955 research outputs found
Large-scale bias in the Universe: bispectrum method
Evidence that the Universe may be close to the critical density, required for
its expansion eventually to be halted, comes principally from dynamical studies
of large-scale structure. These studies either use the observed peculiar
velocity field of galaxies directly, or indirectly by quantifying its
anisotropic effect on galaxy clustering in redshift surveys. A potential
difficulty with both such approaches is that the density parameter
is obtained only in the combination , if linear
perturbation theory is used. The determination of the density parameter
is therefore compromised by the lack of a good measurement of the
bias parameter , which relates the clustering of sample galaxies to the
clustering of mass.
In this paper, we develop an idea of Fry (1994), using second-order
perturbation theory to investigate how to measure the bias parameter on large
scales. The use of higher-order statistics allows the degeneracy between
and to be lifted, and an unambiguous determination of
then becomes possible. We apply a likelihood approach to the bispectrum, the
three-point function in Fourier space. This paper is the first step in turning
the idea into a practical proposition for redshift surveys, and is principally
concerned with noise properties of the bispectrum, which are non-trivial. The
calculation of the required bispectrum covariances involves the six-point
function, including many noise terms, for which we have developed a generating
functional approach which will be of value in calculating high-order statistics
in general.Comment: 12 pages, latex, 7 postscript figures included. Accepted by MNRAS.
(Minor numerical typesetting errors corrected: results unchanged
Young and middle age pulsar light-curve morphology: Comparison of Fermi observations with gamma-ray and radio emission geometries
Thanks to the huge amount of gamma-ray pulsar photons collected by the Fermi
Large Area Telescope since June 2008, it is now possible to constrain gamma-ray
geometrical models by comparing simulated and observed light-curve
morphological characteristics. We assumed vacuum-retarded dipole pulsar
magnetic field and tested simulated and observed morphological light-curve
characteristics in the framework of two pole emission geometries, Polar Cap
(PC), radio, and Slot Gap (SG), and Outer Gap (OG)/One Pole Caustic (OPC)
emission geometries. We compared simulated and observed/estimated light-curve
morphological parameters as a function of observable and non-observable pulsar
parameters. The PC model gives the poorest description of the LAT pulsar
light-curve morphology. The OPC best explains both the observed gamma-ray peak
multiplicity and shape classes. The OPC and SG models describe the observed
gamma-ray peak-separation distribution for low- and high-peak separations,
respectively. This suggests that the OPC geometry best explains the single-peak
structure but does not manage to describe the widely separated peaks predicted
in the framework of the SG model as the emission from the two magnetic
hemispheres. The OPC radio-lag distribution shows higher agreement with
observations suggesting that assuming polar radio emission, the gamma-ray
emission regions are likely to be located in the outer magnetosphere. The
larger agreement between simulated and LAT estimations in the framework of the
OPC suggests that the OPC model best predicts the observed variety of profile
shapes. The larger agreement between observations and the OPC model jointly
with the need to explain the abundant 0.5 separated peaks with two-pole
emission geometries, calls for thin OPC gaps to explain the single-peak
geometry but highlights the need of two-pole caustic emission geometry to
explain widely separated peaks.Comment: 28 pages, 20 figures, 8 tables; accepted for publication in Astronomy
and Astrophysic
The VIMOS VLT Deep Survey: Evolution of the non-linear galaxy bias up to z=1.5
We present the first measurements of the Probability Distribution Function
(PDF) of galaxy fluctuations in the VIMOS-VLT Deep Survey (VVDS) cone, covering
0.4x0.4 deg between 0.4<z<1.5. The second moment of the PDF, i.e. the rms
fluctuations of the galaxy density field, is with good approximation constant
over the full redshift baseline investigated: we find that, in redshift space,
sigma_8 for galaxies brighter than M=-20+5log h has a mean value of 0.94\pm0.07
in the redshift interval 0.7<z<1.5. The third moment, i.e. the skewness,
increases with cosmic time: we find that the probability of having underdense
regions is greater at z~0.7 than it was at z~1.5. By comparing the PDF of
galaxy density contrasts with the theoretically predicted PDF of mass
fluctuations we infer the redshift-, density-, and scale-dependence of the
biasing function b(z, \delta, R) between galaxy and matter overdensities up to
redshift z=1.5. Our results can be summarized as follows: i) the galaxy bias is
an increasing function of redshift: evolution is marginal up to z~0.8 and more
pronounced for z>0.8; ii) the formation of bright galaxies is inhibited below a
characteristic mass-overdensity threshold whose amplitude increases with
redshift and luminosity; iii) the biasing function is non linear in all the
redshift bins investigated with non-linear effects of the order of a few to 10%
on scales >5Mpc.Comment: 30 pages, 17 figs, Accepted by A&
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