2,246 research outputs found
The Halo Formation Rate and its link to the Global Star Formation Rate
The star formation history of the universe shows strong evolution with
cosmological epoch. Although we know mergers between galaxies can cause
luminous bursts of star formation, the relative importance of such mergers to
the global star formation rate (SFR) is unknown. We present a simple analytic
formula for the rate at which halos merge to form higher-mass systems, derived
from Press-Schechter theory and confirmed by numerical simulations (for high
halo masses). A comparison of the evolution in halo formation rate with the
observed evolution in the global SFR indicates that the latter is largely
driven by halo mergers at z>1. Recent numerical simulations by Kolatt et al.
(1999) and Knebe & Muller (1999) show how merging systems are strongly biased
tracers of mass fluctuations, thereby explaining the strong clustering observed
for Lyman-break galaxies without any need to assume that Lyman-break galaxies
are associated only with the most massive systems at z~3.Comment: 4 pages, 2 figures. To appear in `The Hy-redshift universe: Galaxy
formation and evolution at high redshift' eds. A.J. Bunker and W.J.M. van
Breuge
Measuring the cosmological constant with redshift surveys
It has been proposed that the cosmological constant might be
measured from geometric effects on large-scale structure. A positive vacuum
density leads to correlation-function contours which are squashed in the radial
direction when calculated assuming a matter-dominated model. We show that this
effect will be somewhat harder to detect than previous calculations have
suggested: the squashing factor is likely to be , given realistic
constraints on the matter contribution to . Moreover, the geometrical
distortion risks being confused with the redshift-space distortions caused by
the peculiar velocities associated with the growth of galaxy clustering. These
depend on the density and bias parameters via the combination , and we show that the main practical effect of a geometrical
flattening factor is to simulate gravitational instability with . Nevertheless, with datasets of sufficient size it is
possible to distinguish the two effects; we discuss in detail how this should
be done. New-generation redshift surveys of galaxies and quasars are
potentially capable of detecting a non-zero vacuum density, if it exists at a
cosmologically interesting level.Comment: MNRAS in press. 12 pages LaTeX including Postscript figures. Uses
mn.sty and epsf.st
Power Spectrum Analysis of the 2dF QSO Sample Revisited
We revisit the power spectrum analysis of the complete sample of the two
degree field (2dF) QSO redshift (2QZ) survey, as a complementary test of the
work by Outram et al. (2003). A power spectrum consistent with that of the 2QZ
group is obtained. Differently from their approach, fitting of the power
spectrum is investigated incorporating the nonlinear effects, the geometric
distortion and the light-cone effect. It is shown that the QSO power spectrum
is consistent with the cold dark matter (CDM) model with the matter
density parameter . Our constraint on the density
parameter is rather weaker than that of the 2QZ group. We also show that the
constraint slightly depends on the equation of state parameter of the dark
energy. The constraint on from the QSO power spectrum is demonstrated,
though it is not very tight.Comment: 15 pages, 5 figures, accepted for publication in the Astrophysical
Journa
Using galaxy pairs as cosmological tracers
The Alcock-Paczynski (AP) effect uses the fact that, when analyzed with the
correct geometry, we should observe structure that is statistically isotropic
in the Universe. For structure undergoing cosmological expansion with the
background, this constrains the product of the Hubble parameter and the angular
diameter distance. However, the expansion of the Universe is inhomogeneous and
local curvature depends on density. We argue that this distorts the AP effect
on small scales. After analyzing the dynamics of galaxy pairs in the Millennium
simulation, we find an interplay between peculiar velocities, galaxy properties
and local density that affects how pairs trace cosmological expansion. We find
that only low mass, isolated galaxy pairs trace the average expansion with a
minimum "correction" for peculiar velocities. Other pairs require larger, more
cosmology and redshift dependent peculiar velocity corrections and, in the
small-separation limit of being bound in a collapsed system, do not carry
cosmological information.Comment: 15 pages, 14 figures, 1 tabl
Satellite remote sensing can provide semi-automated monitoring to aid coastal decision-making
Coastlines are projected to face unprecedented pressures over the next century due to climate change-induced changes in sea level, storm, wave, and tidal regimes. This projection of increasing pressure is driving a reappraisal of existing shoreline management practices, with both science and policy calling for future strategies to work with the natural protection provided by coastal habitats such as salt marshes. However, we currently lack the understanding of long-term ecosystem dynamics required to incorporate these habitats into the definitive predictions of risk relied on in coastal protection planning. Satellite remote sensing has the potential to provide data that could address this knowledge gap with its frequent repeat times and global coverage facilitating the production of high temporal frequency time-series over large areas. This study sought to explore this potential in one of the largest coastal plain estuaries the in the UK, the Severn Estuary. The Random Forest machine learning algorithm was used to develop a time-series of marsh extents across the estuary from 1985 to 2020 in Google Earth Engine, with widths also extracted as a proxy for the marshes’ protective capacity. These changes were monitored in six areas that contained the most significant areas of salt marsh across the estuary. This analysis revealed a significant increasing trend in extent and widths (p 90% and a strong agreement found between the detected widths and those found in previous surveys. These findings demonstrate that satellite remote sensing combined with machine learning has the potential to provide valuable insights into changes in the extents of marshes and therefore their protective capacity. This information can be useful in the coastal planning process, allowing decision-makers to assess the sustainability of existing defences fronted by marshes, as well as allowing them to make informed decisions about the location of restoration schemes
Forecasting the Cosmological Constraints with Anisotropic Baryon Acoustic Oscillations from Multipole Expansion
Baryon acoustic oscillations (BAOs) imprinted in the galaxy power spectrum
can be used as a standard ruler to determine angular diameter distance and
Hubble parameter at high redshift galaxies. Combining redshift distortion
effect which apparently distorts the galaxy clustering pattern, we can also
constrain the growth rate of large-scale structure formation. Usually, future
forecast for constraining these parameters from galaxy redshift surveys has
been made with a full 2D power spectrum characterized as function of wavenumber
and directional cosine between line-of-sight direction and wave
vector, i.e., . Here, we apply the multipole expansion to the full 2D
power spectrum, and discuss how much cosmological information can be extracted
from the lower-multipole spectra, taking a proper account of the non-linear
effects on gravitational clustering and redshift distortion. The Fisher matrix
analysis reveals that compared to the analysis with full 2D spectrum, a partial
information from the monopole and quadrupole spectra generally degrades the
constraints by a factor of for each parameter. The additional
information from the hexadecapole spectrum helps to improve the constraints,
which lead to an almost comparable result expected from the full 2D spectrum.Comment: 12 pages, 6 figure
A Geometrical Test of the Cosmological Energy Contents Using the Lyman-alpha Forest
In this Letter we explore a version of the test of cosmological geometry
proposed by Alcock and Paczynski (1979), using observations of the Lyman-alpha
forest in the spectra of close quasar pairs. By comparing the correlations in
absorption in one quasar spectrum with correlations between the spectra of
neighboring quasars one can determine the relation of the redshift distance
scale to the angle distance scale at the redshift of the absorbers, . Since this relationship depends on the parameters of the cosmological
model, these parameters may be determined using the Lyman-alpha forest. While
this test is relatively insensitive to the density parameter in a
dust-dominated universe, it is more sensitive to the presence of a matter
component with large negative pressure (such as a cosmological constant
) and its equation of state. With only 25 pairs of quasar spectra at
angular separations , one can discriminate between an open universe () and an flat
(-dominated) universe at the level. The S/N can be enhanced
by considering quasar pairs at smaller angular separations, but requires proper
modeling of nonlinear redshift space distortions. Here the correlations and
redshift space distortions are modeled using linear theory.Comment: 13 pages, 2 ps figures, submitted to ApJ
Can Geometric Test Probe the Cosmic Equation of State ?
Feasibility of the geometric test as a probe of the cosmic equation of state
of the dark energy is discussed assuming the future 2dF QSO sample. We examine
sensitivity of the QSO two-point correlation functions, which are theoretically
computed incorporating the light-cone effect and the redshift distortions, as
well as the nonlinear effect, to a bias model whose evolution is
phenomenologically parameterized. It is shown that the correlation functions
are sensitive on a mean amplitude of the bias and not to the speed of the
redshift evolution. We will also demonstrate that an optimistic geometric test
could suffer from confusion that a signal from the cosmological model can be
confused with that from a stochastic character of the bias.Comment: 11 pages, including 3 figures, accepted for publication in ApJ
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
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