100 research outputs found

### A Bayesian analysis of the 69 highest energy cosmic rays detected by the Pierre Auger Observatory

The origins of ultra-high energy cosmic rays (UHECRs) remain an open
question. Several attempts have been made to cross-correlate the arrival
directions of the UHECRs with catalogs of potential sources, but no definite
conclusion has been reached. We report a Bayesian analysis of the 69 events
from the Pierre Auger Observatory (PAO), that aims to determine the fraction of
the UHECRs that originate from known AGNs in the Veron-Cety & Veron (VCV)
catalog, as well as AGNs detected with the Swift Burst Alert Telescope
(Swift-BAT), galaxies from the 2MASS Redshift Survey (2MRS), and an additional
volume-limited sample of 17 nearby AGNs. The study makes use of a multi-level
Bayesian model of UHECR injection, propagation and detection. We find that for
reasonable ranges of prior parameters, the Bayes factors disfavour a purely
isotropic model. For fiducial values of the model parameters, we report 68%
credible intervals for the fraction of source originating UHECRs of
0.09+0.05-0.04, 0.25+0.09-0.08, 0.24+0.12-0.10, and 0.08+0.04-0.03 for the VCV,
Swift-BAT and 2MRS catalogs, and the sample of 17 AGNs, respectively

### Gravitational lensing in galaxy redshift surveys

Gravitationally-lensed quasars should be discovered as a by-product of large
galaxy redshift surveys, being discovered spectroscopically when a low-redshift
galaxy exhibits high-redshift quasar emission lines. The number of lenses
expected is higher than previously estimated, mainly due to the fact that the
presence of the quasar images brings faint deflector galaxies above the survey
limit. Thus the a posteriori likelihood of the discovery of Q 2237+0305 in the
Center for Astrophysics redshift survey is approximately 0.03. In the future,
the 2 degree Field survey should yield at least 10 lensed quasars, and the
Sloan Digitial Sky Survey up to 100.Comment: Gravitational Lensing: Recent Progress and Future Goals, C.S.
Kochanek & T.G. Brainerd, eds., in press; 2 pages, 1 figur

### Using the 2dF galaxy redshift survey to detect gravitationally-lensed quasars

Galaxy redshift surveys can be used to detect gravitationally-lensed quasars
if the spectra obtained are searched for the quasars' emission lines. Previous
investigations of this possibility have used simple models to show that the 2
degree Field (2dF) redshift survey could yield several tens of new lenses, and
that the larger Sloan Digital Sky Survey should contain an order of magnitude
more. However the particular selection effects of the samples were not included
in these calculations, limiting the robustness of the predictions; thus a more
detailed simulation of the 2dF survey was undertaken here. The use of an
isophotal magnitude limit reduces both the depth of the sample and the expected
number of lenses, but more important is the Automatic Plate Measuring survey's
star-galaxy separation algorithm, used to generate the 2dF input catalogue. It
is found that most quasar lenses are classed as merged stars, with only the few
lenses with low-redshift deflectors likely to be classified as galaxies.
Explicit inclusion of these selection effects implies that the 2dF survey
should contain 10 lenses on average. The largest remaining uncertainty is the
lack of knowledge of the ease with which any underlying quasars can be
extracted from the survey spectra.Comment: MNRAS, in press; 14 pages, 19 figure

### Gravitational lensing by elliptical galaxies

The fraction of high-redshift sources which are multiply-imaged by
intervening galaxies is strongly dependent on the cosmological constant, and so
can be a useful probe of the cosmological model. However its power is limited
by various systematic (and random) uncertainties in the calculation of lensing
probabilities, one of the most important of which is the dynamical
normalisation of elliptical galaxies. Assuming ellipticals' mass distributions
can be modelled as isothermal spheres, the mass normalisation depends on: the
velocity anisotropy; the luminosity density; the core radius; and the area over
which the velocity dispersion is measured. The differences in the lensing
probability and optical depth produced by using the correct normalisation can
be comparable to the differences between even the most extreme cosmological
models. The existing data is not sufficient to determine the correct
normalisation with enough certainty to allow lensing statistics to be used to
their full potential. However, as the correct lensing probability is almost
certainly higher than is usually assumed, upper bounds on the cosmological
constant are not weakened by these possibilities.Comment: MNRAS, in press; 13 pages, 22 figure

### Gravitational lensing in modified Newtonian dynamics

Modified Newtonian dynamics (MOND) is an alternative theory of gravity that
aims to explain large-scale dynamics without recourse to any form of dark
matter. However the theory is incomplete, lacking a relativistic counterpart,
and so makes no definite predictions about gravitational lensing. The most
obvious form that MONDian lensing might take is that photons experience twice
the deflection of massive particles moving at the speed of light, as in general
relativity (GR). In such a theory there is no general thin-lens approximation
(although one can be made for spherically-symmetric deflectors), but the
three-dimensional acceleration of photons is in the same direction as the
relativistic acceleration would be. In regimes where the deflector can
reasonably be approximated as a single point-mass (specifically low-optical
depth microlensing and weak galaxy-galaxy lensing), this naive formulation is
consistent with observations. Forthcoming galaxy-galaxy lensing data and the
possibility of cosmological microlensing have the potential to distinguish
unambiguously between GR and MOND. Some tests can also be performed with
extended deflectors, for example by using surface brightness measurements of
lens galaxies to model quasar lenses, although the breakdown of the thin-lens
approximation allows an extra degree of freedom. Nonetheless, it seems unlikely
that simple ellipsoidal galaxies can explain both constraints. Further, the
low-density universe implied by MOND must be completely dominated by the
cosmological constant (to fit microwave background observations), and such
models are at odds with the low frequency of quasar lenses. These conflicts
might be resolved by a fully consistent relativistic extension to MOND; the
alternative is that MOND is not an accurate description of the universe.Comment: MNRAS, in press; 11 pages, 10 figure

### Gravitational lensing and modified Newtonian dynamics

Gravitational lensing is most often used as a tool to investigate the
distribution of (dark) matter in the universe, but, if the mass distribution is
known a priori, it becomes, at least in principle, a powerful probe of gravity
itself. Lensing observations are a more powerful tool than dynamical
measurements because they allow measurements of the gravitational field far
away from visible matter. For example, modified Newtonian dynamics (MOND) has
no relativistic extension, and so makes no firm lensing predictions, but
galaxy-galaxy lensing data can be used to empirically the deflection law of a
point-mass. MONDian lensing is consistent with general relativity, in so far as
the deflection experienced by a photon is twice that experienced by a massive
particle moving at the speed of light. With the deflection law in place and no
invisible matter, MOND can be tested wherever lensing is observed. The
implications are that either MONDian lensing is completely non-linear or that
MOND is not an accurate description of the universe.Comment: PASA (OzLens edition), in press; 5 pages, 1 figur

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