14 research outputs found
Cosmic Structure and Dynamics of the Local Universe
We present a cosmography analysis of the Local Universe based on the recently
released Two-Micron All-Sky Redshift Survey (2MRS). Our method is based on a
Bayesian Networks Machine Learning algorithm (the Kigen-code) which
self-consistently samples the initial density fluctuations compatible with the
observed galaxy distribution and a structure formation model given by second
order Lagrangian perturbation theory (2LPT). From the initial conditions we
obtain an ensemble of reconstructed density and peculiar velocity fields which
characterize the local cosmic structure with high accuracy unveiling nonlinear
structures like filaments and voids in detail. Coherent redshift space
distortions are consistently corrected within 2LPT. From the ensemble of
cross-correlations between the reconstructions and the galaxy field and the
variance of the recovered density fields we find that our method is extremely
accurate up to k ~ 1 h Mpc^-1 and still yields reliable results down to scales
of about 3-4 h^-1 Mpc. The motion of the local group we obtain within ~ 80 h^-1
Mpc (v_LG=522+-86 km s^-1, l_LG=291^o +- 16^o, b_LG=34^o+-8^o) is in good
agreement with measurements derived from the CMB and from direct observations
of peculiar motions and is consistent with the predictions of LambdaCDM.Comment: 6 pages, 5 figures; accepted at MNRAS after minor correction
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
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 6dF Galaxy Survey: peculiar velocity field and cosmography
We derive peculiar velocities for the 6dF Galaxy Survey (6dFGS) and describe
the velocity field of the nearby () southern hemisphere. The survey
comprises 8885 galaxies for which we have previously reported Fundamental Plane
data. We obtain peculiar velocity probability distributions for the redshift
space positions of each of these galaxies using a Bayesian approach. Accounting
for selection bias, we find that the logarithmic distance uncertainty is 0.11
dex, corresponding to in linear distance. We use adaptive kernel
smoothing to map the observed 6dFGS velocity field out to
\kms, and compare this to the predicted velocity fields from the PSCz Survey
and the 2MASS Redshift Survey. We find a better fit to the PSCz prediction,
although the reduced for the whole sample is approximately unity for
both comparisons. This means that, within the observational uncertainties due
to redshift independent distance errors, observed galaxy velocities and those
predicted by the linear approximation from the density field agree. However, we
find peculiar velocities that are systematically more positive than model
predictions in the direction of the Shapley and Vela superclusters, and
systematically more negative than model predictions in the direction of the
Pisces-Cetus Supercluster, suggesting contributions from volumes not covered by
the models.Comment: 22 pages, 14 figures, accepted for publication in MNRAS. Table 1 is
available in its entirety as an ancillary file. Fully interactive 3D versions
of Figures 11 and 12 are also available as ancillary files. A version of this
paper with the 3D versions of Figs. 11 and 12 embedded within the pdf can
also be accessed from http://www.6dfgs.net/vfield/veldata.pd
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
Modified gravity and large scale flows
Reconstruction of the local velocity field from the overdensity field and a gravitational acceleration that falls off from a point mass as r −2 yields velocities in broad agreement with peculiar velocities measured with galaxy distance indicators. MONDian gravity does not. To quantify this, we introduce the velocity angular correlation function as a diagnostic of peculiar velocity field alignment and coherence as a function of scale. It is independent of the bias parameter of structure formation in the standard model of cosmology and the acceleration parameter of MOND. A modified gravity acceleration consistent with observed large scale structure would need to asymptote to zero at large distances more like r −2, than r −1
The 6dF Galaxy Survey: peculiar velocity field and cosmography
We derive peculiar velocities for the 6dF Galaxy Survey (6dFGS) and describe the velocity field of the nearby (z < 0.055) Southern hemisphere. The survey comprises 8885 galaxies for which we have previously reported Fundamental Plane data. We obtain peculiar velocity probability distributions for the redshift-space positions of each of these galaxies using a Bayesian approach. Accounting for selection bias, we find that the logarithmic distance uncertainty is 0.11 dex, corresponding to 26 per cent in linear distance. We use adaptive kernel smoothing to map the observed 6dFGS velocity field out to cz ∼ 16 000 km s−1, and compare this to the predicted velocity fields from the PSCz Survey and the 2MASS Redshift Survey. We find a better fit to the PSCz prediction, although the reduced χ2 for the whole sample is approximately unity for both comparisons. This means that, within the observational uncertainties due to redshift-independent distance errors, observed galaxy velocities and those predicted by the linear approximation from the density field agree. However, we find peculiar velocities that are systematically more positive than model predictions in the direction of the Shapley and Vela superclusters, and systematically more negative than model predictions in the direction of the Pisces-Cetus Supercluster, suggesting contributions from volumes not covered by the models