signal. Although we are dealing with
simulated dark matter halos, whether there is a signal from real galaxies could
be checked with a nearby sample with known redshifts.Comment: 12 pages, 11 ps figures, emulateapj.sty, submitted to Ap
Tidal Torques and the Orientation of Nearby Disk Galaxies
We use numerical simulations to investigate the orientation of the angular
momentum axis of disk galaxies relative to their surrounding large scale
structure. We find that this is closely related to the spatial configuration at
turnaround of the material destined to form the galaxy, which is often part of
a coherent two-dimensional slab criss-crossed by filaments. The rotation axis
is found to align very well with the intermediate principal axis of the inertia
momentum tensor at this time. This orientation is approximately preserved
during the ensuing collapse, so that the rotation axis of the resulting disk
ends up lying on the plane traced by the protogalactic material at turnaround.
This suggests a tendency for disks to align themselves so that their rotation
axis is perpendicular to the minor axis of the structure defined by surrounding
matter. One example of this trend is provided by our own Galaxy, where the
Galactic plane is almost at right angles with the supergalactic plane (SGP)
drawn by nearby galaxies; indeed, the SGP latitude of the North Galactic Pole
is just 6 degrees. We have searched for a similar signature in catalogs of
nearby disk galaxies, and find a significant excess of edge-on spirals (for
which the orientation of the disk rotation axis may be determined
unambiguously) highly inclined relative to the SGP. This result supports the
view that disk galaxies acquire their angular momentum as a consequence of
early tidal torques acting during the expansion phase of the protogalactic
material.Comment: 5 pages, 2 figures, accepted for publication in ApJ
Some remarks on the angular momenta of galaxies, their clusters and superclusters
We discuss the relation between angular momenta and masses of galaxy
structures base on the Li model of the universe with global rotation. In our
previous paper (God{\l}owski et al 2002) it was shown that the model predicts
the presence of a minimum in this relation. In the present paper we discuss
observational evidence allowing us to verify this relation. We find null
angular momentum J=0 for the masses corresponding to mass of galaxy grups and
non-vanishing angular momenta for other galactic structures. We check these
theoretical predictions analysing Tully's galaxy grups. The existing data
comparing alignment in different galactic structure are consistent with
obtained theoretical relation if we interpret the groving alignment as
the galactic increasing angular momenta in the galactic structure.Comment: 20 pages 1 figure. GRG accepte
Analytic provenance as constructs of behavioural markers for externalizing thinking processes in criminal intelligence analysis
Studying how analysts use interaction in visualization systems is an important part of evaluating how well these interactions support analysis needs, like generating insights or performing tasks. Analytic Provenance commonly known as interaction histories contains information about the sequence of choices that analysts make when exploring data or performing a task. This research work presents a compositional reductionist approach as a way of externalizing analystâs thinking processes by using markers of analytical behaviour extracted from such interaction histories. Set of Behavioural Markers (BMs) have been identified through a workshop with domain experts and a systematic literature review to use them as cognitive attributes of imagination, insight, transparency, fluidity and rigour to enhance performance in criminal intelligence analysis. A low level semantic action sequence computation also has been proposed as a detection approach of identified BMs and found from computation that BMs can act as bridge between human cognition and computation through semantic interaction. This research work has addressed problems of existing qualitative experiments to extract these BMs through cognitive task analysis and found that the proposed computational technique can be a supplementary approach for validating experimental results
Einstein Cluster Alignments Revisited
We have examined whether the major axes of rich galaxy clusters tend to point
toward their nearest neighboring cluster. We have used the data of Ulmer,
McMillan, and Kowalski, who used position angles based on X-ray morphology. We
also studied a subset of this sample with updated positions and distances from
the MX Northern Abell Cluster Survey (for rich clusters () with well
known redshifts). A Kolmogorov-Smirnov (KS) test showed no significant signal
for nonrandom angles on any scale Mpc. However, refining the
null hypothesis with the Wilcoxon rank-sum test, we found a high confidence
signal for alignment. Confidence levels increase to a high of 99.997% as only
near neighbors which are very close are considered. We conclude there is a
strong alignment signal in the data, consistent with gravitational instability
acting on Gaussian perturbations.Comment: Minor revisions. To be published in Ap
Cluster Alignments and Ellipticities in LCDM Cosmology
The ellipticities and alignments of clusters of galaxies, and their evolution
with redshift, are examined in the context of a Lambda-dominated cold dark
matter cosmology. We use a large-scale, high-resolution N-body simulation to
model the matter distribution in a light cone containing ~10^6 clusters out to
redshifts of z=3. Cluster ellipticities are determined as a function of mass,
radius, and redshift, both in 3D and in projection. We find strong cluster
ellipticities: the mean ellipticity increases with redshift from 0.3 at z=0 to
0.5 at z=3, for both 3D and 2D ellipticities; the evolution is well-fit by
e=0.33+0.05z. The ellipticities increase with cluster mass and with cluster
radius; the main cluster body is more elliptical than the cluster cores, but
the increase of ellipticities with redshift is preserved. Using the fitted
cluster ellipsoids, we determine the alignment of clusters as a function of
their separation. We find strong alignment of clusters for separations <100
Mpc/h; the alignment increases with decreasing separation and with increasing
redshift. The evolution of clusters from highly aligned and elongated systems
at early times to lower alignment and elongation at present reflects the
hierarchical and filamentary nature of structure formation. These measures of
cluster ellipticity and alignment will provide a new test of the current
cosmological model when compared with upcoming cluster surveys.Comment: 29 pages including 13 figures, to appear in ApJ Jan. 2005 (corrected
typos, added reference
Intrinsic Inclination of Galaxies embedded in Cosmic Sheets and its Cosmological Implications: An Analytic Calculation
We investigate analytically a large-scale coherence in the orientation of
galaxies embedded in two-dimensional sheet-like structures in the frame of the
tidal torque theory. Assuming that the galaxy spin and the surrounding matter
fields are intrinsically aligned in accordance with the tidal torque model, we
first derive analytically the probability distribution of the galaxy position
angles, and evaluate the degree of their inclinations relative to the plane of
the sheet. Then, we apply our analytic approach to the nearby spirals in the
Local Super Cluster, and provide theoretical explanations about why and to what
degree the nearby spirals are inclined relative to the supergalatic plane.
Finally, we conclude that the observed large-scale coherence in the orientation
of nearby spirals relative to the supergalactic plane can be quantitatively
understood in terms of galaxy intrinsic alignment predicted by the tidal torque
theory, and that the spins of luminous galaxies might be more strongly aligned
with the surrounding matter than the underlying dark halos. If applied to large
scale surveys like Sloan Digital Sky Survey (SDSS), our analytic approach will
allow us to measure accurately the strength of galaxy intrinsic alignment which
plays a role of statistical error in weak lensing searches and can be used as a
fossil record to reconstruct cosmology.Comment: accepted by the ApJ Letters, revised version, typos and mistakes
corrected,12 pages, 3 figure
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