93 research outputs found
Combining cosmological datasets: hyperparameters and Bayesian evidence
A method is presented for performing joint analyses of cosmological datasets,
in which the weight assigned to each dataset is determined directly by it own
statistical properties. The weights are considered in a Bayesian context as a
set of hyperparameters, which are then marginalised over in order to recover
the posterior distribution as a function only of the cosmological parameters of
interest. In the case of a Gaussian likelihood function, this marginalisation
may be performed analytically. Calculation of the Bayesian evidence for the
data, with and without the introduction of hyperparameters, enables a direct
determination of whether the data warrant the introduction of weights into the
analysis; this generalises the standard likelihood ratio approach to model
comparison. The method is illustrated by application to the classic toy problem
of fitting a straight line to a set of data. A cosmological illustration of the
technique is also presented, in which the latest measurements of the cosmic
microwave background power spectrum are used to infer constraints on
cosmological parameters.Comment: 12 pages, 6 figures, submitted to MNRA
The Dipole Anisotropy of the 2mass Redshift Survey
We estimate the flux weighted acceleration on the Local Group (LG) from the
near-infrared Two Micron All Sky Redshift Survey (2MRS). The near-infrared flux
weighted dipoles are very robust because they closely approximate a mass
weighted dipole, bypassing the effects of redshift distortions and require no
preferred reference frame. We use this method with the redshift information to
determine the change in dipole with distance. The LG dipole seemingly converges
by 60 Mpc/h. Assuming convergence, the comparison of the 2MRS flux dipole and
the CMB dipole provides a value for the combination of the mass density and
luminosity bias parameters Omega_m^0.6/b_L= 0.40+/-0.09.Comment: 4 pages, 2 figures, Contribution to Rencontres de Moriond: Contents
and Structures of the Universe, March 18-25, 2006, La Thuil
Luminous superclusters: remnants from inflation
We derive the luminosity and multiplicity functions of superclusters compiled
for the 2dF Galaxy Redshift Survey, the Sloan Digital Sky Survey (Data Release
4), and for three samples of simulated superclusters. We find for all
supercluster samples Density Field (DF) clusters, which represent high-density
peaks of the class of Abell clusters, and use median luminosities/masses of
richness class 1 DF-clusters to calculate relative luminosity/mass functions.
We show that the fraction of very luminous (massive) superclusters in real
samples is more than tenfolds greater than in simulated samples. Superclusters
are generated by large-scale density perturbations which evolve very slowly.
The absence of very luminous superclusters in simulations can be explained
either by non-proper treatment of large-scale perturbations, or by some yet
unknown processes in the very early Universe.Comment: 6 pages, 3 Figures, submitted for Astronomy and Astrophysic
Superclusters of galaxies from the 2dF redshift survey. I. The catalogue
We use the 2dF Galaxy Redshift Survey data to compile catalogues of
superclusters for the Northern and Southern regions of the 2dFGRS, altogether
543 superclusters at redshifts 0.009 < z < 0.2. We analyse methods of compiling
supercluster catalogues and use results of the Millennium Simulation to
investigate possible selection effects and errors. We find that the most
effective method is the density field method using smoothing with an
Epanechnikov kernel of radius 8 Mpc/h. We derive positions of the highest
luminosity density peaks and find the most luminous cluster in the vicinity of
the peak, this cluster is considered as the main cluster and its brightest
galaxy the main galaxy of the supercluster. In catalogues we give equatorial
coordinates and distances of superclusters as determined by positions of their
main clusters. We also calculate the expected total luminosities of the
superclusters.Comment: 16 pages, 11 figures, submitted for Astronomy and Astrophysics.
High-resolution pdf file and supplementary data can be found at
http://www.aai.ee/~maret/2dfscl.htm
The richest superclusters. I. Morphology
We study the morphology of the richest superclusters from the catalogues of
superclusters of galaxies in the 2dF Galaxy Redshift Survey and compare the
morphology of real superclusters with model superclusters in the Millennium
Simulation. We use Minkowski functionals and shapefinders to quantify the
morphology of superclusters: their sizes, shapes, and clumpiness. We generate
empirical models of simple geometry to understand which morphologies correspond
to the supercluster shapefinders. We show that rich superclusters have
elongated, filamentary shapes with high-density clumps in their core regions.
The clumpiness of superclusters is determined using the fourth Minkowski
functional . In the - shapefinder plane the morphology of
superclusters is described by a curve which is characteristic to
multi-branching filaments. We also find that the differences between the fourth
Minkowski functional for the bright and faint galaxies in observed
superclusters are larger than in simulated superclusters.Comment: 14 pages, 8 figures, submitted to Astronomy and Astrophysic
Superclusters of galaxies from the 2dF redshift survey. II. Comparison with simulations
We investigate properties of superclusters of galaxies found on the basis of
the 2dF Galaxy Redshift Survey, and compare them with properties of
superclusters from the Millennium Simulation. We study the dependence of
various characteristics of superclusters on their distance from the observer,
on their total luminosity, and on their multiplicity. The multiplicity is
defined by the number of Density Field (DF) clusters in superclusters. Using
the multiplicity we divide superclusters into four richness classes: poor,
medium, rich and extremely rich. We show that superclusters are asymmetrical
and have multi-branching filamentary structure, with the degree of asymmetry
and filamentarity being higher for the more luminous and richer superclusters.
The comparison of real superclusters with Millennium superclusters shows that
most properties of simulated superclusters agree very well with real data, the
main differences being in the luminosity and multiplicity distributions.Comment: 15 pages, 13 Figures, submitted for Astronomy and Astrophysic
Reconstructed Density and Velocity Fields from the 2MASS Redshift Survey
We present the reconstructed real-space density and the predicted velocity
fields from the Two Mass Redshift Survey (2MRS). The 2MRS is the densest
all-sky redshift survey to date and includes about 23,200 galaxies with
extinction corrected magnitudes brighter than K = 11.25. Our method is based on
the expansion of these fields in Fourier-Bessel functions. Within this
framework, the linear redshift distortions only affect the density field in the
radial direction and can easily be deconvolved using a distortion matrix.
Moreover, in this coordinate system, the velocity field is related to the
density field by a simple linear transformation. The shot noise errors in the
reconstructions are suppressed by means of a Wiener filter which yields a
minimum variance estimate of the density and velocity fields. Using the
reconstructed real-space density fields, we identify all major superclusters
and voids. At 50 Mpc/h, our reconstructed velocity field indicates a back-side
infall to the Great Attractor region of vi = (491 +/- 200)(beta/0.5) km/sec in
the Local Group frame and v = (64 +/- 205)(beta/0.5) km/sec in the cosmic
microwave background (CMB) frame and beta is the redshift distortion parameter.
The direction of the reconstructed dipole agrees well with the dipole derived
by Erdogdu et al. (2006). The misalignment between the reconstructed 2MRS and
the CMB dipoles drops to 13 degrees at around 5000 km/sec but then increases at
larger distances. A version of this paper with high resolution figures can be
obtained from http://www.nottingham.ac.uk/~ppzzpeComment: 21 pages. 22 figures, accepted for publication in MNRAS. The figures
are coarsely resolved, a version of this paper with high resolution figures
can be obtained from http://www.nottingham.ac.uk/~ppzzp
Upper limits on neutrino masses from the 2dFGRS and WMAP: the role of priors
Solar, atmospheric, and reactor neutrino experiments have confirmed neutrino
oscillations, implying that neutrinos have non-zero mass, but without pinning
down their absolute masses. While it is established that the effect of
neutrinos on the evolution of cosmic structure is small, the upper limits
derived from large-scale structure data could help significantly to constrain
the absolute scale of the neutrino masses. In a recent paper the 2dF Galaxy
Redshift Survey (2dFGRS) team provided an upper limit m_nu,tot < 2.2 eV, i.e.
approximately 0.7 eV for each of the three neutrino flavours, or phrased in
terms of their contributioin to the matter density, Omega_nu/Omega_m < 0.16.
Here we discuss this analysis in greater detail, considering issues of assumed
'priors' like the matter density Omega_m and the bias of the galaxy
distribution with respect the dark matter distribution. As the suppression of
the power spectrum depends on the ratio Omega_nu/Omega_m, we find that the
out-of- fashion Mixed Dark Matter Model, with Omega_nu=0.2, Omega_m=1 and no
cosmological constant, fits the 2dFGRS power spectrum and the CMB data
reasonably well, but only for a Hubble constant H_0<50 km/s/Mpc. As a
consequence, excluding low values of the Hubble constant, e.g. with the HST Key
Project is important in order to get a strong constraint on the neutrino
masses. We also comment on the improved limit by the WMAP team, and point out
that the main neutrino signature comes from the 2dFGRS and the Lyman alpha
forest.Comment: 24 pages, 12 figures Minor changes to matched version published in
JCA
Superclusters of galaxies in the 2dF redshift survey. III. The properties of galaxies in superclusters
We use catalogues of superclusters of galaxies from the 2dF Galaxy Redshift
Survey to study the properties of galaxies in superclusters. We compare the
properties of galaxies in high and low density regions of rich superclusters,
in poor superclusters and in the field, as well as in groups, and of isolated
galaxies in superclusters of various richness. We show that in rich
superclusters the values of the luminosity density smoothed on a scale of 8
\Mpc are higher than in poor superclusters: the median density in rich
superclusters is , in poor superclusters . Rich superclusters contain high density cores with densities while in poor superclusters such high density cores are absent. The
properties of galaxies in rich and poor superclusters and in the field are
different: the fraction of early type, passive galaxies in rich superclusters
is slightly larger than in poor superclusters, and is the smallest among the
field galaxies. Most importantly, in high density cores of rich superclusters
() there is an excess of early type, passive galaxies in groups
and clusters, as well as among those which do not belong to groups or clusters.
The main galaxies of superclusters have a rather limited range of absolute
magnitudes. The main galaxies of rich superclusters have larger luminosities
than those of poor superclusters and of groups in the field. Our results show
that both the local (group/cluster) environments and global (supercluster)
environments influence galaxy morphologies and their star formation activity.Comment: 13 pages, 10 figures, submitted to Astronomy and Astrophysic
The Signature of Large Scale Structures on the Very High Energy Gamma-Ray Sky
If the diffuse extragalactic gamma ray emission traces the large scale
structures of the universe, peculiar anisotropy patterns are expected in the
gamma ray sky. In particular, because of the cutoff distance introduced by the
absorption of 0.1-10 TeV photons on the infrared/optical background, prominent
correlations with the local structures within a range of few hundreds Mpc
should be present. We provide detailed predictions of the signal based on the
PSCz map of the local universe. We also use mock N-body catalogues complemented
with the halo model of structures to study some statistical features of the
expected signatures. The results are largely independent from cosmological
details, and depend mostly on the index of correlation (or bias) of the sources
with respect to the large scale distribution of galaxies. For instance, the
predicted signal in the case of a quadratic correlation (as it may happen for a
dark matter annihilation contribution to the diffuse gamma flux) differs
substantially from a linear correlation case, providing a complementary tool to
unveil the nature of the sources of the diffuse gamma ray emission. The chances
of the present and future space and ground based observatories to measure these
features are discussed.Comment: 26 pages, 9 figures; matches published versio
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