321 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
Halo Occupation Distribution Modeling of Clustering of Luminous Red Galaxies
We perform Halo Occupation Distribution (HOD) modeling to interpret
small-scale and intermediate-scale clustering of 35,000 luminous early-type
galaxies and their cross-correlation with a reference imaging sample of normal
L* galaxies in the Sloan Digital Sky Survey. The modeling results show that
most of these luminous red galaxies (LRGs) are central galaxies residing in
massive halos of typical mass M ~ a few times 10^13 to 10^14 Msun/h, while a
few percent of them have to be satellites within halos in order to produce the
strong auto-correlations exhibited on smaller scales. The mean luminosity Lc of
central LRGs increases with the host halo mass, with a rough scaling relation
of Lc \propto M^0.5. The halo mass required to host on average one satellite
LRG above a luminosity threshold is found to be about 10 times higher than that
required to host a central LRG above the same threshold. We find that in
massive halos the distribution of L* galaxies roughly follows that of the dark
matter and their mean occupation number scales with halo mass as M^1.5. The HOD
modeling results also allows for an intuitive understanding of the
scale-dependent luminosity dependence of the cross-correlation between LRGs and
L_* galaxies. Constraints on the LRG HOD provide tests to models of formation
and evolution of massive galaxies, and they are also useful for cosmological
parameter investigations. In one of the appendices, we provide LRG HOD
parameters with dependence on cosmology inferred from modeling the two-point
auto-correlation functions of LRGs.Comment: 21 pages, 10 figures, accepted for publication in Ap
Galaxy clustering and projected density profiles as traced by satellites in photometric surveys: Methodology and luminosity dependence
We develop a new method which measures the projected density distribution
w_p(r_p)n of photometric galaxies surrounding a set of
spectroscopically-identified galaxies, and simultaneously the projected
correlation function w_p(r_p) between the two populations. In this method we
are able to divide the photometric galaxies into subsamples in luminosity
intervals when redshift information is unavailable, enabling us to measure
w_p(r_p)n and w_p(r_p) as a function of not only the luminosity of the
spectroscopic galaxy, but also that of the photometric galaxy. Extensive tests
show that our method can measure w_p(r_p) in a statistically unbiased way. The
accuracy of the measurement depends on the validity of the assumption in the
method that the foreground/background galaxies are randomly distributed and
thus uncorrelated with those galaxies of interest. Therefore, our method can be
applied to the cases where foreground/background galaxies are distributed in
large volumes, which is usually valid in real observations. We applied our
method to data from SDSS including a sample of 10^5 LRGs at z~0.4 and a sample
of about half a million galaxies at z~0.1, both of which are cross-correlated
with a deep photometric sample drawn from the SDSS. On large scales, the
relative bias factor of galaxies measured from w_p(r_p) at z~0.4 depends on
luminosity in a manner similar to what is found at z~0.1, which are usually
probed by autocorrelations of spectroscopic samples. On scales smaller than a
few Mpc and at both z~0.4 and z~0.1, the photometric galaxies of different
luminosities exhibit similar density profiles around spectroscopic galaxies at
fixed luminosity and redshift. This provides clear support for the assumption
commonly-adopted in HOD models that satellite galaxies of different
luminosities are distributed in a similar way, following the dark matter
distribution within their host halos.Comment: 38 pages, 12 figures, published in Ap
A Coarse-Grained Field Theory for Density Fluctuations and Correlation Functions of Galactic Objects
We present a coarse-grained field theory of density fluctuations for a
Newtonian self-gravitating many-body system and apply it to a homogeneous
Universe with small density fluctuations. The theory treats the clustering of
galaxies and clusters in terms of the field of density fluctuations. The Jeans
length , a unique physical scale for a gravitating system, appears
naturally as the characteristic scale underlying the large scale structure.
Under Gaussian approximation the analytic expressions of and
are obtained. The correlation amplitude is proportional to the galactic mass,
and is oscillating over large scales Mpc and damped to
zero. The spectrum amplitude is inversely proportional to the galactic number
density.The preliminary results qualitatively explain some pronounced features
of large scale structures.Comment: 7 pages, 4 figures. to appear in A&
Systematic Effects on Determination of the Growth Factor from Redshift-space Distortions
The linear growth factor of density perturbations is believed to be a
powerful observable of future redshift surveys to probe physical properties of
dark energy and to distinguish among gravity theories. We investigate
systematic effects on determination of the growth factor f from a measurement
of redshift-space distortions. Using N-body simulations we identify dark matter
halos over a broad mass range. We compute the power spectra and correlation
functions for the halos and then examine how well the redshift distortion
parameter beta=f/b can be reconstructed as a function of halo mass. We find
that beta measured for a fixed halo mass is generally a function of scale even
on large scales, in contrast with the common expectation that beta approaches a
constant described by Kaiser's formula on such scales. The scale dependence
depends on the halo mass, being stronger for smaller halos. It also cannot be
easily explained with the well-known distribution function of the halo peculiar
velocities. We demonstrate that the biasing for smaller halos has larger
nonlinearity and stochasticity, thus the linear bias assumption becomes worse
for smaller halos. Only for massive halos with b>1.5, beta approaches the
linear theory prediction on scales of r or pi/k>30Mpc/h. Luminous red galaxies
(LRG), targeted by the SDSS-III's BOSS survey, tend to reside in very massive
halos. Our results indicate that if the LRG is used for the measurement of
redshift distortions, f can be measured unbiasedly. On the other hand, if one
considers to use emission line galaxies, which are targeted by the BigBOSS
survey and inhabited in halos of a broad mass range, the scale dependence of
beta must be taken into account carefully; otherwise one might give incorrect
constraints on dark energy or modified gravity theories. We also find that beta
reconstructed in Fourier space behaves better than that in configuration space.Comment: 12 pages, 9 figures, submitted to ApJ, revised according to referee
repor
Determine the galaxy bias factors on large scales using bispectrum method
We study whether the bias factors of galaxies can be unbiasedly recovered
from their power spectra and bispectra. We use a set of numerical N-body
simulations and construct large mock galaxy catalogs based upon the
semi-analytical model of Croton et al. (2006). We measure the reduced bispectra
for galaxies of different luminosity, and determine the linear and first
nonlinear bias factors from their bispectra. We find that on large scales down
to that of the wavenumber k=0.1h/Mpc, the bias factors b1 and b2 are nearly
constant, and b1 obtained with the bispectrum method agrees very well with the
expected value. The nonlinear bias factor b2 is negative, except for the most
luminous galaxies with M<-23 which have a positive b2. The behavior of b2 of
galaxies is consistent with the b2 mass dependence of their host halos. We show
that it is essential to have an accurate estimation of the dark matter
bispectrum in order to have an unbiased measurement of b1 and b2. We also test
the analytical approach of incorporating halo occupation distribution to model
the galaxy power spectrum and bispectrum. The halo model predictions do not fit
the simulation results well on the precision requirement of current
cosmological studies.Comment: 9 pages, 8 figures, accepted for publication in Ap
Understanding the PSCz Galaxy Power Spectrum with N-body Simulations
By comparing the PSCz galaxy power spectrum with the results of nested pure
dark matter N-body simulations, we try to understand how infrared-selected
galaxies populate dark-matter haloes, paying special attention to the method of
halo identification in the simulations. We thus test the hypothesis that
baryonic physics negligibly affects the distribution of galaxies down to the
smallest scales yet observed. We are successful in reproducing the PSCz power
spectrum on scales < ~40 h/Mpc, near our resolution limit, by imposing a
central density cut-off on simulated haloes, which gives a rough minimum mass
and circular velocity of haloes in which PSCz galaxies formed.Comment: 12 pages, 16 figures (one added), conforms to version in MNRA
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
Scale Dependence of Dark Energy Antigravity
We investigate the effects of negative pressure induced by dark energy
(cosmological constant or quintessence) on the dynamics at various
astrophysical scales. Negative pressure induces a repulsive term (antigravity)
in Newton's law which dominates on large scales. Assuming a value of the
cosmological constant consistent with the recent SnIa data we determine the
critical scale beyond which antigravity dominates the dynamics () and discuss some of the dynamical effects implied. We show that
dynamically induced mass estimates on the scale of the Local Group and beyond
are significantly modified due to negative pressure. We also briefly discuss
possible dynamical tests (eg effects on local Hubble flow) that can be applied
on relatively small scales (a few ) to determine the density and equation
of state of dark energy.Comment: Contributed talk at the 2nd Hellenic Cosmology Workshop at NOA
(Athens) Jan. 2001.To appear in the proceedings. Based on work done in
collaboration with M. Axenides and E. Florato
A galaxy-halo model of large-scale structure
We present a new, galaxy-halo model of large-scale structure, in which the
galaxies entering a given sample are the fundamental objects. Haloes attach to
galaxies, in contrast to the standard halo model, in which galaxies attach to
haloes. The galaxy-halo model pertains mainly to the relationships between the
power spectra of galaxies and mass, and their cross-power spectrum. With
surprisingly little input, an intuition-aiding approximation to the
galaxy-matter cross-correlation coefficient R(k) emerges, in terms of the halo
mass dispersion. This approximation seems valid to mildly non-linear scales (k
< ~3 h/Mpc), allowing measurement of the bias and the matter power spectrum
from measurements of the galaxy and galaxy-matter power spectra (or correlation
functions). This is especially relevant given the recent advances in precision
in measurements of the galaxy-matter correlation function from weak
gravitational lensing. The galaxy-halo model also addresses the issue of
interpreting the galaxy-matter correlation function as an average halo density
profile, and provides a simple description of galaxy bias as a function of
scale.Comment: 13 pages, 9 figures, submitted to MNRAS. Minor changes, suggested by
refere
- …