47 research outputs found
Compatibility of the large quasar groups with the concordance cosmological model
We study the compatibility of large quasar groups with the concordance cosmological model. Large quasar groups are very large spatial associations of quasars in the cosmic web, with sizes of 50â250 hâ1 Mpc. In particular, the largest large quasar group known, named Huge-LQG, has a longest axis of âŒ860 hâ1 Mpc, larger than the scale of homogeneity (âŒ260 Mpc), which has been noted as a possible violation of the cosmological principle. Using mock catalogues
constructed from the Horizon Run 2 cosmological simulation, we found that large quasar groups size, quasar member number and mean overdensity distributions in the mocks agree with observations. The Huge-LQG is found to be a rare group with a probability of 0.3 per cent of finding a group as large or larger than the observed, but an extreme value analysis shows that it is an expected maximum in the sample volume with a probability of 19 per cent of observing a largest quasar group as large or larger than Huge-LQG. The Huge-LQG is expected to be the largest structure in a volume at least 5.3 ± 1 times larger than the one currently studied
Seeking the Local Convergence Depth. I. TF Observations of the Clusters A168, A397, A569, A1139, A1228, and A1983
We present first results of an all-sky observing program designed to improve
the quality of the I band Tully-Fisher (TF) template and to obtain the reflex
motion of the Local Group with respect to clusters to z = 0.06. We are
obtaining between 5 and 15 TF measurements per cluster on a sample of 50
clusters at intermediate redshifts (0.02 < z < 0.06). Presentation of the data
for seven Abell clusters of galaxies is given here. This data incorporates
methods for estimating the true inclination of a spiral disk, an observational
parameter undervalued for small angular-sized galaxies or for galaxies observed
in poor seeing conditions.Comment: 21 pages, uses AAS LaTeX, 3 tables and 8 postscript figures (only
first page of fig. 6 included in this version); to appear in the Astronomical
Journa
Seeking the Local Convergence Depth. The Abell Cluster Dipole Flow to 200 Mpc/h
We have obtained new Tully-Fisher (TF) peculiar velocity measurements for 52
Abell galaxy clusters distributed throughout the sky between ~ 50 and 200
Mpc/h.The measurements are based on I band photometry and optical rotation
curves for a sample of 522 spiral galaxies, from which an accurate TF template
relation has been constructed. Individual cluster TF relations are referred to
the template to compute cluster peculiar motions.
The reflex motion of the Local Group of galaxies is measured with respect to
the reference frame defined by our cluster sample and the distant portion of
the Giovanelli et al. (1998) cluster set. We find the Local Group motion in
this frame to be 565+/-113 km/s in the direction (l,b)=(267,26)+/-10 when
peculiar velocities are weighted according to their errors. After optimizing
the dipole calculation to sample equal volumes equally, the vector is 509+/-195
km/s towards (255,33)+/-22. Both solutions agree, to within 1-sigma or better,
with the Local Group motion as inferred from the cosmic microwave background
(CMB) dipole. Thus, the cluster sample as a whole moves slowly in the CMB
reference frame, its bulk flow being at most 200 km/s.Comment: 11 pages, uses AAS LaTeX; to appear in the Astrophysical Journal
Letter
No Hubble Bubble in the Local Universe
Zehavi et al. (1998) have suggested that the Hubble flow within 70/h Mpc may
be accelerated by the existence of a void centered on the Local Group. Its
underdensity would be ~20 %, which would result in a local Hubble distortion of
about 6.5 %. We have combined the peculiar velocity data of two samples of
clusters of galaxies, SCI and SCII, to investigate the amplitude of Hubble
distortions to 200/h Mpc. Our results are not supportive of that conclusion.
The amplitude of a possible distortion in the Hubble flow within 70/h Mpc in
the SCI+SCII merged data is 0.010\pm0.022. The largest, and still quite
marginal, geocentric deviation from smooth Hubble flow consistent with that
data set is a shell with (Delta H)/H =0.027\pm0.023, centered at hd = 101 Mpc
and extending over some 30/h Mpc. Our results are thus consistent with a Hubble
flow that, on distances in excess of about 50/h Mpc, is remarkably smooth.Comment: 11 pages, 1 tables, 1 figure; uses AAS LaTex; to appear in ApJ Nov 9
Weak lensing mass distributions for 24 X-Ray Abell Clusters
We use the weak gravitational lensing effect to study the mass distribution
and dynamical state of a sample of 24 X-ray luminous clusters of galaxies
() observed with the VLT-FORS1 under homogeneous sky conditions
and subarsecond image quality. The galaxy shapes were measured after
deconvolution with a locally determined point-spread-function, while the
two-dimensional mass distributions of the clusters were computed using an
algorithm based on the maximum entropy method. By comparing the mass and light
distributions of the clusters in our sample, we find that their mass centers,
for the majority of the clusters, is consistent with the positions of optical
centers. We find that some clusters present significant mass substructures
which generally have optical counterparts. At least in one cluster (Abell
1451), we detect a mass substructure without an obvious luminous counterpart.
The radial profile of the shear of the clusters was fitted using circular and
elliptical isothermal elliptical distributions, which allowed the finding of a
strong correlation between the orientation of the major-axis of the matter
distribution and the corresponding major-axes of the brightest cluster galaxy
light-profiles. Estimates of how close to dynamical relaxation are these
clusters were obtained through comparison of our weak-lensing mass measurements
with the x-ray and velocity dispersion determinations available in the
literature. We find that clusters with intra-cluster gas colder than 8 keV show
a good agreement between the different mass determinations, but clusters with
gas hotter than 8 keV present discrepant mass values.Comment: 23 pages, 8 figures. Submitted to ApJ A version of this paper with
all the figures of the appendix A can be found at
http://www.astro.iag.usp.br/~eduardo/shear.ps.g
Shrinking of Cluster Ellipticals: a Tidal Stripping explanation and Implications for the Intra-Cluster Light
We look for evidence of tidal stripping in elliptical galaxies through the
analysis of homogeneous CCD data corresponding to a sample of 228 elliptical
galaxies belonging to 24 clusters of galaxies at . We
investigate departures from the standard magnitude-isophotal size relation, as
a function of environmental (cluster-centric distance, local galaxy density)
and structural (cluster velocity dispersion, Bautz-Morgan type) properties. We
find that, for any particular galaxy luminosity, the ellipticals in the inner
and denser regions of the clusters are about 5% smaller than those in the outer
regions, which is in good agreement with the finding of Strom & Strom (1978)
based on photographic photometry. The null hypothesis (ie., galaxy sizes are
independent of the cluster-centric distance or density) is rejected at a
significance level of better than 99.7%. Numericals models of Aguilar & White
(1986) predict that tidal stripping can lead to changes in the whole structure
of ellipticals producing shrinkage and brightening of the galaxy, qualitatively
consistent with our measurements and also with the findings of Trujillo et al.
(2002), that more centrally concentrated ellipticals populate denser regions.
Our observational results can be interpreted as evidence for stripping of stars
from ellipticals in the central/denser regions of clusters, contributing to the
intra-cluster light observed in these structures.Comment: AJ Accepted, 15 pages, 9 figure