33 research outputs found
The context of the Local Volume: structures and motions in the nearby universe
The 6dF Galaxy Survey (6dFGS) and the 2MASS Redshift Survey (2MRS) provide
the most complete maps of the large-scale structures and motions in the nearby
universe. These maps have been used to reconstruct the density field in the
local volume, and to predict the corresponding velocity field and the dipole of
the Local Group motion.Comment: 4 pages, to appear in "Galaxies in the Local Volume", 2008, eds B.
Koribalski and H. Jerjen, Springer Astrophysics and Space Science Series
(proceedings of conference held in Sydney on 8-13 July 2007
Scale-dependent Galaxy Bias
We present a simple heuristic model to demonstrate how feedback related to
the galaxy formation process can result in a scale-dependent bias of mass
versus light, even on very large scales. The model invokes the idea that
galaxies form initially in locations determined by the local density field, but
the subsequent formation of galaxies is also influenced by the presence of
nearby galaxies that have already formed. The form of bias that results
possesses some features that are usually described in terms of stochastic
effects, but our model is entirely deterministic once the density field is
specified. Features in the large-scale galaxy power spectrum (such as wiggles
that might in an extreme case mimic the effect of baryons on the primordial
transfer function) could, at least in principle, arise from spatial modulations
of the galaxy formation process that arise naturally in our model. We also show
how this fully deterministic model gives rise to apparently stochasticity in
the galaxy distribution.Comment: 14 pages, 2 figures, typos corrected, discussion added and references
corrected; matches version accepted by JCA
Is the misalignment of the Local Group velocity and the 2MASS Redshift Survey dipole typical in a LambdaCDM model?
We predict the acceleration of the Local Group generated by the 2MASS
Redshift Survey within the framework of LambdaCDM and the halo model of
galaxies. We show that as the galaxy fluctuations derived from the halo model
have more power on small scales compared with the mass fluctuations, the
misalignment angle between the CMB velocity vector and the 2MRS dipole is in
reasonable agreement with the observed 21 degrees. This statistical analysis
suggests that it is not necessary to invoke a hypothetical nearby galaxy or a
distant cluster to explain this misalignment.Comment: Extended version, accepted for publication in PRD, 7 pages, 3 figure
"Dark energy" in the Local Void
The unexpected discovery of the accelerated cosmic expansion in 1998 has
filled the Universe with the embarrassing presence of an unidentified "dark
energy", or cosmological constant, devoid of any physical meaning. While this
standard cosmology seems to work well at the global level, improved knowledge
of the kinematics and other properties of our extragalactic neighborhood
indicates the need for a better theory. We investigate whether the recently
suggested repulsive-gravity scenario can account for some of the features that
are unexplained by the standard model. Through simple dynamical considerations,
we find that the Local Void could host an amount of antimatter
() roughly equivalent to the mass of a typical
supercluster, thus restoring the matter-antimatter symmetry. The antigravity
field produced by this "dark repulsor" can explain the anomalous motion of the
Local Sheet away from the Local Void, as well as several other properties of
nearby galaxies that seem to require void evacuation and structure formation
much faster than expected from the standard model. At the global cosmological
level, gravitational repulsion from antimatter hidden in voids can provide more
than enough potential energy to drive both the cosmic expansion and its
acceleration, with no need for an initial "explosion" and dark energy.
Moreover, the discrete distribution of these dark repulsors, in contrast to the
uniformly permeating dark energy, can also explain dark flows and other
recently observed excessive inhomogeneities and anisotropies of the Universe.Comment: 6 pages, accepted as a Letter to the Editor by Astrophysics and Space
Scienc
Local Gravity versus Local Velocity: Solutions for and nonlinear bias
(abridged) We perform a reconstruction of the cosmological large scale flows
in the nearby Universe using two complementary observational sets. The first,
the SFI++ sample of Tully-Fisher (TF) measurements of galaxies, provides a
direct probe of the flows. The second, the whole sky distribution of galaxies
in the 2MASS redshift survey (2MRS), yields a prediction of the flows given the
cosmological density parameter, , and a biasing relation between mass
and galaxies. We aim at an unbiased comparison between the peculiar velocity
fields extracted from the two data sets and its implication on the cosmological
parameters and the biasing relation. We expand the fields in a set of
orthonormal basis functions, each representing a plausible realization of a
cosmological velocity field. Our analysis completely avoids the strong error
covariance in the smoothed TF velocities by the use of orthonormal basis
functions and employs elaborate realistic mock data sets to extensively
calibrate the errors in 2MRS predicted velocities. We relate the 2MRS galaxy
distribution to the mass density field by a linear bias factor, , and
include a luminosity dependent, , galaxy weighting. We assess
the agreement between the fields as a function of and
, where is the growth factor of linear perturbations.
The agreement is excellent with a reasonable per degree of freedom.
For , we derive and ,
respectively, at the 68.3% and 95.4% confidence levels (CLs). For ,
we get and , respectively, at the 68.3% and 95.4%
CLs. We set a constraint on the fluctuation normalization, finding , in very good agreement with the latest WMAP results.Comment: MNRAS accepted versio
SDSS DR7 superclusters. Morphology
We study the morphology of a set of superclusters drawn from the SDSS DR7. We
calculate the luminosity density field to determine superclusters from a flux-
limited sample of galaxies from SDSS DR7, and select superclusters with 300 and
more galaxies for our study. The morphology of superclusters is described with
the fourth Minkowski functional V3, the morphological signature (the curve in
the shapefinder's K1-K2 plane) and the shape parameter (the ratio of the
shapefinders K1/K2). We investigate the supercluster sample using
multidimensional normal mixture modelling, and use Abell clusters to identify
our superclusters with known superclusters and to study the large-scale
distribution of superclusters. The superclusters in our sample form three
chains of superclusters; one of them is the Sloan Great Wall. Most
superclusters have filament-like overall shapes. Superclusters can be divided
into two sets; more elongated superclusters are more luminous, richer, have
larger diameters, and a more complex fine structure than less elongated
superclusters. The fine structure of superclusters can be divided into four
main morphological types: spiders, multispiders, filaments, and multibranching
filaments. We present the 2D and 3D distribution of galaxies and rich groups,
the fourth Minkowski functional, and the morphological signature for all
superclusters. Widely different morphologies of superclusters show that their
evolution has been dissimilar. A study of a larger sample of superclusters from
observations and simulations is needed to understand the morphological variety
of superclusters and the possible connection between the morphology of
superclusters and their large-scale environment.Comment: Comments: 20 pages, 18 figures, accepted for publication in Astronomy
and Astrophysic
Bayesian analysis of cosmic structures
We revise the Bayesian inference steps required to analyse the cosmological
large-scale structure. Here we make special emphasis in the complications which
arise due to the non-Gaussian character of the galaxy and matter distribution.
In particular we investigate the advantages and limitations of the
Poisson-lognormal model and discuss how to extend this work. With the lognormal
prior using the Hamiltonian sampling technique and on scales of about 4 h^{-1}
Mpc we find that the over-dense regions are excellent reconstructed, however,
under-dense regions (void statistics) are quantitatively poorly recovered.
Contrary to the maximum a posteriori (MAP) solution which was shown to
over-estimate the density in the under-dense regions we obtain lower densities
than in N-body simulations. This is due to the fact that the MAP solution is
conservative whereas the full posterior yields samples which are consistent
with the prior statistics. The lognormal prior is not able to capture the full
non-linear regime at scales below ~ 10 h^{-1} Mpc for which higher order
correlations would be required to describe the matter statistics. However, we
confirm as it was recently shown in the context of Ly-alpha forest tomography
that the Poisson-lognormal model provides the correct two-point statistics (or
power-spectrum).Comment: 11 pages, 1 figure, report for the Astrostatistics and Data Mining
workshop, La Palma, Spain, 30 May - 3 June 2011, to appear in Springer Series
on Astrostatistic
Cosmic flows in the nearby universe from Type Ia Supernovae
Peculiar velocities are one of the only probes of very large-scale mass
density fluctuations in the nearby Universe. We present new "minimal variance"
bulk flow measurements based upon the "First Amendment" compilation of 245 Type
Ia supernovae (SNe) peculiar velocities and find a bulk flow of 249 +/- 76 km/s
in the direction l= 319 +/- 18 deg, b = 7 +/- 14 deg. The SNe bulk flow is
consistent with the expectations of \Lambda CDM. However, it is also marginally
consistent with the bulk flow of a larger compilation of non-SNe peculiar
velocities (Watkins, Feldman, & Hudson 2009). By comparing the SNe peculiar
velocities to predictions of the IRAS Point Source Catalog Redshift survey
(PSCz) galaxy density field, we find \Omega_{m}^{0.55} \sigma_{8,lin} = 0.40
+/- 0.07, which is in agreement with \Lambda CDM. However, we also show that
the PSCz density field fails to account for 150 +/- 43 km/s of the SNe bulk
motion.Comment: MNRAS in press, 9 pages, 4 figures, 4 table
The 2dF Galaxy Redshift Survey: Wiener reconstruction of the cosmic web
We reconstruct the underlying density field of the Two-degree Field Galaxy Redshift Survey (2dFGRS) for the redshift range 0.035 < z < 0.200 using the Wiener filtering method. The Wiener filter suppresses shot noise and accounts for selection and incompleteness effects. The method relies on prior knowledge of the 2dF power spectrum of fluctuations and the combination of matter density and bias parameters, however the results are only slightly affected by changes to these parameters. We present maps of the density field. We use a variable smoothing technique with two different effective resolutions: 5 and 10 h−1 Mpc at the median redshift of the survey. We identify all major superclusters and voids in the survey. In particular, we find two large superclusters and two large local voids. The full set of colour maps can be viewed on the World Wide Web a
The 2MASS Redshift Survey - Description and Data Release
We present the results of the 2MASS Redshift Survey (2MRS), a ten-year
project to map the full three-dimensional distribution of galaxies in the
nearby Universe. The 2 Micron All-Sky Survey (2MASS) was completed in 2003 and
its final data products, including an extended source catalog (XSC), are
available on-line. The 2MASS XSC contains nearly a million galaxies with Ks <=
13.5 mag and is essentially complete and mostly unaffected by interstellar
extinction and stellar confusion down to a galactic latitude of |b|=5 deg for
bright galaxies. Near-infrared wavelengths are sensitive to the old stellar
populations that dominate galaxy masses, making 2MASS an excellent starting
point to study the distribution of matter in the nearby Universe.
We selected a sample of 44,599 2MASS galaxies with Ks =5
deg (>= 8 deg towards the Galactic bulge) as the input catalog for our survey.
We obtained spectroscopic observations for 11,000 galaxies and used
previously-obtained velocities for the remainder of the sample to generate a
redshift catalog that is 97.6% complete to well-defined limits and covers 91%
of the sky. This provides an unprecedented census of galaxy (baryonic mass)
concentrations within 300 Mpc.
Earlier versions of our survey have been used in a number of publications
that have studied the bulk motion of the Local Group, mapped the density and
peculiar velocity fields out to 50 Mpc, detected galaxy groups, and estimated
the values of several cosmological parameters.
Additionally, we present morphological types for a nearly-complete sub-sample
of 20,860 galaxies with Ks = 10 deg.Comment: Accepted for publication in The Astrophysical Journal Supplement
Series. The 2MRS catalogs and a version of the paper with higher-resolution
figures can be found at http://tdc-www.harvard.edu/2mrs