132 research outputs found
The distortion of distant galaxy images by large-scale structure
Inhomogeneity in the distribution of mass in the universe on scales ≲ 100 Mpc can generate a coherent shear distortion or polarization of the images of background galaxies. This distortion may be measurable over patches of the sky up to a few square degrees in size. If this distortion is measured, or conversely, if its magnitude is limited, it should help us understand the degree to which luminosity traces the underlying mass over cosmological scales. A prescription is given for quantifying the galaxy distortion and a propagation equation for its evolution in an inhomogeneous universe is derived. The creation of shear by inhomogeneity is illustrated using model kinematic universes comprising random distributions of point masses, spheres and circular discs designed to simulate the superclusters, voids and ‘walls’ reported in galaxy velocity surveys. Using these simulations, we estimate that an rms induced ellipticity of |p|_(rms) ˜ 0.2Ω_(LSS) (where Ω_(LSS) is the fraction of the mass of the universe clustered on the large scale) will be produced. The angular correlation length is ˜ 1.6°.
In an alternative prescription, the universe is modelled using a power spectrum of density fluctuations and the mean correlation function is computed both analytically and numerically. In these simulations we find that |p|_(rms) ˜0.02 for biased cold dark matter models of an Einslein–De Sitter universe, and the effective correlation length is θ_(1/2) ˜ 0.5^ΰ. For a hot dark matter dominated universe the correlation length is θ_(1/2) ˜ 0.7^ΰ.
The faint, blue galaxies discovered by Tyson and collaborators have a surface density of ˜ 3 × 10^5 deg^(−2) and should provide an ideal set of sources for measuring this effect
Warped Galaxies From Misaligned Angular Momenta
A galaxy disk embedded in a rotating halo experiences a dynamical friction
force which causes it to warp when the angular momentum axes of the disk and
halo are misaligned. Our fully self-consistent simulations of this process
induce long-lived warps in the disk which mimic Briggs's rules of warp
behavior. They also demonstrate that random motion within the disk adds
significantly to its stiffness. Moreover, warps generated in this way have no
winding problem and are more pronounced in the extended \h1 disk. As emphasized
by Binney and his co-workers, angular momentum misalignments, which are
expected in hierarchical models of galaxy formation, can account for the high
fraction of warped galaxies. Our simulations exemplify the role of misaligned
spins in warp formation even when the halo density is not significantly
flattened.Comment: 6 pages, 5 figures. Accepted for publication in Ap.J.
Cosmological Model Predictions for Weak Lensing: Linear and Nonlinear Regimes
Weak lensing by large scale structure induces correlated ellipticities in the
images of distant galaxies. The two-point correlation is determined by the
matter power spectrum along the line of sight. We use the fully nonlinear
evolution of the power spectrum to compute the predicted ellipticity
correlation. We present results for different measures of the second moment for
angular scales \theta \simeq 1'-3 degrees and for alternative normalizations of
the power spectrum, in order to explore the best strategy for constraining the
cosmological parameters. Normalizing to observed cluster abundance the rms
amplitude of ellipticity within a 15' radius is \simeq 0.01 z_s^{0.6}, almost
independent of the cosmological model, with z_s being the median redshift of
background galaxies.
Nonlinear effects in the evolution of the power spectrum significantly
enhance the ellipticity for \theta < 10' -- on 1' the rms ellipticity is \simeq
0.05, which is nearly twice the linear prediction. This enhancement means that
the signal to noise for the ellipticity is only weakly increasing with angle
for 2'< \theta < 2 degrees, unlike the expectation from linear theory that it
is strongly peaked on degree scales. The scaling with cosmological parameters
also changes due to nonlinear effects. By measuring the correlations on small
(nonlinear) and large (linear) angular scales, different cosmological
parameters can be independently constrained to obtain a model independent
estimate of both power spectrum amplitude and matter density \Omega_m.
Nonlinear effects also modify the probability distribution of the ellipticity.
Using second order perturbation theory we find that over most of the range of
interest there are significant deviations from a normal distribution.Comment: 38 pages, 11 figures included. Extended discussion of observational
prospects, matches accepted version to appear in Ap
The non-Gaussian tail of cosmic-shear statistics
Due to gravitational instability, an initially Gaussian density field
develops non-Gaussian features as the Universe evolves. The most prominent
non-Gaussian features are massive haloes, visible as clusters of galaxies. The
distortion of high-redshift galaxy images due to the tidal gravitational field
of the large-scale matter distribution, called cosmic shear, can be used to
investigate the statistical properties of the LSS. In particular, non-Gaussian
properties of the LSS will lead to a non-Gaussian distribution of cosmic-shear
statistics. The aperture mass () statistics, recently introduced as
a measure for cosmic shear, is particularly well suited for measuring these
non-Gaussian properties. In this paper we calculate the highly non-Gaussian
tail of the aperture mass probability distribution, assuming Press-Schechter
theory for the halo abundance and the `universal' density profile of haloes as
obtained from numerical simulations. We find that for values of
much larger than its dispersion, this probability distribution is closely
approximated by an exponential, rather than a Gaussian. We determine the
amplitude and shape of this exponential for various cosmological models and
aperture sizes, and show that wide-field imaging surveys can be used to
distinguish between some of the currently most popular cosmogonies. Our study
here is complementary to earlier cosmic-shear investigations which focussed
more on two-point statistical properties.Comment: 9 pages, 5 figures, submitted to MNRA
Statistics of Weak Gravitational Lensing in Cold Dark Matter Models; Magnification Bias on Quasar Luminosity Functions
We compute statistical properties of weak gravitational lensing by
large-scale structure in three Cold Dark Matter models. We use a PM
-body code to simulate the formation and evolution of large-scale structure
in the universe. We perform ray-tracing experiments for each
model using the multiple lens-plane algorithm. From the results of these
experiments, we calculate the probability distribution functions (PDF) of the
convergences, shears, and magnifications, and their root-mean-square (rms)
values. We find that the rms values of the convergence and shear agree with the
predictions of a nonlinear analytical model. We also find that the PDFs of the
magnifications have a peak at values slightly smaller than , and
are strongly skewed toward large magnifications. In particular, for the
high-density model, a power-law tail appears in the magnification distribution
at large magnifications for sources at redshifts . The rms values of the
magnifications essentially agree with the nonlinear analytical predictions for
sources at low redshift, but exceed these predictions for high redshift
sources, once the power-law tail appears.
We study the effect of magnification bias on the luminosity functions of
high-redshift quasars, using the calculated PDFs of the magnifications. We show
that the magnification bias is moderate in the absence of the power-law tail in
the magnification distribution, but depends strongly on the value of the
density parameter. In presence of the power-law tail, the bias becomes
considerable, especially at the bright end of the luminosity functions.Comment: 24 pages, 9 figures, LaTex using epsfig.sty. Submitted to the The
Astrophysical Journa
Comparison of analyses of the QTLMAS XII common dataset. I: Genomic selection
<p>Abstract</p> <p>A dataset was simulated and distributed to participants of the QTLMAS XII workshop who were invited to develop genomic selection models. Each contributing group was asked to describe the model development and validation as well as to submit genomic predictions for three generations of individuals, for which they only knew the genotypes. The organisers used these genomic predictions to perform the final validation by comparison to the true breeding values, which were known only to the organisers. Methods used by the 5 groups fell in 3 classes 1) fixed effects models 2) BLUP models, and 3) Bayesian MCMC based models. The Bayesian analyses gave the highest accuracies, followed by the BLUP models, while the fixed effects models generally had low accuracies and large error variance. The best BLUP models as well as the best Bayesian models gave unbiased predictions. The BLUP models are clearly sensitive to the assumed SNP variance, because they do not estimate SNP variance, but take the specified variance as the true variance. The current comparison suggests that Bayesian analyses on haplotypes or SNPs are the most promising approach for Genomic selection although the BLUP models may provide a computationally attractive alternative with little loss of efficiency. On the other hand fixed effect type models are unlikely to provide any gain over traditional pedigree indexes for selection.</p
The distortion of distant galaxy images by large-scale structure
Inhomogeneity in the distribution of mass in the universe on scales ≲ 100 Mpc can generate a coherent shear distortion or polarization of the images of background galaxies. This distortion may be measurable over patches of the sky up to a few square degrees in size. If this distortion is measured, or conversely, if its magnitude is limited, it should help us understand the degree to which luminosity traces the underlying mass over cosmological scales. A prescription is given for quantifying the galaxy distortion and a propagation equation for its evolution in an inhomogeneous universe is derived. The creation of shear by inhomogeneity is illustrated using model kinematic universes comprising random distributions of point masses, spheres and circular discs designed to simulate the superclusters, voids and ‘walls’ reported in galaxy velocity surveys. Using these simulations, we estimate that an rms induced ellipticity of |p|_(rms) ˜ 0.2Ω_(LSS) (where Ω_(LSS) is the fraction of the mass of the universe clustered on the large scale) will be produced. The angular correlation length is ˜ 1.6°.
In an alternative prescription, the universe is modelled using a power spectrum of density fluctuations and the mean correlation function is computed both analytically and numerically. In these simulations we find that |p|_(rms) ˜0.02 for biased cold dark matter models of an Einslein–De Sitter universe, and the effective correlation length is θ_(1/2) ˜ 0.5^ΰ. For a hot dark matter dominated universe the correlation length is θ_(1/2) ˜ 0.7^ΰ.
The faint, blue galaxies discovered by Tyson and collaborators have a surface density of ˜ 3 × 10^5 deg^(−2) and should provide an ideal set of sources for measuring this effect
Bars and Dark Matter Halo Cores
Self-consistent bars that form in galaxies embedded within cuspy halos are
unable to flatten the cusp. Short bars form in models with quasi-flat rotation
curves. They lose angular momentum to the halo through dynamical friction, but
the continuous concentration of mass within the disk as the bar grows actually
compresses the halo further, overwhelming any density reduction due to the
modest angular momentum transfer to the halo. Thus the Weinberg-Katz proposed
solution to the non-existence of the predicted cuspy halos from CDM simulations
would seem to be unworkable. I also find that the concerns over the performance
of N-body codes raised by these authors do not apply to the methods used here.Comment: Latex 11 pages (uses emulateapj.sty), 8 figures, revised version to
appear ApJ, very minor change
A Comparison of Simple Mass Estimators for Galaxy Clusters
High-resolution N-body simulations are used to investigate systematic trends
in the mass profiles and total masses of clusters as derived from 3 simple
estimators: (1) the weak gravitational lensing shear field under the assumption
of an isothermal cluster potential, (2) the dynamical mass obtained from the
measured velocity dispersion under the assumption of an isothermal cluster
potential, and (3) the classical virial estimator. The clusters consist of
order 2.5e+05 particles of mass m_p \simeq 10^{10} \Msun, have triaxial mass
distributions, and significant substructure exists within their virial radii.
Not surprisingly, the level of agreement between the mass profiles obtained
from the various estimators and the actual mass profiles is found to be
scale-dependent.
The virial estimator yields a good measurement of the total cluster mass,
though it is systematically underestimated by of order 10%. This result
suggests that, at least in the limit of ideal data, the virial estimator is
quite robust to deviations from pure spherical symmetry and the presence of
substructure. The dynamical mass estimate based upon a measurement of the
cluster velocity dispersion and an assumption of an isothermal potential yields
a poor measurement of the total mass. The weak lensing estimate yields a very
good measurement of the total mass, provided the mean shear used to determine
the equivalent cluster velocity dispersion is computed from an average of the
lensing signal over the entire cluster (i.e. the mean shear is computed
interior to the virial radius). [abridged]Comment: Accepted for publication in The Astrophysical Journal. Complete
paper, including 3 large colour figures can also be obtained from
http://bu-ast.bu.edu/~brainerd/preprints
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