37,671 research outputs found
Cosmological redshift distortion: deceleration, bias and density parameters from future redshift surveys of galaxies
The observed two-point correlation functions of galaxies in redshift space
become anisotropic due to the geometry of the universe as well as due to the
presence of the peculiar velocity field. On the basis of linear perturbation
theory, we expand the induced anisotropies of the correlation functions with
respect to the redshift , and obtain analytic formulae to infer the
deceleration parameter , the density parameter and the
derivative of the bias parameter at in terms of the
observable statistical quantities. The present method does not require any
assumption of the shape and amplitude of the underlying fluctuation spectrum,
and thus can be applied to future redshift surveys of galaxies including the
Sloan Digital Sky Survey. We also evaluate quantitatively the systematic error
in estimating the value of from a galaxy
redshift survey on the basis of a conventional estimator for which
neglects both the geometrical distortion effect and the time evolution of the
parameter . If the magnitude limit of the survey is as faint as 18.5
(in B-band) as in the case of the Sloan Digital Sky Survey, the systematic
error ranges between -20% and 10% depending on the cosmological parameters.
Although such systematic errors are smaller than the statistical errors in the
current surveys, they will dominate the expected statistical error for future
surveys.Comment: 9 pages, 5 figs, aastex, ApJ in press, replaced version includes
minor correction
Velocity bias in a LCDM model
We use N-body simulations to study the velocity bias of dark matter halos,
the difference in the velocity fields of dark matter and halos, in a flat low-
density LCDM model. The high force, 2kpc/h, and mass, 10^9Msun/h, resolution
allows dark matter halos to survive in very dense environments of groups and
clusters making it possible to use halos as galaxy tracers. We find that the
velocity bias pvb measured as a ratio of pairwise velocities of the halos to
that of the dark matter evolves with time and depends on scale. At high
redshifts (z ~5) halos move generally faster than the dark matter almost on all
scales: pvb(r)~1.2, r>0.5Mpc/h. At later moments the bias decreases and gets
below unity on scales less than r=5Mpc/h: pvb(r)~(0.6-0.8) at z=0. We find that
the evolution of the pairwise velocity bias follows and probably is defined by
the spatial antibias of the dark matter halos at small scales. One-point
velocity bias b_v, defined as the ratio of the rms velocities of halos and dark
matter, provides a more direct measure of the difference in velocities because
it is less sensitive to the spatial bias. We analyze b_v in clusters of
galaxies and find that halos are ``hotter'' than the dark matter: b_v=(1.2-1.3)
for r=(0.2-0.8)r_vir, where r_vir is the virial radius. At larger radii, b_v
decreases and approaches unity at r=(1-2)r_vir. We argue that dynamical
friction may be responsible for this small positive velocity bias b_v>1 found
in the central parts of clusters. We do not find significant difference in the
velocity anisotropy of halos and the dark matter. The dark matter the velocity
anisotropy can be approximated as beta(x)=0.15 +2x/(x^2+4), where x is measured
in units of the virial radius.Comment: 13 pages, Latex, AASTeXv5 and natbi
Majorana spinors and extended Lorentz symmetry in four-dimensional theory
An extended local Lorentz symmetry in four-dimensional (4D) theory is
considered. A source of this symmetry is a group of general linear
transformations of four-component Majorana spinors GL(4,M) which is isomorphic
to GL(4,R) and is the covering of an extended Lorentz group in a 6D Minkowski
space M(3,3) including superluminal and scaling transformations. Physical
space-time is assumed to be a 4D pseudo-Riemannian manifold. To connect the
extended Lorentz symmetry in the M(3,3) space with the physical space-time, a
fiber bundle over the 4D manifold is introduced with M(3,3) as a typical fiber.
The action is constructed which is invariant with respect to both general 4D
coordinate and local GL(4,M) spinor transformations. The components of the
metric on the 6D fiber are expressed in terms of the 4D pseudo-Riemannian
metric and two extra complex fields: 4D vector and scalar ones. These extra
fields describe in the general case massive particles interacting with an extra
U(1) gauge field and weakly interacting with ordinary particles, i.e.
possessing properties of invisible (dark) matter.Comment: 24 page
Computer system for monitoring radiorepirometry data
System monitors expired breath patterns simultaneously from four small animals after they have been injected with carbon-14 substrates. It has revealed significant quantitative differences in oxidation patterns of glucose following such mild treatments of rats as a change in diet or environment
Mean age gradient and asymmetry in the star formation history of the Small Magellanic Cloud
We derive the star formation history in four regions of the Small Magellanic
Cloud (SMC) using the deepest VI color-magnitude diagrams (CMDs) ever obtained
for this galaxy. The images were obtained with the Advanced Camera for Surveys
onboard the Hubble Space Telescope and are located at projected distances of
0.5-2 degrees from the SMC center, probing the main body and the wing of the
galaxy. We derived the star-formation histories (SFH) of the four fields using
two independent procedures to fit synthetic CMDs to the data. We compare the
SFHs derived here with our earlier results for the SMC bar to create a deep
pencil-beam survey of the global history of the central SMC. We find in all the
six fields observed with HST a slow star formation pace from 13 to 5-7 Gyr ago,
followed by a ~ 2-3 times higher activity. This is remarkable because dynamical
models do not predict a strong influence of either the LMC or the Milky Way
(MW) at that time. The level of the intermediate-age SFR enhancement
systematically increases towards the center, resulting in a gradient in the
mean age of the population, with the bar fields being systematically younger
than the outer ones. Star formation over the most recent 500 Myr is strongly
concentrated in the bar, the only exception being the area of the SMC wing. The
strong current activity of the latter is likely driven by interaction with the
LMC. At a given age, there is no significant difference in metallicity between
the inner and outer fields, implying that metals are well mixed throughout the
SMC. The age-metallicity relations we infer from our best fitting models are
monotonically increasing with time, with no evidence of dips. This may argue
against the major merger scenario proposed by Tsujimoto and Bekki 2009,
although a minor merger cannot be ruled out.Comment: 30 pages, 16 figures, accepted for publication in Ap
The Effects of Age on Red Giant Metallicities Derived from the Near-Infrared Ca II Triplet
We have obtained spectra with resolution 2.5 Angstroms in the region
7500-9500 Angstroms for 116 red giants in 5 Galactic globular clusters and 6
old open clusters (5 with published metallicities, and one previously
unmeasured). The signal-to-noise ranges from 20 to 85. We measure the
equivalent widths of the infrared Ca II triplet absorption lines in each stars
and compare to cluster metallicities taken from the literature. With globular
cluster abundances on the Carretta & Gratton scale, and open cluster abundances
taken from the compilation of Friel and collaborators, we find a linear
relation between [Fe/H] and Ca II line strength spanning the range -2 < [Fe/H]
< -0.2 and ages from 2.5 - 13 Gyr. No evidence for an age effect on the
metallicity calibration is observed. Using this calibration, we find the
metallicity of the old open cluster Trumpler 5 to be [Fe/H] = -0.56 +/-0.11.
Considering the 10 clusters of known metallicity shifted to a common distance
and reddening, we find that the additional metallicity error introduced by the
variation of horizontal branch/red clump magnitude with metallicity and age is
of order +/-0.05 dex, which can be neglected in comparison to the intrinsic
scatter in our method. The results are discussed in the context of abundance
determinations for red giants in Local Group galaxies.Comment: Accepted by MNRAS; 21 pages in LaTeX MNRAS style, 6 tables, 6 figure
Dependence of the Inner DM Profile on the Halo Mass
I compare the density profile of dark matter (DM) halos in cold dark matter
(CDM) N-body simulations with 1 Mpc, 32 Mpc, 256 Mpc and 1024 Mpc box sizes. In
dimensionless units the simulations differ only for the initial power spectrum
of density perturbations. I compare the profiles when the most massive halos
are composed of about 10^5 DM particles. The DM density profiles of the halos
in the 1 Mpc box show systematically shallower cores with respect to the
corresponding halos in the 32 Mpc simulation that have masses, M_{dm}, typical
of the Milky Way and are fitted by a NFW profile. The DM density profiles of
the halos in the 256 Mpc box are consistent with having steeper cores than the
corresponding halos in the 32 Mpc simulation, but higher mass resolution
simulations are needed to strengthen this result. Combined, these results
indicate that the density profile of DM halos is not universal, presenting
shallower cores in dwarf galaxies and steeper cores in clusters. Physically the
result sustains the hypothesis that the mass function of the accreting
satellites determines the inner slope of the DM profile. In comoving
coordinates, r, the profile \rho_{dm} \propto 1/(X^\alpha(1+X)^{3-\alpha}),
with X=c_\Delta r/r_\Delta, r_\Delta is the virial radius and \alpha
=\alpha(M_{dm}), provides a good fit to all the DM halos from dwarf galaxies to
clusters at any redshift with the same concentration parameter c_\Delta ~ 7.
The slope, \gamma, of the outer parts of the halo appears to depend on the
acceleration of the universe: when the scale parameter is a=(1+z)^{-1} < 1, the
slope is \gamma ~ 3 as in the NFW profile, but \gamma ~ 4 at a > 1 when
\Omega_\Lambda ~ 1 and the universe is inflating.[abridged]Comment: Accepted for publication in MNRAS. 13 pages, including 11 figures and
2 tables. The revised version has an additional discussion section and work
on the velocity dispersion anisotrop
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