7,259 research outputs found
The Correlation Function of Rich Clusters of Galaxies in CDM-like Models
We use ensembles of high-resolution CDM simulations to investigate the shape
and amplitude of the two point correlation function of rich clusters. The
standard scale-invariant CDM model with provides a poor description
of the clustering measured from the APM rich cluster redshift survey, which is
better fitted by models with more power at large scales. The amplitudes of the
rich cluster correlation functions measured from our models depend weakly on
cluster richness. Analytic calculations of the clustering of peaks in a
Gaussian density field overestimate the amplitude of the N-body cluster
correlation functions, but reproduce qualitatively the weak trend with cluster
richness. Our results suggest that the high amplitude measured for the
correlation function of richness class Abell clusters is either an
artefact arising from incompleteness in the Abell catalogue, or an indication
that the density perturbations in the early universe were very non-Gaussian.Comment: uuencoded compressed postscript ,MNRAS, in press, OUAST-93-1
Reconstruction of cosmological density and velocity fields in the Lagrangian Zel'dovich Approximation
We present a method for reconstructing cosmological densityn for and velocity
fields using the Lagrangian Zel'dovich formalism. . The method involves finding
the least action solution for straight line particle paths in an evolving
density field. Our starting point is the final, evolved density , so that we
are in effect carrying out the standard Zel'dovich Approximation based process
in reverse. Using a simple numerical algorithm we are able to minimise the
action for the trajectories of several million particles. We apply our method
to the evolved density taken from N-body simulations of different cold dark
matter dominated universes, testing both the prediction for the present day
velocity field and for the initial density field. The method is easy to apply,
reproduces the accuracy of the forward Zel'dovich Approximation, and also works
directly in redshift space with minimal modification.Comment: 13 pages with only 2 (out 9) figures. MNRAS in press. New Appendix
shows the relation between shell crossing and PIZA. A completed version with
all 9 figures available by anonymous ftp at
ftp://bessel.mps.ohio-state.edu/pub/racc/piza.ps.gz (USA) or
ftp://ftp-astro.physics.ox.ac.uk/pub/eg/piza3.ps.gz (UK
Lyman-alpha forest-CMB cross-correlation and the search for the ionized baryons at high redshift
The intergalactic neutral hydrogen which is responsible for the Lyman alpha
forest of quasar absorption is a tracer of much larger amounts of ionised
hydrogen. The ionised component has yet to be detected directly, but is
expected to scatter CMB photons via the Sunyaev-Zel'dovich (SZ) effect. We use
hydrodynamic simulations of a LambdaCDM universe to create mock quasar spectra
and CMB sky maps. We find that the high-z Lya forest gas causes temperature
fluctuations of the order of 1 muK rms in the CMB on arcmin scales. The kinetic
and thermal SZ effects have a similar magnitude at z=3, with the thermal effect
becoming relatively weaker as expected at higher z. The CMB signal associated
with lines of sight having HI column densities > 10^18 cm^-2 is only marginally
stronger than that for lower column densities. The strong dependence of rms
temperature fluctuation on mean Lya absorbed flux, however, suggests that the
CMB signal effectively arises in lower density material. We investigate the use
of the cross-correlation of the Lya forest and the microwave background to
detect the SZ effect at redshifts 2-4. In so doing we are able to set direct
limits on the density of diffuse ionised intergalactic baryons. We carry out a
preliminary comparison at a mean redshift z=3 of 3488 quasar spectra from SDSS
Data Release 3 and the WMAP first year data. Assuming that the baryons are
clustered as in a LambdaCDM cosmology, and have the same mean temperature, the
cross-correlation yields a weak limit on the cosmic density of ionised baryons
Omega_(b,I), which is Omega_(b,I) < 0.8 at 95% confidence. With data from
upcoming CMB telescopes, we anticipate that a direct detection of the high
redshift ionised IGM will soon be possible, providing an important consistency
check on cosmological models.Comment: 14 pages, 10 figures, submitted to MNRA
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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