1,365 research outputs found

### Statistics of mass substructure from strong gravitational lensing: quantifying the mass fraction and mass function

A Bayesian statistical formalism is developed to quantify the level at which
the mass function slope (alpha) and the projected cumulative mass fraction (f)
of (CDM) substructure in strong gravitational-lens galaxies, with arcs or
Einstein rings, can be recovered as function of the lens-survey parameters and
the detection threshold of the substructure mass. The method is applied to
different sets of mock data to explore a range of observational limits: (i) the
number of lens galaxies in the survey, (ii) the mass threshold, Mlow, for the
detection of substructures and (iii) the uncertainty of the measured
substructure masses. We explore two different priors on the mass function
slope: a uniform prior and a Gaussian prior with alpha = 1.90+-0.1. With a
substructure detection threshold Mlow=3x10^8 Msun, the number of lenses
available now (n_l=30), a true dark-matter mass fraction in (CDM) substructure
<=1.0% and a prior of alpha = 1.90+-0.1, we find that the upper limit of f can
be constrained down to a level <=1.0% (95% CL). In the case of a Gaussian prior
on alpha, it is always possible to set stringent constraints on both
parameters. We also find that lowering the detection threshold has the largest
impact on the ability to recover alpha, because of the (expected) steep
mass-function slope. In the future, thanks to new surveys with telescopes, such
as SKA, LSST and JDEM and follow-up telescopes with high-fidelity data, a
significant increase in the number of known lenses will allow us to recover the
satellite population in its completeness. For example, a sample of 200 lenses,
equivalent in data-quality to the Sloan Lens ACS Survey and a detection
threshold of 10^8 Msun, allows one to determine f=0.5+-0.1% (68% CL) and
alpha=1.90+-0.2 (68% CL).Comment: MNRAS (in press

### The Kinematics of High Proper Motion Halo White Dwarfs

We analyse the kinematics of the entire spectroscopic sample of 99 recently
discovered high proper-motion white dwarfs by Oppenheimer et al. using a
maximum-likelihood analysis, and discuss the claim that the high-velocity white
dwarfs are members of a halo population with a local density at least ten times
greater than traditionally assumed. We argue that the observations, as
reported, are consistent with the presence of an almost undetected thin disc
plus a thick disc, with densities as conventionally assumed. In addition, there
is a kinematically distinct, flattened, halo population at the more than 99%
confidence level. Surprisingly, the thick disc and halo populations are
indistinguishable in terms of luminosity, color and apparent age (1-10 Gyr).
Adopting a bimodal, Schwarzschild model for the local velocity ellipsoid, with
the ratios sigma_U:sigma_V:sigma_W=1:2/3:1/2, we infer radial velocity
dispersions of sigma_U=62(+8/-10) km/s and 150(+80/-40) km/s (90% C.L.) for the
local thick disc and halo populations, respectively. The thick disc result
agrees with the empirical relation between asymmetric drift and radial velocity
dispersion, inferred from local stellar populations. The local thick-disc plus
halo density of white dwarfs is n^{td+h}=(1.9+-0.5)x10^-3 pc^-3 (90% C.L.), of
which n^{h}=1.1(+2.1/-0.7)x10^-4 pc^-3 (90% C.L.) belongs to the halo, a
density about five times higher than previously thought. (Abridged)Comment: 19 pages, 11 figures; submitted to MNRA

### White Dwarfs: Contributors and Tracers of the Galactic Dark-Matter Halo

We examine the claim by Oppenheimer et al. (2001) that the local halo density
of white dwarfs is an order of magnitude higher than previously thought. As it
stands, the observational data support the presence of a kinematically distinct
population of halo white dwarfs at the >99% confidence level. A
maximum-likelihood analysis gives a radial velocity dispersion of
sigma^h_U=150(+80/-40) km/s and an asymmetric drift of v_a^h=176(+102/-80)
km/s, for a Schwarzschild velocity distribution function with
sigma_U:sigma_V:sigma_W=1:2/3:1/2. Halo white dwarfs have a local number
density of 1.1(+2.1/-0.7)x10^-4 pc-3, which amounts to 0.8(+1.6/-0.5) per cent
of the nominal local dark-matter halo density and is 5.0(+9.5/-3.2) times
higher and thus only marginally in agreement with previous estimates (all
errors indicate the 90% C.L.). We discuss several direct consequences of this
white-dwarf population (e.g. microlensing) and postulate a potential mechanism
to eject young white dwarfs from the disc to the halo, through the orbital
instabilities in triple or multiple stellar systems.Comment: 5 pages, to appear in the proceedings of the Yale Cosmology Workshop
"The Shapes of Galaxies and their Halos" (ed. Priya Natarajan); revised
numerical results, using a corrected likelihood function (thanks to David
Graff and Andy Gould); general conclusions remain simila

### Quantifying Suppression of the Cosmological 21-cm Signal due to Direction Dependent Gain Calibration in Radio Interferometers

The 21-cm signal of neutral hydrogen - emitted during the Epoch of
Reionization - promises to be an important source of information for the study
of the infant universe. However, its detection is impossible without sufficient
mitigation of other strong signals in the data, which requires an accurate
knowledge of the instrument. Using the result of instrument calibration, a
large part of the contaminating signals are removed and the resulting residual
data is further analyzed in order to detect the 21-cm signal. Direction
dependent calibration (DDC) can strongly affect the 21-cm signal, however, its
effect has not been precisely quantified.
In the analysis presented here we show how to exactly calculate what part of
the 21-cm signal is removed as a result of the DDC. We also show how a-priori
information about the frequency behavior of the instrument can be used to
reduce signal suppression. The theoretical results are tested using a realistic
simulation based on the LOFAR setup. Our results show that low-order smooth
gain functions (e.g. polynomials) over a bandwidth of ~10\,MHz - over which the
signal is expected to be stationary - is sufficient to allow for calibration
with limited, quantifiable, signal suppression in its power spectrum. We also
show mathematically and in simulations that more incomplete sky models lead to
larger 21-cm signal suppression, even if the gain models are enforced to be
fully smooth. This result has immediate consequences for current and future
radio telescopes with non-identical station beams, where DDC might be necessary
(e.g. SKA-low).Comment: Submitted to MNRAS on 10-Aug-201

### Micro & strong lensing with the Square Kilometer Array: The mass--function of compact objects in high--redshift galaxies

We present the results from recent VLA 8.5-GHz and WSRT 1.4 and 4.9-GHz
monitoring campaigns of the CLASS gravitational lens B1600+434 and show how the
observed variations argue strongly in favor of microlensing by MACHOs in the
halo of a dark-matter dominated edge-on disk galaxy at z=0.4. The population of
flat-spectrum radio sources with micro-Jy flux-densities detected with the
Square-Kilometer-Array is expected to have dimensions of micro-arcsec. They
will therefore vary rapidly as a result of Galactic scintillation (diffractive
and refractive). However, when positioned behind distant galaxies they will
also show variations due to microlensing, even more strongly than in the case
of B1600+434. Relativistic or superluminal motion in these background sources
typically leads to temporal variations on time scales of days to weeks.
Scintillation and microlensing can be distinguished, and separated, by their
different characteristic time scales and the frequency dependence of their
modulations. Monitoring studies with Square-Kilometer-Array at GHz frequencies
will thus probe both microscopic and macroscopic properties of dark matter and
its mass-function as a function of redshift, information very hard to obtain by
any other method.Comment: 8 pages, 5 figures, to appear in Perspectives in Radio Astronomy:
Scientific Imperatives at cm and m Wavelengths (Dwingeloo: NFRA), Edited by:
M.P. van Haarlem & J.M. van der Huls

### Robust Foregrounds Removal for 21-cm Experiments

Direct detection of the Epoch of Reionization via the redshifted 21-cm line
will have unprecedented implications on the study of structure formation in the
early Universe. To fulfill this promise current and future 21-cm experiments
will need to detect the weak 21-cm signal over foregrounds several order of
magnitude greater. This requires accurate modeling of the galactic and
extragalactic emission and of its contaminants due to instrument chromaticity,
ionosphere and imperfect calibration. To solve for this complex modeling, we
propose a new method based on Gaussian Process Regression (GPR) which is able
to cleanly separate the cosmological signal from most of the foregrounds
contaminants. We also propose a new imaging method based on a maximum
likelihood framework which solves for the interferometric equation directly on
the sphere. Using this method, chromatic effects causing the so-called "wedge"
are effectively eliminated (i.e. deconvolved) in the cylindrical ($k_{\perp},
k_{\parallel}$) power spectrum.Comment: Subbmited to the Proceedings of the IAUS333, Peering Towards Cosmic
Dawn, 4 pages, 2 figure

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