2,603 research outputs found
On the "initial" Angular Momentum of Galaxies
Spherical density profiles and specific angular momentum profiles of Dark
Matter halos found in cosmological N-body simulations have been measured
extensively. The distribution of the total angular momentum of dark matter
halos is also used routinely in semi-analytic modeling of the formation of disk
galaxies. However, it is unclear whether the initial (i.e. at the time the halo
is assembled) angular momentum distributions of baryons is related to the dark
matter at all. Theoretical models for ellipticities in weak lensing studies
often rely on an assumed correlation of the angular momentum vectors of dark
matter and gas in galaxies. Both of these assumptions are shown to be in
reasonable agreement with high resolution cosmological smoothed particle
hydrodynamical simulations that follow the dark matter as long as only
adiabatic gas physics are included. However, we argue that in more realistic
models of galaxy formation one expects pressure forces to play a significant
role at turn--around. Consequently the torquing force on DM and baryons will be
uncorrelated and their respective angular momenta are not expected to align. An
SPH simulation with ad-hoc feedback is presented that illustrates these
effects. Massive low redshift elliptical galaxies may be a notable exception
where "light may trace mass".Comment: 4 latex pages (uses sprocl.sty), 1 eps figure. To appear in the
proceedings of "The Shapes of Galaxies and Their Halos", Yale, May 200
Spatial curvature endgame: Reaching the limit of curvature determination
Current constraints on spatial curvature show that it is dynamically
negligible: (95% CL). Neglecting
it as a cosmological parameter would be premature however, as more stringent
constraints on at around the level would offer
valuable tests of eternal inflation models and probe novel large-scale
structure phenomena. This precision also represents the "curvature floor",
beyond which constraints cannot be meaningfully improved due to the cosmic
variance of horizon-scale perturbations. In this paper, we discuss what future
experiments will need to do in order to measure spatial curvature to this
maximum accuracy. Our conservative forecasts show that the curvature floor is
unreachable - by an order of magnitude - even with Stage IV experiments, unless
strong assumptions are made about dark energy evolution and the CDM
parameter values. We also discuss some of the novel problems that arise when
attempting to constrain a global cosmological parameter like
with such high precision. Measuring curvature down to this level would be an
important validation of systematics characterisation in high-precision
cosmological analyses.Comment: 9 pages, 1 figure. Updated to match version published in Phys. Rev.
INFOBazaar--electronic shopping and service providing for the masses
Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1995.Includes bibliographical references (p. 89-90).by Rupert C. Young.M.S
Weak lensing of the Lyman-alpha forest
The angular positions of quasars are deflected by the gravitational lensing
effect of foreground matter. The Lyman-alpha forest seen in the spectra of
these quasars is therefore also lensed. We propose that the signature of weak
gravitational lensing of the forest could be measured using similar techniques
that have been applied to the lensed Cosmic Microwave Background, and which
have also been proposed for application to spectral data from 21cm radio
telescopes. As with 21cm data, the forest has the advantage of spectral
information, potentially yielding many lensed "slices" at different redshifts.
We perform an illustrative idealized test, generating a high resolution angular
grid of quasars (of order arcminute separation), and lensing the
Lyman-alphaforest spectra at redshifts z=2-3 using a foreground density field.
We find that standard quadratic estimators can be used to reconstruct images of
the foreground mass distribution at z~1. There currently exists a wealth of Lya
forest data from quasar and galaxy spectral surveys, with smaller sightline
separations expected in the future. Lyman-alpha forest lensing is sensitive to
the foreground mass distribution at redshifts intermediate between CMB lensing
and galaxy shear, and avoids the difficulties of shape measurement associated
with the latter. With further refinement and application of mass reconstruction
techniques, weak gravitational lensing of the high redshift Lya forest may
become a useful new cosmological probe.Comment: 9 pages, 7 figures, submitted to MNRA
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