4,647 research outputs found
Sub-milliarcsec-scale structure of the gravitational lens B1600+434
In the gravitational lens system B1600+434 the brighter image, A, is known to
show rapid variability which is not detected in the weaker image, B (Koopmans &
de Bruyn 2000). Since correlated variability is one of the fundamental
properties of gravitational lensing, it has been proposed that image A is
microlensed by stars in the halo of the lensing galaxy (Koopmans & de Bruyn
2000). We present VLBA observations of B1600+434 at 15 GHz with a resolution of
0.5 milliarcsec to determine the source structure at high spatial resolution.
The surface brightness of the images are significantly different, with image A
being more compact. This is in apparent contradiction with the required
property of gravitational lensing that surface brightness be preserved. Our
results suggest that both the lensed images may show two-sided elongation at
this resolution, a morphology which does not necessarily favour superluminal
motion. Instead these data may suggest that image B is scatter-broadened at the
lens so that its size is larger than that of A, and hence scintillates less
than image A.Comment: 4 pages, 2 figures, accepted in AA Letter
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
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
[Review of: G.C. Loury, T. Modood (2005) Ethnicity, social mobility and public policy]
International audienc
Testing Verlinde's emergent gravity in early-type galaxies
Verlinde derived gravity as an emergent force from the information flow,
through two-dimensional surfaces and recently, by a priori postulating the
entanglement of information in 3D space, he derived the effect of the
gravitational potential from dark matter (DM) as the entropy displacement of
dark energy by baryonic matter. In Emergent Gravity (EG) this apparent DM
depends only on the baryonic mass distribution and the present-day value of the
Hubble parameter. In this paper we test the EG proposition, formalized by
Verlinde for a spherical and isolated mass distribution, using the central
velocity dispersion, and the light distribution in a sample of 4260
massive and local early-type galaxies (ETGs) from the SPIDER sample. Our
results remain unaltered if we consider the sample of 807 roundest field
galaxies. We derive the predictions by EG for the stellar mass-to-light ratio
(M/L) and the Initial Mass Function (IMF), and compare them with the same
inferences derived from a) DM-based models, b) MOND and c) stellar population
models. We demonstrate that, consistently with a classical Newtonian framework
with a DM halo component, or alternative theories of gravity as MOND, the
central dynamics can be fitted if the IMF is assumed non-universal. The results
can be interpreted with a IMF lighter than a standard Chabrier at low-,
and bottom-heavier IMFs at larger . We find lower, but still
acceptable, stellar M/L in EG theory, if compared with the DM-based NFW model
and with MOND. The results from EG are comparable to what is found if the DM
haloes are adiabatically contracted and with expectations from spectral
gravity-sensitive features. If the strain caused by the entropy displacement
would be not maximal, as adopted in the current formulation, then the dynamics
of ETGs could be reproduced with larger M/L. (abridged)Comment: 12 pages, 2 figures, submitted to MNRAS. The updated manuscript
presents significantly altered conclusions, after discovering an internal bug
in an older version of the Mathematica package, leading to incorrect
numerical results when calculating the derivatives of Gamma function
Gravitational lensing statistics with extragalactic surveys. II. Analysis of the Jodrell Bank-VLA Astrometric Survey
We present constraints on the cosmological constant from
gravitational lensing statistics of the Jodrell Bank-VLA Astrometric Survey
(JVAS). Although this is the largest gravitational lens survey which has been
analysed, cosmological constraints are only comparable to those from optical
surveys. This is due to the fact that the median source redshifts of JVAS are
lower, which leads to both relatively fewer lenses in the survey and a weaker
dependence on the cosmological parameters. Although more approximations have to
be made than is the case for optical surveys, the consistency of the results
with those from optical gravitational lens surveys and other cosmological tests
indicate that this is not a major source of uncertainty in the results.
However, joint constraints from a combination of radio and optical data are
much tighter. Thus, a similar analysis of the much larger Cosmic Lens All-Sky
Survey should provide even tighter constraints on the cosmological constant,
especially when combined with data from optical lens surveys.
At 95% confidence, our lower and upper limits on ,
using the JVAS lensing statistics information alone, are respectively -2.69 and
0.68. For a flat universe, these correspond to lower and upper limits on
\lambda_{0} of respectively -0.85 and 0.84. Using the combination of JVAS
lensing statistics and lensing statistics from the literature as discussed in
Quast & Helbig (Paper I) the corresponding values are
-1.78 and 0.27. For a flat universe, these correspond to lower and upper limits
on of respectively -0.39 and 0.64.Comment: LaTeX, 9 pages, 18 PostScript files in 6 figures. Paper version
available on request. Data available from
http://gladia.astro.rug.nl:8000/ceres/data_from_papers/papers.htm
The QED Structure of the Photon
Measurements of the QED structure of the photon based on the reaction ee -->
ee \gamma(*)(P^2)\gamma*(Q^2) --> ee mumu are discussed.
This review is an update of the discussion of the results on the QED
structure of the photon presented in Refs.[1], and covers the published
measurements of the photon structure functions F_2, F_A nd F_B and of the
differential cross-section dsig/dx for the exchange of two virtual photons.Comment: Invited talk given at the 7th International Workshop on Deep
Inelastic Scattering and QCD, April 19 to 23, 1999, Zeuthen, to appear in the
proceedings. 8pages 4 figure
Cool White Dwarfs Revisited -- New Spectroscopy and Photometry
In this paper we present new and improved data on 38 cool white dwarfs
identified by Oppenheimer et al. 2001 (OHDHS) as candidate dark halo objects.
Using the high-res spectra obtained with LRIS, we measure radial velocities for
13 WDs that show an H alpha line. We show that the knowledge of RVs decreases
the UV-plane velocities by only 6%. The radial velocity sample has a W-velocity
dispersion of sig_W = 59 km/s--in between the values associated with the thick
disk and the stellar halo. We also see indications for the presence of two
populations by analyzing the velocities in the UV plane. In addition, we
present CCD photometry for half of the sample, and with it recalibrate the
photographic photometry of the remaining WDs. Using the new photometry in
standard bands, and by applying the appropriate color-magnitude relations for H
and He atmospheres, we obtain new distance estimates. New distances of the WDs
that were not originally selected as halo candidates yield 13 new candidates.
On average, new distances produce velocities in the UV plane that are larger by
10%, with already fast objects gaining more. Using the new data, while applying
the same UV-velocity cut (94 km/s) as in OHDHS, we find a density of cool WDs
of 1.7e-4 pc^-3, confirming the value of OHDHS. In addition, we derive the
density as a function of the UV-velocity cutoff. The density (corrected for
losses due to higher UV cuts) starts to flatten out at 150 km/s (0.4e-4 pc^-3),
and is minimized (thus minimizing a possible non-halo contamination) at 190
km/s (0.3e-4 pc^-3). These densities are in a rough agreement with the
estimates for the stellar halo WDs, corresponding to a factor of 1.9 and 1.4
higher values.Comment: Accepted to ApJ. New version contains some additional data. Results
unchange
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