226 research outputs found
Femtolens Imaging of a Quasar Central Engine Using a Dwarf Star Telescope
We show that it is possible to image the structure of a distant quasar on
scales of AU by constructing a telescope which uses a nearby dwarf
star as its ``primary lens'' together with a satellite-borne ``secondary''. The
image produced by the primary is magnified by in one direction but
is contracted by 0.5 in the other, and therefore contains highly degenerate
one-dimensional information about the two-dimensional source. We discuss
various methods for extracting information about the second dimension including
``femtolens interferometry'' where one measures the interference between
different parts of the one-dimensional image with each other. Assuming that the
satellite could be dispatched to a position along a star-quasar line of sight
at a distance from the Sun, the nearest available dwarf-star primary is
likely to be at \sim 15\,\pc\,(r/40\,\rm AU)^{-2}. The secondary should
consist of a one-dimensional array of mirrors extending m to
achieve 1 AU resolution, or m to achieve 4 AU resolution.Comment: 12 pages including 3 embedded figure
The Gravitational Lensing in Redshift-space Correlation Functions of Galaxies and Quasars
The gravitational lensing, as well as the velocity field and the cosmological
light-cone warp, changes the observed correlation function of high-redshift
objects. We present an analytical expression of 3D correlation function,
simultaneously including those three effects. When two objects are separated
over several hundreds Mpc along a line of sight, the observed correlation
function is dominated by the effect of gravitational lensing rather than the
intrinsic clustering. For a canonical lambda-CDM model, the lensing signals in
the galaxy-galaxy and galaxy-QSO correlations are beyond noise levels in
large-scale redshift surveys like the Sloan Digital Sky Survey.Comment: 10 pages, 1 figure, submitted to ApJ
The distortion of distant galaxy images by large-scale structure
Inhomogeneity in the distribution of mass in the universe on scales âČ 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 âwallsâ reported in galaxy velocity surveys. Using these simulations, we estimate that an rms induced ellipticity of |p|_(rms) Ë 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 Ë 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) Ë0.02 for biased cold dark matter models of an EinsleinâDe Sitter universe, and the effective correlation length is Ξ_(1/2) Ë 0.5^ΰ. For a hot dark matter dominated universe the correlation length is Ξ_(1/2) Ë 0.7^ΰ.
The faint, blue galaxies discovered by Tyson and collaborators have a surface density of Ë 3 Ă 10^5 deg^(â2) and should provide an ideal set of sources for measuring this effect
Magnification-Temperature Correlation: the Dark Side of ISW Measurements
Integrated Sachs-Wolfe (ISW) measurements, which involve cross-correlating
the CMB with the foreground large-scale structure (e.g. galaxies/quasars), have
proven to be an interesting probe of dark energy. We show that magnification
bias, which is the inevitable modulation of the foreground number counts by
gravitational lensing, alters both the shape and amplitude of the observed ISW
signal. This is true especially at high redshifts because (1) the intrinsic
galaxy-temperature signal diminishes greatly back in the matter dominated era,
(2) the lensing efficiency increases with redshift and (3) the number count
slope generally steepens with redshift in a magnitude limited sample. At z >~
2, the magnification-temperature correlation dominates over the intrinsic
galaxy-temperature correlation and causes the observed ISW signal to increase
with z, despite dark energy subdominance -- a result of the fact that
magnification probes structures between the observer and the sources. Ignoring
magnification bias can then lead to erroneous conclusions about dark energy.
While the lensing modulation opens up an interesting high z window for ISW
measurements, high z measurements are not expected to add much new information
to low z ones if dark energy is the cosmological constant. This is because
lensing introduces significant covariance across redshifts. The most compelling
reason to pursue high z ISW measurements is to look for a potential surprise
such as early dark energy domination or the signature of modified gravity. We
conclude with a discussion of existing measurements, the highest z of which is
at the margin of being sensitive to magnification bias. We also develop a
formalism which might be of general interest: to predict biases in estimating
parameters when certain physical effects are ignored in interpreting data.Comment: 14 pages, 12 figures, references added, minor typos corrected,
accepted for publication by PR
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
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.
Testing a new analytic model for gravitational lensing probabilities
We study gravitational lensing with a multiple lens plane approach, proposing
a simple analytical model for the probability distribution function (PDF) of
the dark matter convergence, kappa, for the different lens planes in a given
cosmology as a function of redshift and smoothing angle, theta. The model is
fixed solely by the variance of kappa, which in turn is fixed by the amplitude
of the power spectrum, sigma_8. We test the PDF against a high resolution
Tree-Particle-Mesh simulation and find that it is far superior to the Gaussian
or the lognormal, especially for small values of theta << 1 arcmin and at large
values of kappa relevant to strong lensing. With this model, we predict the
probabilities of strong lensing by a single plane or by multiple planes. We
find that for theta ~ 10 arcsec, a single plane accounts for almost all (~ 98%)
of the strong lensing cases for source redshift unity. However, for a more
typical source redshift of 4, about 12% of the strong lensing cases will result
from the contribution of a secondary clump of matter along the line of sight,
introducing a systematic error in the determination of the surface density of
clusters, typically overestimating it by about 2-5%. We also find that matter
inhomogenieties introduce a dispersion in the value of the angular diameter
distance about its cosmological mean. The probable error relative to the mean
increases with redshift to a value of about 8% for z ~ 6 and theta ~ 10 arcsec.Comment: Accepted for publication in ApJ, 13 pages, 12 figures, revised
version, references added, section 6 expande
The Origin of the Brightest Cluster Galaxies
Most clusters and groups of galaxies contain a giant elliptical galaxy in
their centres which far outshines and outweighs normal ellipticals. The origin
of these brightest cluster galaxies is intimately related to the collapse and
formation of the cluster. Using an N-body simulation of a cluster of galaxies
in a hierarchical cosmological model, we show that galaxy merging naturally
produces a massive, central galaxy with surface brightness and velocity
dispersion profiles similar to observed BCG's. To enhance the resolution of the
simulation, 100 dark halos at are replaced with self-consistent
disk+bulge+halo galaxy models following a Tully-Fisher relation using 100000
particles for the 20 largest galaxies and 10000 particles for the remaining
ones. This technique allows us to analyze the stellar and dark matter
components independently. The central galaxy forms through the merger of
several massive galaxies along a filament early in the cluster's history.
Galactic cannibalism of smaller galaxies through dynamical friction over a
Hubble time only accounts for a small fraction of the accreted mass. The galaxy
is a flattened, triaxial object whose long axis aligns with the primordial
filament and the long axis of the cluster galaxy distribution agreeing with
observed trends for galaxy-cluster alignment.Comment: Revised and accepted in ApJ, 25 pages, 10 figures, online version
available at http://www.cita.utoronto.ca/~dubinski/bcg
The Robustness of Dark Matter Density Profiles in Dissipationless Mergers
We present a comprehensive series of dissipationless N-body simulations to
investigate the evolution of density distribution in equal-mass mergers between
dark matter (DM) halos and multicomponent galaxies. The DM halo models are
constructed with various asymptotic power-law indices ranging from steep cusps
to core-like profiles and the structural properties of the galaxy models are
motivated by the LCDM paradigm of structure formation. The adopted force
resolution allows robust density profile estimates in the inner ~1% of the
virial radii of the simulated systems. We demonstrate that the central slopes
and overall shapes of the remnant density profiles are virtually identical to
those of the initial systems suggesting that the remnants retain a remarkable
memory of the density structure of their progenitors, despite the relaxation
that accompanies merger activity. We also find that halo concentrations remain
approximately constant through hierarchical merging involving identical systems
and show that remnants contain significant fractions of their bound mass well
beyond their formal virial radii. These conclusions hold for a wide variety of
initial asymptotic density slopes, orbital energies, and encounter
configurations, including sequences of consecutive merger events, simultaneous
mergers of severals ystems, and mergers of halos with embedded cold baryonic
components in the form of disks, spheroids, or both. As an immediate
consequence, the net effect of gas cooling, which contracts and steepens the
inner density profiles of DM halos, should be preserved through a period of
dissipationless major merging. Our results imply that the characteristic
universal shape of DM density profiles may be set early in the evolution of
halos.Comment: Accepted for publication in ApJ, 20 pages, 10 figures, LaTeX (uses
emulateapj.cls
Ray Tracing Simulations of Weak Lensing by Large-Scale Structure
We investigate weak lensing by large-scale structure using ray tracing
through N-body simulations. Photon trajectories are followed through high
resolution simulations of structure formation to make simulated maps of shear
and convergence on the sky. Tests with varying numerical parameters are used to
calibrate the accuracy of computed lensing statistics on angular scales from
about 1 arcminute to a few degrees. Various aspects of the weak lensing
approximation are also tested. For fields a few degrees on a side the shear
power spectrum is almost entirely in the nonlinear regime and agrees well with
nonlinear analytical predictions. Sampling fluctuations in power spectrum
estimates are investigated by comparing several ray tracing realizations of a
given model. For survey areas smaller than a degree on a side the main source
of scatter is nonlinear coupling to modes larger than the survey. We develop a
method which uses this effect to estimate the mass density parameter Omega from
the scatter in power spectrum estimates for subregions of a larger survey. We
show that the power spectrum can be measured accurately from realistically
noisy data on scales corresponding to 1-10 Mpc/h. Non-Gaussian features in the
one point distribution function of the weak lensing convergence (reconstructed
from the shear) are also sensitive to Omega. We suggest several techniques for
estimating Omega in the presence of noise and compare their statistical power,
robustness and simplicity. With realistic noise Omega can be determined to
within 0.1-0.2 from a deep survey of several square degrees.Comment: 59 pages, 22 figures included. Matches version accepted for Ap
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