arXiv subjects

L. V. E. Koopmans

Publications and source records attributed to L. V. E. Koopmans.

At least 199 records · Page 11Linked to original sources

arXiv2002

The Cosmic Lens All-Sky Survey:II. Gravitational lens candidate selection and follow-up

We report the final results of the search for gravitationally lensed flat-spectrum radio sources found in the combination of CLASS (Cosmic Lens All Sky Survey) and JVAS (Jodrell-Bank VLA Astrometric Survey). VLA observations of 16,503 sources have been made, resulting in the largest sample of arcsec-scale lens systems available. Contained within the 16,503 sources is a complete sample of 11,685 sources having two-point spectral indices between 1.4 and 5 GHz flatter than -0.5 and 5 GHz flux densities $\geq$30 mJy. A subset of 8,958 sources form a well-defined statistical sample suitable for analysis of the lens statistics. We describe the systematic process by which 149 candidate lensed sources were picked from the statistical sample on the basis of possessing multiple compact components in the 0.2 arcsec-resolution VLA maps. Candidates were followed up with 0.05 arcsec resolution MERLIN and 0.003 arcsec VLBA observations at 5 GHz and rejected as lens systems if they failed well-defined surface brightness and/or morphological tests. Maps for all the candidates can be found on the World Wide Web at http://www.jb.man.ac.uk/research/gravlens/index.html We summarize the properties of each of the 22 gravitational lens systems in JVAS/CLASS. Twelve are double-image systems, nine are four-image systems and one is a six-image system. Thirteen constitute a statistically well-defined sample giving a point-source lensing rate of 1:690$\pm$190. The interpretation of the results in terms of the properties of the lensing galaxy population and cosmological parameters will be published elsewhere. (Abridged)

astro-ph↗

arXiv2002

The Cosmic Lens All-Sky Survey: I. Source selection and observations

The Cosmic Lens All-Sky Survey (CLASS) is an international collaborative program which has obtained high-resolution radio images of over 10000 flat-spectrum radio sources in order to create the largest and best studied statistical sample of radio-loud gravitationally lensed systems. With this survey, combined with detailed studies of the lenses found therein, constraints can be placed on the expansion rate, matter density, and dark energy (e.g. cosmological constant, quintessence) content of the Universe that are complementary to and independent of those obtained through other methods. CLASS is aimed at identifying lenses where multiple images are formed from compact flat-spectrum radio sources, which should be easily identifiable in the radio maps. Because CLASS is radio-based, dust obscuration in lensing galaxies is not a factor, and the relative insensitivity of the instrument to environmental conditions leads to nearly uniform sensitivity and resolution over the entire survey. In four observing seasons from 1994-1999, CLASS has observed 13783 radio sources with the VLA at 8.4 GHz at 0.2 arcsecond resolution. When combined with the JVAS survey, the CLASS sample contains over 16,000 images. A complete sample of 11685 flat-spectrum sources was observed, selected from GB6 catalogue at 4.85 GHz and the NVSS at 1.4 GHz. So far, CLASS has found 16 new gravitational lens systems, and the JVAS/CLASS survey contains a total of 22 lenses. (Abridged)

astro-ph↗

arXiv2002

B0850+054: a new gravitational lens system from CLASS

We report the discovery of a new gravitational lens system from the CLASS survey. Radio observations with the VLA, the WSRT and MERLIN show that the radio source B0850+054 is comprised of two compact components with identical spectra, a separation of 0.7 arcsec and a flux density ratio of 6:1. VLBA observations at 5 GHz reveal structures that are consistent with the gravitational lens hypothesis. The brighter of the two images is resolved into a linear string of at least six sub-components whilst the weaker image is radially stretched towards the lens galaxy. UKIRT K-band imaging detects an 18.7 mag extended object, but the resolution of the observations is not sufficient to resolve the lensed images and the lens galaxy. Mass modelling has not been possible with the present data and the acquisition of high-resolution optical data is a priority for this system.

astro-ph↗

arXiv2002

CLASS B0445+123: a new two-image gravitational lens system

A new two-image gravitational lens system has been discovered as a result of the Cosmic Lens All-Sky Survey (CLASS). Radio observations with the VLA, MERLIN and the VLBA at increasingly higher resolutions all show two components with a flux density ratio of ~7:1 and a separation of 1.34". Both components are compact and have the same spectral index. Followup observations made with the VLA at 8.4 GHz show evidence of a feature to the south-east of the brighter component and a corresponding extension of the weaker component to the north-west. Optical observations with the WHT show ~1.7" extended emission aligned in approximately the same direction as the separation between the radio components with an R-band magnitude of 21.8 +/- 0.4.

astro-ph↗

arXiv2002

The Structure and Dynamics of Luminous and Dark Matter in the Early-Type Lens Galaxy of 0047-281 at z=0.485

We have measured the kinematic profile of the early-type (E/S0) lens galaxy in the system 0047-281 (z=0.485) with the {\sl Echelle Spectrograph and Imager} (ESI) on the W.M. Keck--II Telescope, as part of the {\sl Lenses Structure and Dynamics (LSD) Survey}. The central velocity dispersion is σ=229\pm 15 \kms, and the dispersion profile is nearly flat to beyond one effective radius (R_e). Surface photometry of the lens galaxy is measured from {\sl Hubble Space Telescope} images. From the offset from the local Fundamental Plane (FP), we measure an evolution of the effective mass-to-light ratio of Δ\log M/L_B=-0.37\pm0.06 between z=0 and z=0.485, consistent with the observed evolution of field E/S0 galaxies. (We assume h_{65}=1,Ω_{\rm m}=0.3 and Ω_Λ=0.7 throughout.) Gravitational lens models provide a mass of M_{\rmE}=(4.06\pm0.20)\times 10^{11} h_{65}^{-1}M_\odot inside the Einstein radius of R_{\rm E}=(8.70\pm0.07) h_{65}^{-1} kpc. This allows us to break the degeneracy between velocity anisotropy and density profile, typical of dynamical models for E/S0 galaxies. We find that constant M/L model, even with strongly tangential anisotropy of the stellar velocity ellipsoid, are excluded at >99.9%CL. The total mass distribution inside R_{\rm E} can be described by a single power-law density profile, ρ_t\propto r^{-γ'}, with an effective slope γ'=1.90^{+0.05}_{-0.23} (68%CL; \pm0.1 systematic error). Two-component models yield an upper limit (68% CL) of γ\le 1.55(1.12) on the power-law slope of the dark-matter density profile and a projected dark-matter mass fraction of 0.41(0.54)^{+0.15}_{-0.05}(^{+0.09}_{-0.06}) (68% CL) inside R_{\rm E}, for Osipkov--Merritt models with anisotropy radius r_i=\infty(R_e).

astro-ph↗

arXiv2002

Constraints on Cosmological Parameters from the Analysis of the Cosmic Lens All Sky Survey Radio-Selected Gravitational Lens Statistics

We derive constraints on cosmological parameters and the properties of the lensing galaxies from gravitational lens statistics based on the final Cosmic Lens All Sky Survey (CLASS) data. For a flat universe with a classical cosmological constant, we find that the present matter fraction of the critical density is $Ω_{\rm m}=0.31^{+0.27}_{-0.14}$ (68%) $^{+0.12}_{-0.10}$ (systematic). For a flat universe with a constant equation of state for dark energy $w = p_x({pressure})/ρ_x({energy density})$, we find $w < -0.55^{+0.18}_{-0.11}$ (68%).

astro-ph↗

arXiv2002

A Determination of H_0 with the CLASS Gravitational Lens B1608+656: III. A Significant Improvement in the Precision of the Time Delay Measurements

The gravitational lens CLASS B1608+656 is the only four-image lens system for which all three independent time delays have been measured. This makes the system an excellent candidate for a high-quality determination of H_0 at cosmological distances. However, the original measurements of the time delays had large (12-20%) uncertainties, due to the low level of variability of the background source during the monitoring campaign. In this paper, we present results from two additional VLA monitoring campaigns. In contrast to the ~5% variations seen during the first season of monitoring, the source flux density changed by 25-30% in each of the subsequent two seasons. We analyzed the combined data set from all three seasons of monitoring to improve significantly the precision of the time delay measurements; the delays are consistent with those found in the original measurements, but the uncertainties have decreased by factors of two to three. We combined the delays with revised isothermal mass models to derive a measurement of H_0. Depending on the positions of the galaxy centroids, which vary by up to 0.1 arcsec in HST images obtained with different filters, we obtain H_0 = 61-65 km/s/Mpc, for (Omega_M,Omega_L) = (0.3,0.7). The value of H_0 decreases by 6% if (Omega_M,Omega_L) = (1.0,0.0). The formal uncertainties on H_0 due to the time delay measurements are +/- 1 (+/- 2) km/s/Mpc for the 1-sigma (2-sigma) confidence limits. Thus, the systematic uncertainties due to the lens model, which are on the order of +/- 15 km/s/Mpc, now dominate the error budget for this system. In order to improve the measurement of H_0 with this lens, new models that incorporate the constraints provided by stellar dynamics and the optical/infrared Einstein ring seen in HST images must be developed.

astro-ph↗

arXiv2002

Probing dark matter distribution with gravitational lensing and stellar dynamics

The Lenses Structure and Dynamics (LSD) Survey aims at measuring the luminous and dark matter distribution of high redshift early-type galaxies by combining gravitational lens analysis with newly determined velocity dispersion profiles. Here we describe the first results from the LSD survey, a measurement of the internal structure of the lens galaxy in MG2016+112 (z=1.004). The relevance of this measurement to the cosmological model is briefly discussed, in particular in the context of E/S0 galaxy formation and evolution, the existence and shape of universal dark matter profiles, and the measurement of the Hubble constant from gravitational time delay.

astro-ph↗

arXiv2002

The stellar velocity dispersion of the lens galaxy in MG2016+112 at z=1.004

We present a direct measurement of the stellar velocity dispersion of the early-type lens galaxy D in the system MG2016+112 (z=1.004), determined from a spectrum obtained with the Echelle Spectrograph and Imager (ESI) on the W.M. Keck-II Telescope, as part of the Lenses Structure and Dynamics (LSD) Survey. We find a velocity dispersion of sigma_ap=304+-27 km/s inside an effective circular aperture with a radius of 0.65'', corresponding to a central velocity dispersion of sigma=328+-32 km/s. From a Hubble Space Telescope F160W-band image, we measure the effective radius and effective surface brightness in order to determine the offset of the lens galaxy with respect to the local Fundamental Plane. The offset corresponds to an evolution of the rest-frame effective mass-to-light ratio of Delta log(M/L_B)=-0.62+-0.08 from z=0 to z=1.004. By interpreting colors and offset of the FP with two independent stellar population synthesis models, we obtain a single-burst equivalent age of 2.8+-0.8 Gyr (i.e. z_f>1.9) and a supersolar metallicity of log[Z/Z_sun]=0.25+-0.25. The lens galaxy is therefore a massive elliptical dominated by an old and metal rich stellar population at z>1. The excellent agreement of the stellar velocity dispersion with that predicted from recent lens models confirms that the angular separation of the multiple images of the background QSO is predominantly due to the lens galaxy, and not to a massive ``dark cluster'', in agreement with recent weak lensing and X-ray observations. However, the significant overdensity of galaxies in the field might indicate that this system is a proto-cluster, in formation around galaxy D, responsible for the ~10% external shear inferred from the strong lens models.

astro-ph↗

arXiv2002

2016+112: A Gravitationally Lensed Type-II Quasar

A single-screen model of the gravitational lens system 2016+112 is proposed, that explains recent Hubble Space Telescope} (HST) infrared (NICMOS-F160W) observations and new high-resolution European VLBI Network (EVN) 5-GHz radio observations, presented in this paper. In particular, we find that a massive `dark' structure at the lens position, previously suggested by X-ray, optical and spectroscopic observations of the field around 2016+112, is not necessarily required to accommodate the strong lensing constraints. A massive structure to the north-west of the lens system, suggested from a weak-lensing analysis of the field, is included in the model. The lensed source is an X-ray bright active galaxy at z=3.273 with a central bright optical continuum core and strong narrow emission lines, suggestive of a type-II quasar. The EVN 5-GHz radio maps show a radio-jet structure with at least two compact subcomponents. We propose that the diamond caustic crosses the counter-jet of the radio source, so that part of the counter-jet, host galaxy and narrow-line emission regions are quadruply imaged. The remainder of the radio source, including the core, is doubly imaged. Our lens model predicts a very high magnification (mu~300) at the bightness peaks of the inner two radio components of complex C. If the jet exhibits relativistic velocities on micro-arsecond scales, it might result in apparent hyperluminal motion. However, the lack of strong radio variability and the peaked radio spectrum imply that these motions need not be present in the source. Our model furthermore implies that the optical spectrum of C' can only show features of the AGN and its host galaxy.

astro-ph↗

arXiv2001

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.

astro-ph↗

arXiv2001

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)

astro-ph↗

arXiv2001

The new two-image gravitational lens system CLASS B2319+051

We report the discovery of a new two-image gravitational lens system from the Cosmic Lens All-Sky Survey, CLASS B2319+051. Radio imaging with the Very Large Array (VLA) and Multi-Element Radio-Linked Interferometer Network (MERLIN) shows two compact components with a flux density ratio of 5:1, separated by 1.36 arcsec. Observations with the Very Long Baseline Array (VLBA) resolve each of the radio components into a pair of parity-reversed subcomponents. Hubble Space Telescope (HST) observations with the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) show a bright elliptical galaxy (G1) coincident with the radio position, and a second irregular galaxy (G2) 3.4 arcsec to the northwest. Previous spectroscopic studies have indicated that these galaxies are at different redshifts: z(G1) = 0.624, z(G2) = 0.588. Infrared counterparts to the lensed radio components are not detected in the NICMOS image, and the source redshift has not yet been determined. Preliminary mass modeling based on the VLBA subcomponent data indicates that the lensing potential includes a strong external shear contribution. A VLA monitoring program is currently being undertaken to measure the differential time delay.

astro-ph↗

arXiv2001

On the Probability of Microlensing in GRB Afterglows

The declining lightcurve of the optical afterglow of gamma-ray burst GRB000301C showed rapid variability with one particularly bright feature at about t-t0=3.8 days. This event was interpreted as gravitational microlensing by Garnavich, Loeb & Stanek (2000) and subsequently used to derive constraints on the structure of the GRB optical afterglow. In this paper, we use these structural parameters to calculate the probability of such a microlensing event in a realistic scenario, where all compact objects in the universe are associated with observable galaxies. For GRB000301C at a redshift of z=2.04, the `a posteriori' probability for a microlensing event with an amplitude of dm>=0.95 mag (as observed) is 0.7% (2.7%) for the most plausible scenario of a flat Lambda-dominated FRW universe with Omega_m=0.3 and a fraction f_*=0.2 (1.0) of dark-matter in the form of compact objects. If we lower the magnification threshold to dm>=0.10 mag, the probabilities for microlensing events of GRB afterglows increase to 17% (57%). We emphasise that this low probability for a microlensing signature of almost a magnitude does not exclude that the observed event in the afterglow lightcurve of GRB000301C was caused by microlensing, especially in light of the fact that a galaxy was found within 2 arcsec from the GRB. In that case, however, a more robust upper limit on the `a posteriori' probability of ~5% is found. It does show, however, that it will not be easy to create a large sample of strong GRB afterglow microlensing events for statistical studies of their physical conditions on micro-arcsec scales.

astro-ph↗

arXiv2001

The Hubble Constant from (CLASS) Gravitational Lenses

One of the main objectives of the Cosmic Lens All-Sky Survey (CLASS) collaboration has been to find gravitational lens (GL) systems at radio wavelengths that are suitable for the determination of time delays between image pairs. The survey is now near completion and at least 18 GL systems have been found. Here, I will discuss our efforts to measure time delays from several of these systems with the ultimate aim of constraining the Hubble Constant (H0). Thus far three CLASS GL systems (i.e. B0218+357, B1600+434 and B1608+656) have yielded measurements of time delays, from which values of H0=60-70 km/s/Mpc have been estimated. Although most GL systems give similar values of H0, statistical and systematic uncertainties are still considerable. To reduce these uncertainties, I will shortly mention two monitoring programs that we are undertaking to (re)measure time delays in 15 CLASS GL systems and address several important issues for the near future.

astro-ph↗

arXiv2000

Radio Microlensing: Past, Present & Near Future

Strongly correlated non-intrinsic variability between 5 and 8.5 GHz has been observed in one of the lensed images of the gravitational lens B1600+434. These non-intrinsic (i.e. `external') variations are interpreted as `radio-microlensing' of relativistic micro-arsec-scale jet components in the source at a redshift of z=1.59 by massive compact objects in the halo of the edge-on disk lens galaxy at z=0.41. We shortly summarize these observations and discuss several new observational and theoretical programs to investigate this new phenomenon in more detail.

astro-ph↗

arXiv2000

A New Quadruple Gravitational Lens System: CLASS B0128+437

High resolution MERLIN observations of a newly-discovered four-image gravitational lens system, B0128+437, are presented. The system was found after a careful re-analysis of the entire CLASS dataset. The MERLIN observations resolve four components in a characteristic quadruple-image configuration; the maximum image separation is 542 mas and the total flux density is 48 mJy at 5 GHz. A best-fit lens model with a singular isothermal ellipsoid results in large errors in the image positions. A significantly improved fit is obtained after the addition of a shear component, suggesting that the lensing system is more complex and may consist of multiple deflectors. The integrated radio spectrum of the background source indicates that it is a GigaHertz-Peaked Spectrum (GPS) source. It may therefore be possible to resolve structure within the radio images with deep VLBI observations and thus better constrain the lensing mass distribution.

astro-ph↗

arXiv2000

CLASS B0739+366: A new two-image gravitational lens system

We present the discovery of CLASS B0739+366, a new gravitational lens system from the Cosmic Lens All-Sky Survey. Radio imaging of the source with the Very Large Array (VLA) shows two compact components separated by $0\farcs54$, with a flux density ratio of $\sim$ 6:1. High-resolution follow-up observations using the Very Long Baseline Array (VLBA) at 1.7 GHz detect weak, parity-reversed jet emission from each of the radio components. Hubble Space Telescope NICMOS F160W observations detect infrared counterparts to the lensed images, as well as an extended object between them which we identify as the lensing galaxy. Redshifts for the galaxy and lensed source have not yet been obtained. For typical lens and source redshifts of $z=0.5$ and $z=1.5$, respectively, preliminary mass modeling predicts a time delay of $\sim7h^{-1}$ days in a flat $Ω_{M}=1.0$ universe. The small predicted time delay and weak radio components will make CLASS B0739+366 a challenging target for Hubble constant determination.

astro-ph↗