18,294 research outputs found
Lensing galaxies: light or dark?
In a recent paper, Hawkins (1997) argues on the basis of statistical studies
of double-image gravitational lenses and lens candidates that a large
population of dark lenses exists and that these outnumber galaxies with more
normal mass-to-light ratios by a factor of 3:1. If correct, this is a very
important result for many areas of astronomy including galaxy formation and
cosmology. In this paper we discuss our new radio-selected gravitational lens
sample, JVAS/CLASS, in order to test and constrain this proposition. We have
obtained ground-based and HST images of all multiple-image lens systems in our
sample and in 12 cases out of 12 we find the lensing galaxies in the optical
and/or near infrared. Our success in finding lensing galaxies creates problems
for the dark lens hypothesis. If it is to survive, ad hoc modifications seem to
be necessary: only very massive galaxies (more than about one trillion solar
masses) can be dark, and the cutoff in mass must be sharp. Our finding of lens
galaxies in all the JVAS/CLASS systems is complementary evidence which supports
the conclusion of Kochanek et al. (1997) that many of the wide-separation
optically-selected pairs are physically distinct quasars rather than
gravitational lens systems.Comment: 4 pages, 2 included figures, accepted for publication in Astronomy
and Astrophysics. Paper version available on request. This replacement amends
the text to allow more discussion of the overlap with astro-ph/971016
Artificial dielectric optical structures: A challenge for nanofabrication
Diffractive optical components can be made using multiple level kinoforms or single level artificial dielectric structures. The latter require the fabrication of pillars of equal depth but differing width and spacing. As a demonstration device, the diffractive optic equivalent of a wedge has been made in GaAs for use at 1.15 μm. The need for all pillars to have the same height was met by using a selective etch and a very thin etch-stop layer on AlGaAs. The experimental diffraction efficiency was 87.8%, among the best ever obtained and close to the theoretical maximum of 97.6%. © 1998 American Vacuum Society
A multi-epoch VLBI survey of the kinematics of CFJ sources
Context. This is the second in a series of papers presenting VLBI observations of the 293 Caltech-Jodrell Bank Flat-spectrum (hereafter CJF) sources and their analysis.
Aims. We obtain a consistent motion dataset large enough to allow the systematic properties of the population to be studied.
Methods. We present detailed kinematic analysis of the complete flux-density limited CJF survey. We computed 2D kinematic models based on the optimal model-fitting parameters of multi-epoch VLBA observations. This allows us to calculate not only radial, but also orthogonal motions, and thus to study curvature and acceleration. Statistical tests of the motions measured and their reliability were performed. A correlation analysis between the derived apparent motions, luminosities, spectral indices, and core dominance and the resulting consequences is described.
Results. With at least one velocity in each of the 237 sources, this sample is much larger than any available before, so it allows a meaningful statistical investigation of apparent motions and any possible correlations with other parameters in AGN jets. The main results to emerge are as follows: - In general motions are not consistent with a single uniform velocity applicable to all components along a jet. - We find a slight trend towards a positive outward acceleration and also adduce some evidence for greater acceleration in the innermost regions. - We find a lack of fast components at physical distances less than a few pc from the reference feature. - Only ~4% of the components from galaxies and <2% of those from quasars undergo large bends i.e. within 15° of ± 90°. - The distribution of radial velocities shows a broad distribution of velocities (apparent velocities up to 30 c). Fifteen percent of the best-sampled jet components exhibit low velocities that may need to be explained in a different manner to the fast motions. - Some negative superluminal motions are seen, and in 15 cases (6%) these are definitely significant. - We find a strong correlation between the 5 GHz luminosity and the apparent velocity. - The CJF galaxies, on average, show slower apparent jet-component velocities than the quasars. - The mean velocity in the VLBA 2 cm survey (Kellermann et al. 2004, ApJ, 609, 539) is substantially higher than in the CJF survey, the ratio could be roughly a factor of 1.5-2. This supports the observed trend toward increasing apparent velocity with increasing observing frequency.
Conclusions. This AGN survey provides the basis for any statistical analysis of jet and jet-component properties
A Determination of H_0 with the CLASS Gravitational Lens B1608+656: I. Time Delay Measurements with the VLA
We present the results of a program to monitor the four-image gravitational
lens B1608+656 with the VLA. The system was observed over a seven month period
from 1996 October to 1997 May. The 64 epochs of observation have an average
spacing of 3.6~d. The light curves of the four images of the background source
show that the flux density of the background source has varied at the ~5%
level. We measure time delays in the system based on common features that are
seen in all four light curves. The three independent time delays in the system
are found to be Delta t_{BA} = 31 +/- 7~d, Delta t_{BC} = 36 +/- 7~d, and Delta
t_{BD} = 76^{+9}_{-10}~d at 95% confidence. This is the first gravitational
lens system for which three independent time delays have been measured. A
companion paper presents a mass model for the lensing galaxy which correctly
reproduces the observed image positions, flux density ratios, and time delay
ratios. The last condition is crucial for determining H_0 with a four-image
lens. We combine the time delays with the model to obtain a value for the
Hubble constant of H_0 = 59^{+8}_{-7} km/s/Mpc at 95% confidence (statistical)
for (Omega_M, Omega_{Lambda}) = (1,0). In addition, there is an estimated
systematic uncertainty of +/- 15 km/s/Mpc from uncertainties in modeling the
radial mass profiles of the lensing galaxies. The value of H_0 presented in
this paper is comparable to recent measurements of H_0 from the gravitational
lenses 0957+561, PG1115+080, B0218+357, and PKS1830-211.Comment: Accepted for publication in ApJ. 20 pages, 13 figure
Characterization of Landau-Zener Transitions in Systems with Complex Spectra
This paper is concerned with the study of one-body dissipation effects in
idealized models resembling a nucleus. In particular, we study the quantum
mechanics of a free particle that collides elastically with the slowly moving
walls of a Bunimovich stadium billiard. Our results are twofold. First, we
develop a method to solve in a simple way the quantum mechanical evolution of
planar billiards with moving walls. The formalism is based on the {\it scaling
method} \cite{ver} which enables the resolution of the problem in terms of
quantities defined over the boundary of the billiard. The second result is
related to the quantum aspects of dissipation in systems with complex spectra.
We conclude that in a slowly varying evolution the energy is transferred from
the boundary to the particle through LandauZener transitions.Comment: 24 pages (including 7 postcript figures), Revtex. Submitted to PR
The measurement of low pay in the UK labour force survey
Consideration of the National Minimum Wage requires estimates of the distribution of hourly pay. The UK Labour Force Survey (LFS) is a key source of such estimates. The approach most frequently adopted by researchers has been to measure hourly earnings from several questions on pay and hours. The Office for National Statistics is now applying a new approach, based on an alternative more direct measurement introduced in March 1999. These two measures do not produce identical values and this paper investigates sources of discrepancies and concludes that the new variable is more accurate. The difficulty with using the new variable is that it is only available on a subset of respondents. An approach is developed in which missing values of the new variable are replaced by imputed values. The assumptions underlying this imputation approach and results of applying it to LFS data are presented. The relation to weighting approaches is also discussed
CLASS B1152+199 and B1359+154: Two New Gravitational Lens Systems Discovered in the Cosmic Lens All-Sky Survey
The third phase of the Cosmic Lens All-Sky Survey (CLASS) has recently been
completed, bringing the total number of sources imaged to over 15000 in the
CLASS and JVAS combined survey. In the VLA observations carried out in March
and April of 1998, two new candidate lensed systems were discovered: CLASS
B1152+199 and B1359+154. B1152+199 is a 1.6 arcsecond double, with a background
quasar at z=1.019 lensed by a foreground galaxy at z=0.439. The relatively flat
radio spectra of the lensed images, combined with a previous ROSAT detection of
the source, make B1152+199 a strong candidate for time delay studies at both
radio and X-ray wavelengths. B1359+154 is a quadruply lensed quasar at z=3.235,
with a maximum image separation of 1.7 arcseconds. As yet, the redshift of the
lensing object in this system is undetermined. The steep spectral index of the
source suggests that B1359+154 will not exhibit strong variability, and is
therefore unlikely to be useful for determining the Hubble constant from
measured time delays.Comment: accepted for publication in The Astronomical Journa
The Discovery of Argon in Comet C/1995 O1 (Hale-Bopp)
On 30.14 March 1997 we observed the EUV spectrum of the bright comet C/1995
O1 (Hale-Bopp) at the time of its perihelion, using our EUVS sounding rocket
telescope/spectrometer. The spectra reveal the presence H Ly beta, O+, and,
most notably, Argon. Modelling of the retrieved Ar production rates indicates
that comet Hale-Bopp is enriched in Ar relative to cosmogonic expectations.
This in turn indicates that Hale-Bopp's deep interior has never been exposed to
the 35-40 K temperatures necessary to deplete the comet's primordial argon
supply.Comment: 9 pages, 2 figures. ApJ, 545, in press (2000
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