117 research outputs found
Reconstructing cosmological initial conditions from galaxy peculiar velocities. II. The effect of observational errors
The Reverse Zeldovich Approximation (RZA) is a reconstruction method which
allows to estimate the cosmic displacement field from galaxy peculiar velocity
data and to constrain initial conditions for cosmological simulations of the
Local Universe. In this paper, we investigate the effect of different
observational errors on the reconstruction quality of this method. For this, we
build a set of mock catalogues from a cosmological simulation, varying
different error sources like the galaxy distance measurement error (0 - 20%),
the sparseness of the data points, and the maximum catalogue radius (3000 -
6000 km/s). We perform the RZA reconstruction of the initial conditions on
these mock catalogues and compare with the actual initial conditions of the
simulation. We also investigate the impact of the fact that only the radial
part of the peculiar velocity is observationally accessible. We find that the
sparseness of a dataset has the highest detrimental effect on RZA
reconstruction quality. Observational distance errors also have a significant
influence, but it is possible to compensate this relatively well with Wiener
Filter reconstruction. We also investigate the effect of different object
selection criteria and find that distance catalogues distributed randomly and
homogeneously across the sky (such as spiral galaxies selected for the
Tully-Fisher method) allow for a higher reconstruction quality than if when
data is preferentially drawn from massive objects or dense environments (such
as elliptical galaxies). We find that the error of estimating the initial
conditions with RZA is always dominated by the inherent non-linearity of data
observed at z=0 rather than by the combined effect of the observational errors.
Even an extremely sparse dataset with high observational errors still leads to
a good reconstruction of the initial conditions on a scale of about 5 Mpc/h.Comment: Accepted for MNRAS 2012 december 1
Reconstructing cosmological initial conditions from galaxy peculiar velocities. I. Reverse Zeldovich Approximation
We propose a new method to recover the cosmological initial conditions of the
presently observed galaxy distribution, which can serve to run constrained
simulations of the Local Universe. Our method, the Reverse Zeldovich
Approximation (RZA), can be applied to radial galaxy peculiar velocity data and
extends the previously used Constrained Realizations (CR) method by adding a
Lagrangian reconstruction step. The RZA method consists of applying the
Zeldovich approximation in reverse to galaxy peculiar velocities to estimate
the cosmic displacement field and the initial linear matter distribution from
which the present-day Local Universe evolved.We test our method with a mock
survey taken from a cosmological simulation. We show that the halo peculiar
velocities at z = 0 are close to the linear prediction of the Zeldovich
approximation, if a grouping is applied to the data to remove virial motions.
We find that the addition of RZA to the CR method significantly improves the
reconstruction of the initial conditions. The RZA is able to recover the
correct initial positions of the velocity tracers with a median error of only
1.36 Mpc/h in our test simulation. For realistic sparse and noisy data, this
median increases to 5 Mpc/h. This is a significant improvement over the
previous approach of neglecting the displacement field, which introduces errors
on a scale of 10 Mpc/h or even higher. Applying the RZA method to the upcoming
high-quality observational peculiar velocity catalogues will generate much more
precise constrained simulations of the Local Universe.Comment: Accepted for MNRAS 2012 December 1
Cosmic Bulk Flow and the Local Motion from Cosmicflows-2
Full sky surveys of peculiar velocity are arguably the best way to map the
large scale structure out to distances of a few times 100 Mpc/h. Using the
largest and most accurate ever catalog of galaxy peculiar velocities
"Cosmicflows-2", the large scale structure has been reconstructed by means of
the Wiener filter and constrained realizations assuming as a Bayesian prior
model the LCDM model with the WMAP inferred cosmological parameters. The
present paper focuses on studying the bulk flow of the local flow field,
defined as the mean velocity of top-hat spheres with radii ranging out to R=500
Mpc/h. The estimated large scale structures, in general, and the bulk flow, in
particular, are determined by the tension between the observational data and
the assumed prior model. A prerequisite for such an analysis is the requirement
that the estimated bulk flow is consistent with the prior model. Such a
consistency is found here. At R=50(150) Mpc/h the estimated bulk velocity is
250+/-21 (239+/-38) km/s. The corresponding cosmic variance at these radii is
126(60)km/s, which implies that these estimated bulk flows are dominated by the
data and not by the assumed prior model. The estimated bulk velocity is
dominated by the data out to R~200 Mpc/h, where the cosmic variance on the
individual Supergalactic Cartesian components (of the r.m.s. values) exceeds
the variance of the Constrained Realizations by at least a factor of 2. The
supergalactic SGX and SGY components of the CMB dipole velocity are recovered
by the Wiener filter velocity field down to a very few km/s. The SGZ component
of the estimated velocity, the one that is most affected by the Zone of
Avoidance, is off by 126 km/s (an almost 2 sigma discrepancy).Comment: 10 pages, accepted for MNRA
Goodness-of-fit analysis of the Cosmicflows-2 database of velocities
The goodness-of-fit (GoF) of the Cosmicflows-2 (CF2) database of peculiar
velocities with the LCDM standard model of cosmology is presented. Standard
application of the Chi^2 statistics of the full database, of its 4,838 data
points, is hampered by the small scale nonlinear dynamics which is not
accounted for by the (linear regime) velocity power spectrum. The bulk velocity
constitutes a highly compressed representation of the data which filters out
the small scales non-linear modes. Hence the statistics of the bulk flow
provides an efficient tool for assessing the GoF of the data given a model. The
particular approach introduced here is to use the (spherical top-hat window)
bulk velocity extracted from the Wiener filter reconstruction of the 3D
velocity field as a linear low pass filtered highly compressed representation
of the CF2 data. An ensemble 2250 random linear realizations of the WMAP/LCDM
model has been used to calculate the bulk velocity auto-covariance matrix. We
find that the CF2 data is consistent with the WMAP/LCDM model to better than
the 2 sigma confidence limits. This provides a further validation that the CF2
database is consistent with the standard model of cosmology.Comment: submitted to MNRAS, V2 : solved page sizing proble
The Arrowhead Mini-Supercluster of Galaxies
Superclusters of galaxies can be defined kinematically from local evaluations
of the velocity shear tensor. The location where the smallest eigenvalue of the
shear is positive and maximal defines the center of a basin of attraction.
Velocity and density fields are reconstructed with Wiener Filter techniques.
Local velocities due to the density field in a restricted region can be
separated from external tidal flows, permitting the identification of
boundaries separating inward flows toward a basin of attraction and outward
flows. This methodology was used to define the Laniakea Supercluster that
includes the Milky Way. Large adjacent structures include Perseus-Pisces, Coma,
Hercules, and Shapley but current kinematic data are insufficient to capture
their full domains. However there is a small region trapped between Laniakea,
Perseus-Pisces, and Coma that is close enough to be reliably characterized and
that satisfies the kinematic definition of a supercluster. Because of its
shape, it is given the name the Arrowhead Supercluster. This entity does not
contain any major clusters. A characteristic dimension is ~25 Mpc and the
contained mass is only ~10^15 Msun.Comment: Accepted for publication in The Astrophysical Journal. Video can be
viewed at http://irfu.cea.fr/arrowhea
Filaments from the galaxy distribution and from the velocity field in the local universe
The cosmic web that characterizes the large-scale structure of the Universe
can be quantified by a variety of methods. For example, large redshift surveys
can be used in combination with point process algorithms to extract long
curvilinear filaments in the galaxy distribution. Alternatively, given a full
3D reconstruction of the velocity field, kinematic techniques can be used to
decompose the web into voids, sheets, filaments and knots. In this paper we
look at how two such algorithms - the Bisous model and the velocity shear web -
compare with each other in the local Universe (within 100 Mpc), finding good
agreement. This is both remarkable and comforting, given that the two methods
are radically different in ideology and applied to completely independent and
different data sets. Unsurprisingly, the methods are in better agreement when
applied to unbiased and complete data sets, like cosmological simulations, than
when applied to observational samples. We conclude that more observational data
is needed to improve on these methods, but that both methods are most likely
properly tracing the underlying distribution of matter in the Universe.Comment: 6 Pages, 2 figures, Submitted to MNRAS Letter
The tangential velocity of M31: CLUES from constrained simulations
Determining the precise value of the tangential component of the velocity of
M31 is a non trivial astrophysical issue, that relies on complicated modeling.
This has recently lead to con- flicting estimates, obtained by several groups
that used different methodologies and assump- tions. This letter addresses the
issue by computing a Bayesian posterior distribution function of this quantity,
in order to measure the compatibility of those estimates with LambdaCDM. This
is achieved using an ensemble of local group (LG) look-alikes collected from a
set of Con- strained Simulations (CSs) of the local Universe, and a standard
unconstrained LambdaCDM. The latter allows us to build a control sample of
LG-like pairs and to single out the influence of the environment in our
results. We find that neither estimate is at odds with LambdaCDM; how- ever,
whereas CSs favour higher values of vtan , the reverse is true for estimates
based on LG samples gathered from unconstrained simulations, overlooking the
environmental elementComment: 5 pages, 2 figures, accepted for publications in MNRAS Letter
Planes of satellite galaxies and the cosmic web
Recent observational studies have demonstrated that the majority of satellite
galaxies tend to orbit their hosts on highly flattened, vast, possibly
co-rotating planes. Two nearly parallel planes of satellites have been
confirmed around the M31 galaxy and around the Centaurus A galaxy, while the
Milky Way also sports a plane of satellites. It has been argued that such an
alignment of satellites on vast planes is unexpected in the standard
({\Lambda}CDM) model of cosmology if not even in contradiction to its generic
predictions. Guided by {\Lambda}CDM numerical simulations, which suggest that
satellites are channeled towards hosts along the axis of the slowest collapse
as dictated by the ambient velocity shear tensor, we re-examine the planes of
local satellites systems within the framework of the local shear tensor derived
from the Cosmicflows-2 dataset. The analysis reveals that the Local Group and
Centaurus A reside in a filament stretched by the Virgo cluster and compressed
by the expansion of the Local Void. Four out of five thin planes of satellite
galaxies are indeed closely aligned with the axis of compression induced by the
Local Void. Being the less massive system, the moderate misalignment of the
Milky Way's satellite plane can likely be ascribed to its greater
susceptibility to tidal torques, as suggested by numerical simulations. The
alignment of satellite systems in the local universe with the ambient shear
field is thus in general agreement with predictions of the {\Lambda}CDM model.Comment: 9 pages, 3 figures, 3 tables. Accepted by MNRAS, 9 June 201
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