503 research outputs found
Intrinsic Galaxy Alignments and Weak Gravitational Lensing
Gravitational lensing causes background galaxy images to become aligned, and
the statistical characteristics of the image alignments can then be used to
constrain the power spectrum of mass fluctuations. Analyses of gravitational
lensing assume that intrinsic galaxy alignments are negligible, but if this
assumption does not hold, then the interpretation of image alignments will be
in error. As gravitational lensing experiments become more ambitious and seek
very low-level alignments arising from lensing by large-scale structure, it
becomes more important to estimate the level of intrinsic alignment in the
galaxy population. In this article, I review the cluster of independent
theoretical studies of this issue, as well as the current observational status.
Theoretically, the calculation of intrinsic alignments is by no means
straightforward, but some consensus has emerged from the existing works,
despite each making very different assumptions. This consensus is that a)
intrinsic alignments are a small but non-negligible (< 10%) contaminant of the
lensing ellipticity correlation function, for samples with a median redshift z
= 1; b) intrinsic alignments dominate the signal for low-redshift samples (z =
0.1), as expected in the SuperCOSMOS lensing survey and the Sloan Digital Sky
SurveyComment: 8 pages. Invited talk at Yale Workshop on `The Shapes of Galaxies and
their halos', May 200
The nonlinear redshift-space power spectrum of galaxies
We study the power spectrum of galaxies in redshift space, with third order
perturbation theory to include corrections that are absent in linear theory. We
assume a local bias for the galaxies: i.e. the galaxy density is sampled from
some local function of the underlying mass distribution. We find that the
effect of the nonlinear bias in real space is to introduce two new features:
first, there is a contribution to the power which is constant with wavenumber,
whose nature we reveal as essentially a shot-noise term. In principle this
contribution can mask the primordial power spectrum, and could limit the
accuracy with which the latter might be measured on very large scales.
Secondly, the effect of second- and third-order bias is to modify the effective
bias (defined as the square root of the ratio of galaxy power spectrum to
matter power spectrum). The effective bias is almost scale-independent over a
wide range of scales. These general conclusions also hold in redshift space. In
addition, we have investigated the distortion of the power spectrum by peculiar
velocities, which may be used to constrain the density of the Universe. We look
at the quadrupole-to-monopole ratio, and find that higher-order terms can mimic
linear theory bias, but the bias implied is neither the linear bias, nor the
effective bias referred to above. We test the theory with biased N-body
simulations, and find excellent agreement in both real and redshift space,
providing the local biasing is applied on a scale whose fractional r.m.s.
density fluctuations are .Comment: 13 pages, 7 figures. Accepted by MNRA
3D Weak Gravitational Lensing of the CMB and Galaxies
In this paper we present a power spectrum formalism that combines the full
three-dimensional information from the galaxy ellipticity field, with
information from the cosmic microwave background (CMB). We include in this
approach galaxy cosmic shear and galaxy intrinsic alignments, CMB deflection,
CMB temperature and CMB polarisation data; including the inter-datum power
spectra between all quantities. We apply this to forecasting cosmological
parameter errors for CMB and imaging surveys for Euclid-like, Planck, ACTPoL,
and CoRE-like experiments. We show that the additional covariance between the
CMB and ellipticity measurements can improve dark energy equation of state
measurements by 15%, and the combination of cosmic shear and the CMB, from
Euclid-like and CoRE-like experiments, could in principle measure the sum of
neutrino masses with an error of 0.003 eV.Comment: Accepted to MNRA
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
Estimating non-gaussianity in the microwave background
The bispectrum of the microwave background sky is a possible discriminator
between inflationary and defect models of structure formation in the Universe.
The bispectrum, which is the analogue of the temperature 3-point correlation
function in harmonic space, is zero for most inflationary models, but non-zero
for non-gaussian models. The expected departures from zero are small, and
easily masked by noise, so it is important to be able to estimate the
bispectrum coefficients as accurately as possible, and to know the errors and
correlations between the estimates so they may be used in combination as a
diagnostic to rule out non-gaussian models. This paper presents a method for
estimating in an unbiased way the bispectrum from a microwave background map in
the near-gaussian limit. The method is optimal, in the sense that no other
method can have smaller error bars, and in addition, the covariances between
the bispectrum estimates are calculated explicitly. The method deals
automatically with partial sky coverage and arbitrary noise correlations
without modification. A preliminary application to the Cosmic Background
Explorer 4-year dataset shows no evidence for non-gaussian behaviour.Comment: 5 pages. No figures. To appear in MNRA
Loss of star forming gas in SDSS galaxies
Using the star formation rates from the SDSS galaxy sample, extracted using
the MOPED algorithm, and the empirical Kennicutt law relating star formation
rate to gas density, we calculate the time evolution of the gas fraction as a
function of the present stellar mass. We show how the gas-to-stars ratio varies
with stellar mass, finding good agreement with previous results for smaller
samples at the present epoch. For the first time we show clear evidence for
progressive gas loss with cosmic epoch, especially in low-mass systems. We find
that galaxies with small stellar masses have lost almost all of their cold
baryons over time, whereas the most massive galaxies have lost little. Our
results also show that the most massive galaxies have evolved faster and turned
most of their gas into stars at an early time, thus strongly supporting a
downsizing scenario for galaxy evolution.Comment: 29 pages, 9 figures, ApJ, accepte
Measuring dark energy properties with 3D cosmic shear
We present parameter estimation forecasts for present and future 3D cosmic
shear surveys. We demonstrate that, in conjunction with results from cosmic
microwave background (CMB) experiments, the properties of dark energy can be
estimated with very high precision with large-scale, fully 3D weak lensing
surveys. In particular, a 5-band, 10,000 square degree ground-based survey to a
median redshift of zm=0.7 could achieve 1- marginal statistical errors,
in combination with the constraints expected from the CMB Planck Surveyor, of
w0=0.108 and wa=0.099 where we parameterize w by
w(a)=w0+wa(1-a) where a is the scale factor. Such a survey is achievable with a
wide-field camera on a 4 metre class telescope. The error on the value of w at
an intermediate pivot redshift of z=0.368 is constrained to
w(z=0.368)=0.0175. We compare and combine the 3D weak lensing
constraints with the cosmological and dark energy parameters measured from
planned Baryon Acoustic Oscillation (BAO) and supernova Type Ia experiments,
and find that 3D weak lensing significantly improves the marginalized errors. A
combination of 3D weak lensing, CMB and BAO experiments could achieve
w0=0.037 and wa=0.099. Fully 3D weak shear analysis avoids the
loss of information inherent in tomographic binning, and we show that the
sensitivity to systematic errors is much less. In conjunction with the fact
that the physics of lensing is very soundly based, this analysis demonstrates
that deep, wide-angle 3D weak lensing surveys are extremely promising for
measuring dark energy properties.Comment: 18 pages, 16 figures. Accepted to MNRAS. Figures now in grayscale.
Further discussions on non-Gaussianity and photometric redshift errors. Some
references adde
Wide Angle Redshift Distortions Revisited
We explore linear redshift distortions in wide angle surveys from the point
of view of symmetries. We show that the redshift space two-point correlation
function can be expanded into tripolar spherical harmonics of zero total
angular momentum . The
coefficients of the expansion are analogous to the 's of
the angular power spectrum, and express the anisotropy of the redshift space
correlation function. Moreover, only a handful of are
non-zero: the resulting formulae reveal a hidden simplicity comparable to
distant observer limit. The depend on spherical Bessel
moments of the power spectrum and . In the plane parallel
limit, the results of \cite{Kaiser1987} and \cite{Hamilton1993} are recovered.
The general formalism is used to derive useful new expressions. We present a
particularly simple trigonometric polynomial expansion, which is arguably the
most compact expression of wide angle redshift distortions. These formulae are
suitable to inversion due to the orthogonality of the basis functions. An
alternative Legendre polynomial expansion was obtained as well. This can be
shown to be equivalent to the results of \cite{SzalayEtal1998}. The simplicity
of the underlying theory will admit similar calculations for higher order
statistics as well.Comment: 6 pages, 1 figure, ApJL submitte
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