1,548 research outputs found

### Lensing Sunyaev-Zel'dovich Clusters

Full-sky microwave surveys like the upcoming Planck satellite mission will
detect of order 10^4 galaxy clusters through their thermal Sunyaev-Zel'dovich
effect. I investigate the properties of the gravitationally lensing subsample
of these clusters. The main results are: (1) The combined sample comprises
>~70% of the complete sample. (2) It is confined to redshifts 0.2+-0.1, and to
masses (5+-3) x 10^14 solar masses. (3) Using a particular measure for the weak
lensing effect, viz. the aperture mass, cluster masses can be determined with a
relative accuracy of ~20% if their density profile is known. Consequently, the
mass function of the combined sample can accurately be measured. (4) For
low-density universes, I predict a sharp peak in the measured (aperture) mass
function near 5 x 10^14 solar masses and explain its origin, showing that the
peak will be absent in high-density universes. (5) The location of the peak and
the exponential decrease of the mass function on its high-mass side will allow
the determination of the amplitude of the dark-matter power spectrum on the
cluster scale and the baryon fraction in clusters, and constrain the thermal
history of the intracluster gas.Comment: submitted to Astronomy & Astrophysic

### Cosmological Information from Quasar-Galaxy Correlations induced by Weak Lensing

The magnification bias of large-scale structures, combined with galaxy
biasing, leads to a cross-correlation of distant quasars with foreground
galaxies on angular scales of the order of arc minutes and larger. The
amplitude and angular shape of the cross-correlation function w_QG contain
information on cosmological parameters and the galaxy bias factor. While the
existence of this cross-correlation has firmly been established, existing data
did not allow an accurate measurement of w_QG yet, but wide area surveys like
the Sloan Digital Sky Survey now provide an ideal database for measuring it.
However, w_QG depends on several cosmological parameters and the galaxy bias
factor. We study in detail the sensitivity of w_QG to these parameters and
develop a strategy for using the data. We show that the parameter space can be
reduced to the bias factor b, Omega_0 and sigma_8, and compute the accuracy
with which these parameters can be deduced from SDSS data. Under reasonable
assumptions, it should be possible to reach relative accuracies of the order of
5%-15% for b, Omega_0, and sigma_8. This method is complementary to other
weak-lensing analyses based on cosmic shear.Comment: 11 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic

### Triaxial collapse and virialisation of dark-matter haloes

We reconsider the ellipsoidal-collapse model and extend it in two ways: We
modify the treatment of the external gravitational shear field, introducing a
hybrid model in between linear and non-linear evolution, and we introduce a
virialisation criterion derived from the tensor virial theorem to replace the
ad-hoc criterion employed so far. We compute the collapse parameters delta_c
and Delta_v and find that they increase with ellipticity e and decrease with
prolaticity p. We marginalise them over the appropriate distribution of e and p
and show the marginalised results as functions of halo mass and virialisation
redshift. While the hybrid model for the external shear gives results very
similar to those obtained from the non-linear model, ellipsoidal collapse
changes the collapse parameters typically by (20...50)%, in a way increasing
with decreasing halo mass and decreasing virialisation redshift. We
qualitatively confirm the dependence on mass and virialisation redshift of a
fitting formula for delta_c, but find noticeable quantitative differences in
particular at low mass and high redshift. The derived mass function is in good
agreement with mass functions recently proposed in the literature.Comment: 9 pages, 9 figures, published in Astronomy and Astrophysics; slight
modifications to match the published versio

### QSO-galaxy correlations due to weak lensing in arbitrary Friedmann-Lemaitre cosmologies

We calculate the angular cross-correlation function between background QSOs
and foreground galaxies induced by the weak lensing effect of large-scale
structures. Results are given for arbitrary Friedmann-Lemaitre cosmologies. The
non-linear growth of density perturbations is included. Compared to the linear
growth, the non-linear growth increases the correlation amplitude by about an
order of magnitude in an Einstein-de Sitter universe, and by even more for
lower Omega_0. The dependence of the correlation amplitude on the cosmological
parameters strongly depends on the normalization of the power spectrum. The
QSO-galaxy cross-correlation function is most sensitive to density structures
on scales in the range (1-10) Mpc/h, where the normalization of the power
spectrum to the observed cluster abundance appears most appropriate. In that
case, the correlation strength changes by less than a factor of <~ 2 when
Omega_0 varies between 0.3 and 1, quite independent of the value of
Omega_Lambda. For Omega_0 <~ 0.3, the correlation strength increases with
decreasing Omega_0, and it scales approximately linearly with the Hubble
constant h.Comment: revised version, accepted by MNRA

### The lens parallax method: determining redshifts of faint blue galaxies through gravitational lensing

We propose a new technique, which we call the lens parallax method, to
determine simultaneously the redshift distribution of the faint blue galaxies
and the mass distributions of foreground clusters of galaxies. The method is
based on gravitational lensing and makes use of the following: (1) the
amplitude of lensing-induced distortions of background galaxies increases with
redshift; (2) the surface brightnesses of galaxies decrease steeply with
redshift. The distortions of galaxy images due to lensing are thus expected to
be inversely correlated with surface brightness, allowing us to obtain relative
distances to galaxies as a function of surface brightness. If the redshifts of
the brightest galaxies are measured, then the relative distance scale can be
converted to mean galaxy redshifts as a function of surface brightness.
Further, by comparing the angular sizes of lensed galaxies with those of
similar galaxies in empty control fields, it is possible to break the so-called
mass sheet degeneracy inherent to cluster mass reconstruction techniques which
are based purely on image ellipticities. This allows an unambiguous
determination of the surface density of a lensing cluster. We describe an
iterative algorithm based on these ideas and present numerical simulations
which show that the proposed techniques are feasible with a sample of ~ 10 rich
clusters at moderate redshifts ~ 0.3-0.4 and an equal number of control fields.
The numerical tests show that the method can be used to determine the redshifts
of galaxies with an accuracy of dz ~ 0.1-0.2 at z ~ 1-1.7, and to measure the
masses of lensing clusters to about 5% accuracy.Comment: 31 pages, uuencoded compressed postscript file containing 10 figures,
to be published in the Sep. 20 issue of Ap

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