Gravitational lensing statistics with extragalactic surveys. II. Analysis of the Jodrell Bank-VLA Astrometric Survey

We present constraints on the cosmological constant $\lambda_{0}$ 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 $\lambda_{0}-\Omega_{0}$, 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 $\lambda_{0}-\Omega_{0}$ values are -1.78 and 0.27. For a flat universe, these correspond to lower and upper limits on $\lambda_{0}$ 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

Influence of laser-excited electron distributions on the x-ray magnetic circular dichroism spectra: Implications for femtosecond demagnetization in Ni

In pump-probe experiments an intensive laser pulse creates non-equilibrium excited electron distributions in the first few hundred femtoseconds after the pulse. The influence of non-equilibrium electron distributions caused by a pump laser on the apparent X-ray magnetic circular dichroism (XMCD) signal of Ni is investigated theoretically here for the first time, considering electron distributions immediately after the pulse as well as thermalized ones, that are not in equilibrium with the lattice or spin systems. The XMCD signal is shown not to be simply proportional to the spin momentum in these situations. The computed spectra are compared to recent pump-probe XMCD experiments on Ni. We find that the majority of experimentally observed features considered to be a proof of ultrafast spin momentum transfer to the lattice can alternatively be attributed to non-equilibrium electron distributions. Furthermore, we find the XMCD sum rules for the atomic spin and orbital magnetic moment to remain valid, even for the laser induced non-equilibrium electron distributions.Comment: 6 pages, 3 figure

The Hubble Constant from the Gravitational Lens B1608+656

We present a refined gravitational lens model of the four-image lens system B1608+656 based on new and improved observational constraints: (i) the three independent time-delays and flux-ratios from VLA observations, (ii) the radio-image positions from VLBA observations, (iii) the shape of the deconvolved Einstein Ring from optical and infrared HST images, (iv) the extinction-corrected lens-galaxy centroids and structural parameters, and (v) a stellar velocity dispersion, sigma_ap=247+-35 km/s, of the primary lens galaxy (G1), obtained from an echelle spectrum taken with the Keck--II telescope. The lens mass model consists of two elliptical mass distributions with power-law density profiles and an external shear, totaling 22 free parameters, including the density slopes which are the key parameters to determine the value of H_0 from lens time delays. This has required the development of a new lens code that is highly optimized for speed. The minimum-chi^2 model reproduces all observations very well, including the stellar velocity dispersion and the shape of the Einstein Ring. A combined gravitational-lens and stellar dynamical analysis leads to a value of the Hubble Constant of H_0=75(+7/-6) km/s/Mpc (68 percent CL; Omega_m=0.3, Omega_Lambda=0.7. The non-linear error analysis includes correlations between all free parameters, in particular the density slopes of G1 and G2, yielding an accurate determination of the random error on H_0. The lens galaxy G1 is ~5 times more massive than the secondary lens galaxy (G2), and has a mass density slope of gamma_G1=2.03(+0.14/-0.14) +- 0.03 (68 percent CL) for rho~r^-gamma', very close to isothermal (gamma'=2). (Abridged)Comment: 17 pages, 6 figures, 5 tables; revised version with correct fig.6 and clarified text based on referee report; conclusions unchange

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

Correlation between magnetism and spin-dependent transport in CoFeB alloys

We report a correlation between the spin polarization of the tunneling electrons (TSP) and the magnetic moment of amorphous CoFeB alloys. Such a correlation is surprising since the TSP involves s-like electrons close to the Fermi level (EF), while the magnetic moment mainly arises due to all d-electrons below EF. We show that probing the s and d-bands individually provides clear and crucial evidence for such a correlation to exist through s-d hybridization, and demonstrate the tuneability of the electronic and magnetic properties of CoFeB alloys.Comment: Accepted for publication in Physical Review Letters. Letter (4 pages) and Supplementary material (4 pages

Redshifts of CLASS Radio Sources

Spectroscopic observations of a sample of 42 flat-spectrum radio sources from the Cosmic Lens All-Sky Survey (CLASS) have yielded a mean redshift of $= 1.27$ with an RMS spread of 0.95, at a completeness level of 64%. The sample consists of sources with a 5-GHz flux density of 25-50 mJy, making it the faintest flat-spectrum radio sample for which the redshift distribution has been studied. The spectra, obtained with the Willam Herschel Telescope (WHT), consist mainly of broad-line quasars at $z>1$ and narrow-line galaxies at $z<0.5$. Though the mean redshift of flat-spectrum radio sources exhibits little variation over more than two orders of magnitude in radio flux density, there is evidence for a decreasing fraction of quasars at weaker flux levels. In this paper we present the results of our spectroscopic observations, and discuss the implications for constraining cosmological parameters with statistical analyses of the CLASS survey.Comment: 10 pages, AJ accepte

The Sloan Lens ACS Survey. IX. Colors, Lensing and Stellar Masses of Early-type Galaxies

We present the current photometric dataset for the Sloan Lens ACS (SLACS) Survey, including HST photometry from ACS, WFPC2, and NICMOS. These data have enabled the confirmation of an additional 15 grade `A' (certain) lens systems, bringing the number of SLACS grade `A' lenses to 85; including 13 grade `B' (likely) systems, SLACS has identified nearly 100 lenses and lens candidates. Approximately 80% of the grade `A' systems have elliptical morphologies while ~10% show spiral structure; the remaining lenses have lenticular morphologies. Spectroscopic redshifts for the lens and source are available for every system, making SLACS the largest homogeneous dataset of galaxy-scale lenses to date. We have developed a novel Bayesian stellar population analysis code to determine robust stellar masses with accurate error estimates. We apply this code to deep, high-resolution HST imaging and determine stellar masses with typical statistical errors of 0.1 dex; we find that these stellar masses are unbiased compared to estimates obtained using SDSS photometry, provided that informative priors are used. The stellar masses range from 10^10.5 to 10^11.8 M$_\odot$ and the typical stellar mass fraction within the Einstein radius is 0.4, assuming a Chabrier IMF. The ensemble properties of the SLACS lens galaxies, e.g. stellar masses and projected ellipticities, appear to be indistinguishable from other SDSS galaxies with similar stellar velocity dispersions. This further supports that SLACS lenses are representative of the overall population of massive early-type galaxies with M* >~ 10^11 M$_\odot$, and are therefore an ideal dataset to investigate the kpc-scale distribution of luminous and dark matter in galaxies out to z ~ 0.5.Comment: 20 pages, 18 figures, 5 tables, published in Ap

The remittances behaviour of the second generation in Europe: altruism or self-interest?

Whereas most research on remittances focuses on first-generation migrants, the aim of this paper is to investigate the remitting behaviour of the host country-born children of migrants - the second generation - in various European cities. Some important studies found that migrant transnationalism is not only a phenomenon for the first generation, but also apply to the second and higher generations, through, among other things, family visits, elder care, and remittances. At the same time, the maintenance of a strong ethnic identity in the ‘host’ society does not necessarily mean that second-generation migrants have strong transnational ties to their ‘home’ country. The data used in this paper is from “The Integration of the European Second Generation” (TIES) project. The survey collected information on approximately 6,250 individuals aged 18-35 with at least one migrant parent from Morocco, Turkey or former Yugoslavia, in 15 European cities, regrouped in 8 ‘countries’. For the purpose of this paper, only analyses for Austria (Linz and Vienna); Switzerland (Basle and Zurich); Germany (Berlin and Frankfurt); France (Paris and Strasbourg); the Netherlands (Amsterdam and Rotterdam); Spain (Barcelona and Madrid); and Sweden (Stockholm) will be presented.