266 research outputs found
The APM cluster-galaxy cross-correlation function : Constraints on Omega and galaxy bias
(abridged) We estimate the cluster-galaxy cross-correlation function (Xi_cg),
from the APM galaxy and galaxy cluster surveys, both in real space from the
inversion of projected statistics and in redshift space using the galaxy and
cluster redshift samples. The amplitude of Xi_cg is found to be almost
independent of cluster richness. At large separations, r >~5 h^-1 Mpc, Xi_cg
has a similar shape to the galaxy-galaxy and cluster-cluster autocorrelation
functions. Xi_cg in redshift space can be related to the real space Xi_cg by
convolution with an appropriate velocity field model. Here we apply a spherical
collapse model, which we have tested against N-body simulations, finding that
it provides a surprisingly accurate description of the averaged infall velocity
of matter into galaxy clusters. We use this model to estimate beta
(Omega^{0.6}/b) and find that it tends to overestimate the true result in
simulations by only ~10-30%. Application to the APM results yields beta=0.43
with beta < 0.87 at 95% confidence. We also compare the APM Xi_cg and galaxy
autocorrelations to results from popular cosmological models and derive two
independent estimates of the galaxy biasing expected as a function of scale.
Both low and critical density CDM models require anti-biasing by a factor ~2 on
scales r <~ 2 h^-1Mpc and an MDM model is consistent with a constant biasing
factor on all scales. We use the velocity fields predicted from the different
models to distort the APM real space cross-correlation function. Comparison
with the APM redshift space Xi_cg yields an estimate of the value of Omega^0.6
needed in each model. Only the low Omega model is fully consistent with
observations, with MDM marginally excluded at the ~2 sigma level.Comment: Latex (mn.sty), 17 pages, 16 ps figs, submitted to MNRA
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
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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