100 research outputs found
Strong Gravitational Lensing with SKA
The advent of new observational facilities in the last two decades has
allowed the rapid discovery and high-resolution optical imaging of many strong
lens systems from galaxy to cluster scales, as well as their spectroscopic
follow-up. Radio telescopes have played the dominant role in the systematic
detection of dozens of new arcsec-scale lens systems. For the future, we expect
nothing less! The next major ground- and space-based facilities, especially the
Square Kilometer Array can discover tens of thousands of new lens systems in
large sky surveys. For optical imaging and spectroscopic follow-up a strong
synergy with planned optical facilities is needed. Here, we discuss the field
where strong gravitational lensing is expected to play the dominant role and
where SKA can have a major impact: The study of the internal mass structure and
evolution of galaxies and clusters to z~1. In addition, studies of more exotic
phenomena are contemplated. For example, milli- and microlensing can provide a
way to measure the mass-functions of stars and CDM substructure at cosmological
distances. All-sky radio monitoring will also rapidly develop the field of
time-domain lensing.Comment: 9 pages, 3 figures; to appear in "Science with the Square Kilometer
Array," eds. C. Carilli and S. Rawlings, New Astronomy Reviews (Elsevier:
Amsterdam
Observational constraints on the sub-galactic matter-power spectrum from galaxy-galaxy strong gravitational lensing
SEAGLE - III: towards resolving the mismatch in the dark-matter fraction in early-type galaxies between simulations and observations
Galaxie
Cosmological distance indicators
We review three distance measurement techniques beyond the local universe:
(1) gravitational lens time delays, (2) baryon acoustic oscillation (BAO), and
(3) HI intensity mapping. We describe the principles and theory behind each
method, the ingredients needed for measuring such distances, the current
observational results, and future prospects. Time delays from strongly lensed
quasars currently provide constraints on with < 4% uncertainty, and with
1% within reach from ongoing surveys and efforts. Recent exciting discoveries
of strongly lensed supernovae hold great promise for time-delay cosmography.
BAO features have been detected in redshift surveys up to z <~ 0.8 with
galaxies and z ~ 2 with Ly- forest, providing precise distance
measurements and with < 2% uncertainty in flat CDM. Future BAO
surveys will probe the distance scale with percent-level precision. HI
intensity mapping has great potential to map BAO distances at z ~ 0.8 and
beyond with precisions of a few percent. The next years ahead will be exciting
as various cosmological probes reach 1% uncertainty in determining , to
assess the current tension in measurements that could indicate new
physics.Comment: Review article accepted for publication in Space Science Reviews
(Springer), 45 pages, 10 figures. Chapter of a special collection resulting
from the May 2016 ISSI-BJ workshop on Astronomical Distance Determination in
the Space Ag
Analytic models of plausible gravitational lens potentials
Gravitational lenses on galaxy scales are plausibly modelled as having
ellipsoidal symmetry and a universal dark matter density profile, with a Sersic
profile to describe the distribution of baryonic matter. Predicting all lensing
effects requires knowledge of the total lens potential: in this work we give
analytic forms for that of the above hybrid model. Emphasising that complex
lens potentials can be constructed from simpler components in linear
combination, we provide a recipe for attaining elliptical symmetry in either
projected mass or lens potential. We also provide analytic formulae for the
lens potentials of Sersic profiles for integer and half-integer index. We then
present formulae describing the gravitational lensing effects due to
smoothly-truncated universal density profiles in cold dark matter model. For
our isolated haloes the density profile falls off as radius to the minus fifth
or seventh power beyond the tidal radius, functional forms that allow all
orders of lens potential derivatives to be calculated analytically, while
ensuring a non-divergent total mass. We show how the observables predicted by
this profile differ from that of the original infinite-mass NFW profile.
Expressions for the gravitational flexion are highlighted. We show how
decreasing the tidal radius allows stripped haloes to be modelled, providing a
framework for a fuller investigation of dark matter substructure in galaxies
and clusters. Finally we remark on the need for finite mass halo profiles when
doing cosmological ray-tracing simulations, and the need for readily-calculable
higher order derivatives of the lens potential when studying catastrophes in
strong lenses.Comment: 24 pages, 10 figures, matches published versio
The scale of the problem:Recovering images of reionization with Generalized Morphological Component Analysis
The accurate and precise removal of 21-cm foregrounds from Epoch of
Reionization redshifted 21-cm emission data is essential if we are to gain
insight into an unexplored cosmological era. We apply a non-parametric
technique, Generalized Morphological Component Analysis or GMCA, to simulated
LOFAR-EoR data and show that it has the ability to clean the foregrounds with
high accuracy. We recover the 21-cm 1D, 2D and 3D power spectra with high
accuracy across an impressive range of frequencies and scales. We show that
GMCA preserves the 21-cm phase information, especially when the smallest
spatial scale data is discarded. While it has been shown that LOFAR-EoR image
recovery is theoretically possible using image smoothing, we add that wavelet
decomposition is an efficient way of recovering 21-cm signal maps to the same
or greater order of accuracy with more flexibility. By comparing the GMCA
output residual maps (equal to the noise, 21-cm signal and any foreground
fitting errors) with the 21-cm maps at one frequency and discarding the smaller
wavelet scale information, we find a correlation coefficient of 0.689, compared
to 0.588 for the equivalently smoothed image. Considering only the central 50%
of the maps, these coefficients improve to 0.905 and 0.605 respectively and we
conclude that wavelet decomposition is a significantly more powerful method to
denoise reconstructed 21-cm maps than smoothing.Comment: 13 pages, 12 figures, accepted by MNRA
Unified dark energy models : a phenomenological approach
A phenomenological approach is proposed to the problem of universe
accelerated expansion and of the dark energy nature. A general class of models
is introduced whose energy density depends on the redshift in such a way
that a smooth transition among the three main phases of the universe evolution
(radiation era, matter domination, asymptotical de Sitter state) is naturally
achieved. We use the estimated age of the universe, the Hubble diagram of Type
Ia Supernovae and the angular size - redshift relation for compact and
ultracompact radio structures to test whether the model is in agreement with
astrophysical observation and to constrain its main parameters. Although
phenomenologically motivated, the model may be straightforwardly interpreted as
a two fluids scenario in which the quintessence is generated by a suitably
chosen scalar field potential. On the other hand, the same model may also be
read in the context of unified dark energy models or in the framework of
modified Friedmann equation theories.Comment: 12 pages, 10 figures, accepted for publication on Physical Review
Degree-scale galactic radio emission at 122 MHz around the North Celestial Pole with LOFAR-AARTFAAC
Large scale structure and cosmolog
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