412 research outputs found
Power law correlations in galaxy distribution and finite volume effects from the Sloan Digital Sky Survey Data Release Four
We discuss the estimation of galaxy correlation properties in several volume
limited samples, in different sky regions, obtained from the Fourth Data
Release of the Sloan Digital Sky Survey. The small scale properties are
characterized through the determination of the nearest neighbor probability
distribution. By using a very conservative statistical analysis, in the range
of scales [0.5,~30] Mpc/h we detect power-law correlations in the conditional
density in redshift space, with an exponent \gamma=1.0 \pm 0.1. This behavior
is stable in all different samples we considered thus it does not depend on
galaxy luminosity. In the range of scales [~30,~100] Mpc/h we find evidences
for systematic unaveraged fluctuations and we discuss in detail the problems
induced by finite volume effects on the determination of the conditional
density. We conclude that in such range of scales there is an evidence for a
smaller power-law index of the conditional density. However we cannot
distinguish between two possibilities: (i) that a crossover to homogeneity
(corresponding to \gamma=0 in the conditional density) occurs before 100 Mpc/h,
(ii) that correlations extend to scales of order 100 Mpc/h (with a smaller
exponent 0 < \gamma <1). We emphasize that galaxy distributions in these
samples present large fluctuations at the largest scales probed, corresponding
to the presence of large scale structures extending up to the boundaries of the
present survey. Finally we discuss several differences between the behavior of
the conditional density in mock galaxy catalogs built from cosmological N-body
simulations and real data. We discuss some theoretical implications of such a
fact considering also the super-homogeneous features of primordial density
fields.Comment: 13 pages, 19 figures, to be publsihed in Astronomy and Astrophysic
Cosmological Parameters from Velocities, CMB and Supernovae
We compare and combine likelihood functions of the cosmological parameters
Omega_m, h and sigma_8, from peculiar velocities, CMB and type Ia supernovae.
These three data sets directly probe the mass in the Universe, without the need
to relate the galaxy distribution to the underlying mass via a "biasing"
relation. We include the recent results from the CMB experiments BOOMERANG and
MAXIMA-1. Our analysis assumes a flat Lambda CDM cosmology with a
scale-invariant adiabatic initial power spectrum and baryonic fraction as
inferred from big-bang nucleosynthesis. We find that all three data sets agree
well, overlapping significantly at the 2 sigma level. This therefore justifies
a joint analysis, in which we find a joint best fit point and 95 per cent
confidence limits of Omega_m=0.28 (0.17,0.39), h=0.74 (0.64,0.86), and
sigma_8=1.17 (0.98,1.37). In terms of the natural parameter combinations for
these data sigma_8 Omega_m^0.6 = 0.54 (0.40,0.73), Omega_m h = 0.21
(0.16,0.27). Also for the best fit point, Q_rms-ps = 19.7 muK and the age of
the universe is 13.2 Gyr.Comment: 8 pages, 5 figures. Submitted to MNRA
Extension and estimation of correlations in Cold Dark Matter models
We discuss the large scale properties of standard cold dark matter
cosmological models characterizing the main features of the power-spectrum, of
the two-point correlation function and of the mass variance. Both the
real-space statistics have a very well defined behavior on large enough scales,
where their amplitudes become smaller than unity. The correlation function, in
the range 0<\xi(r)<1, is characterized by a typical length-scale r_c, at which
\xi(r_c)=0, which is fixed by the physics of the early universe: beyond this
scale it becomes negative, going to zero with a tail proportional to -(r^{-4}).
These anti-correlations represent thus an important observational challenge to
verify models in real space. The same length scale r_c characterizes the
behavior of the mass variance which decays, for r>r_c, as r^{-4}, the fastest
decay for any mass distribution. The length-scale r_c defines the maximum
extension of (positively correlated) structures in these models. These are the
features expected for the dark matter field: galaxies, which represent a biased
field, however may have differences with respect to these behaviors, which we
analyze. We then discuss the detectability of these real space features by
considering several estimators of the two-point correlation function. By making
tests on numerical simulations we emphasize the important role of finite size
effects which should always be controlled for careful measurements.Comment: 18 pages, 27 figures, accepted for publication in Astronomy and
Astrophysic
A Local Hubble Bubble from SNe Ia?
We analyze the monopole in the peculiar velocities of 44 Type Ia supernovae
(SNe Ia) to test for a local void. The sample extends from 20 to 300 Mpc/h,
with distances, deduced from light-curve shapes, accurate to ~6%. Assuming
Omega_m=1 and Omega_lambda=0, the most significant deviation we find from the
Hubble law is an outwards flow of (6.6+/-2.2)% inside a sphere of radius 70
Mpc/h as would be produced by a void of ~20% underdensity surrounded by a dense
shell. This shell roughly coincides with the local Great Walls. Monte Carlo
analyses, using Gaussian errors or bootstrap resampling, show the probability
for chance occurrence of this result out of a pure Hubble flow to be ~2%. The
monopole could be contaminated by higher moments of the velocity field,
especially a quadrupole, which are not properly probed by the current limited
sky coverage. The void would be less significant if Omega_m is low and
Omega_lambda is high. It would be more significant if one outlier is removed
from the sample, or if the size of the void is constrained a-priori. This
putative void is not in significant conflict with any of the standard
cosmological scenarios. It suggests that the Hubble constant as determined
within 70 Mpc/h could be overestimated by ~6% and the local value of Omega may
be underestimated by ~20%. While the present evidence for a local void is
marginal in this data set, the analysis shows that the accumulation of SNe Ia
distances will soon provide useful constraints on elusive and important aspects
of regional cosmic dynamics.Comment: 21 pages, 3 figures. Slightly revised version. To appear in ApJ, 503,
Aug. 20, 199
Large Scale Power Spectrum from Peculiar Velocities Via Likelihood Analysis
The power spectrum (PS) of mass density fluctuations, independent of
`biasing', is estimated from the Mark III catalog of peculiar velocities using
Bayesian statistics. A parametric model is assumed for the PS, and the free
parameters are determined by maximizing the probability of the model given the
data. The method has been tested using detailed mock catalogs. It has been
applied to generalized CDM models with and without COBE normalization.
The robust result for all the models is a relatively high PS, with at . An
extrapolation to smaller scales using the different CDM models yields . The peak is weakly constrained to the range
. These results are consistent with a direct
computation of the PS (Kolatt & Dekel 1996). When compared to galaxy-density
surveys, the implied values for () are of order
unity to within 25%.
The parameters of the COBE-normalized, flat CDM model are confined by a 90%
likelihood contour of the sort , where
and for models with and without tensor
fluctuations respectively. For open CDM the powers are and (no tensor fluctuations). A -shape model free of COBE
normalization yields only a weak constraint: .Comment: 19 pages, 8 figures, 2 tables. Accepted for publication in The
Astrophysical Journa
Cosmological Density and Power Spectrum from Peculiar Velocities: Nonlinear Corrections and PCA
We allow for nonlinear effects in the likelihood analysis of galaxy peculiar
velocities, and obtain ~35%-lower values for the cosmological density parameter
Om and the amplitude of mass-density fluctuations. The power spectrum in the
linear regime is assumed to be a flat LCDM model (h=0.65, n=1, COBE) with only
Om as a free parameter. Since the likelihood is driven by the nonlinear regime,
we "break" the power spectrum at k_b=0.2 h/Mpc and fit a power law at k>k_b.
This allows for independent matching of the nonlinear behavior and an unbiased
fit in the linear regime. The analysis assumes Gaussian fluctuations and
errors, and a linear relation between velocity and density. Tests using proper
mock catalogs demonstrate a reduced bias and a better fit. We find for the
Mark3 and SFI data Om_m=0.32+-0.06 and 0.37+-0.09 respectively, with
sigma_8*Om^0.6 = 0.49+-0.06 and 0.63+-0.08, in agreement with constraints from
other data. The quoted 90% errors include cosmic variance. The improvement in
likelihood due to the nonlinear correction is very significant for Mark3 and
moderately so for SFI. When allowing deviations from LCDM, we find an
indication for a wiggle in the power spectrum: an excess near k=0.05 and a
deficiency at k=0.1 (cold flow). This may be related to the wiggle seen in the
power spectrum from redshift surveys and the second peak in the CMB anisotropy.
A chi^2 test applied to modes of a Principal Component Analysis (PCA) shows
that the nonlinear procedure improves the goodness of fit and reduces a spatial
gradient of concern in the linear analysis. The PCA allows addressing spatial
features of the data and fine-tuning the theoretical and error models. It shows
that the models used are appropriate for the cosmological parameter estimation
performed. We address the potential for optimal data compression using PCA.Comment: 18 pages, LaTex, uses emulateapj.sty, ApJ in press (August 10, 2001),
improvements to text and figures, updated reference
Large scale correlations in galaxy clustering from the Two degree Field Galaxy Redshift Survey
We study galaxy correlations from samples extracted from the 2dFGRS final
release. Statistical properties are characterized by studying the nearest
neighbor probability density, the conditional density and the reduced two-point
correlation function. The result is that the conditional density has a
power-law behavior in redshift space described by an exponent \gamma=0.8 \pm
0.2 in the interval from about 1 Mpc/h, the average distance between nearest
galaxies, up to about 40 Mpc/h, corresponding to radius of the largest sphere
contained in the samples. These results are consistent with other studies of
the conditional density and are useful to clarify the subtle role of
finite-size effects on the determination of the two-point correlation function
in redshift and real spaceComment: 11 pages, 14 figures. Accepted for publication in Astronomy and
Astrophysic
A New Statistic for Analyzing Baryon Acoustic Oscillations
We introduce a new statistic omega_l for measuring and analyzing large-scale
structure and particularly the baryon acoustic oscillations. omega_l is a
band-filtered, configuration space statistic that is easily implemented and has
advantages over the traditional power spectrum and correlation function
estimators. Unlike these estimators, omega_l can localize most of the acoustic
information into a single dip at the acoustic scale while also avoiding
sensitivity to the poorly constrained large scale power (i.e., the integral
constraint) through the use of a localized and compensated filter. It is also
sensitive to anisotropic clustering through pair counting and does not require
any binning. We measure the shift in the acoustic peak due to nonlinear effects
using the monopole omega_0 derived from subsampled dark matter catalogues as
well as from mock galaxy catalogues created via halo occupation distribution
(HOD) modeling. All of these are drawn from 44 realizations of 1024^3 particle
dark matter simulations in a 1h^{-1}Gpc box at z=1. We compare these shifts
with those obtained from the power spectrum and conclude that the results
agree. This indicates that any distance measurements obtained from omega_0 and
P(k) will be consistent with each other. We also show that it is possible to
extract the same amount of acoustic information using either omega_0 or P(k)
from equal volume surveys.Comment: 12 pages, 7 figures. ApJ accepted. Edit: Now updated with final
accepted versio
Constraining the LRG Halo Occupation Distribution using Counts-in-Cylinders
The low number density of the Sloan Digital Sky Survey (SDSS) Luminous Red
Galaxies (LRGs) suggests that LRGs occupying the same dark matter halo can be
separated from pairs occupying distinct dark matter halos with high fidelity.
We present a new technique, Counts-in-Cylinders (CiC), to constrain the
parameters of the satellite contribution to the LRG Halo-Occupation
Distribution (HOD). For a fiber collision-corrected SDSS spectroscopic LRG
subsample at 0.16 < z < 0.36, we find the CiC multiplicity function is fit by a
halo model where the average number of satellites in a halo of mass M is
= ((M - Mcut)/M1)^alpha with Mcut = 5.0 +1.5/-1.3 (+2.9/-2.6) X 10^13
Msun, M1 = 4.95 +0.37/-0.26 (+0.79/-0.53) X 10^14 Msun, and alpha = 1.035
+0.10/-0.17 (+0.24/-0.31) at the 68% and 95% confidence levels using a WMAP3
cosmology and z=0.2 halo catalog.
Our method tightly constrains the fraction of LRGs that are satellite
galaxies, 6.36 +0.38/-0.39, and the combination Mcut/10^{14} Msun + alpha =
1.53 +0.08/-0.09 at the 95% confidence level. We also find that mocks based on
a halo catalog produced by a spherical overdensity (SO) finder reproduce both
the measured CiC multiplicity function and the projected correlation function,
while mocks based on a Friends-of-Friends (FoF) halo catalog has a deficit of
close pairs at ~1 Mpc/h separations. Because the CiC method relies on higher
order statistics of close pairs, it is robust to the choice of halo finder. In
a companion paper we will apply this technique to optimize Finger-of-God (FOG)
compression to eliminate the 1-halo contribution to the LRG power spectrum.Comment: 40 pages, 9 figures, submitted to Astrophysical Journa
Absence of anti-correlations and of baryon acoustic oscillations in the galaxy correlation function from the Sloan Digital Sky Survey DR7
One of the most striking features predicted by standard models of galaxy
formation is the presence of anti-correlations in the matter distribution at
large enough scales (r>r_c). Simple arguments show that the location of the
length-scale r_c, marking the transition from positive to negative
correlations, is the same for any class of objects as for the full matter
distribution, i.e. it is invariant under biasing. This scale is predicted by
models to be at about the same distance of the scale signaling the baryonic
acoustic oscillation scale r_{bao}. We test these predictions in the newest
SDSS galaxy samples.We find that, in several MG samples, the correlation
function remains positive at scales >250 Mpc/h, while in the concordance LCDM
it should be negative beyond r_c\approx 120 Mpc/h. In other samples the
correlation function becomes negative at scales <50 Mpc/h. To investigate the
origin of these differences we consider in detail the propagation of errors on
the sample density into the estimation of the correlation function. We conclude
that these are important at large enough separations, and that they are
responsible for the observed differences between different estimators and for
the measured sample to sample variations of the correlation function. We
conclude that, in the newest SDSS samples, the large scale behavior of the
galaxy correlation function is affected by intrinsic errors andv
olume-dependent systematic effects which make the detection of correlations to
be only an estimate of a lower limit of their amplitude, spatial extension and
statistical errors. We point out that these results represent an important
challenge to LCDM models as they largely differ from its predictions.(Abridged
version).Comment: Version accepted for publication in Astronomy and Astrophysics; 10
pages, 13 .eps figures. Substantial changes with respect to version v1, more
detailed explanations of the methods and results. Main results are unchanged.
Version v3 with few typos correcte
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