Browsing by Author "Kneib, JP"

Now showing 1 - 7 of 7
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    A search for clusters and groups of galaxies on the line of sight towards 8 lensed quasars
    (2004) Faure, C; Alloin, D; Kneib, JP; Courbin, F
    In this paper we present new ESO/VLT FORS1 and ISAAC images of the fields around eight gravitationally lensed quasars: CTQ 414, HE 0230-2130, LBQS 1009-0252, B 1030+074, HE 1104-1805, B 1359+154, H 1413+117 and HE 2149-2745. When available and deep enough, HST/WFPC2 data were also used to infer the photometric redshifts of the galaxies around the quasars. The search of galaxy overdensities in space and redshift, as well as a weak-shear analysis and a mass reconstruction are presented in this paper. We find that there are most probably galaxy groups towards CTQ 414, HE 0230-2130, B 1359+154, H 1413+117 and HE 2149-2745, with masses <4x10(14) M-circle dot h(-1). Considering its photometric redshift, the galaxy group discovered in the field around HE 1104-1805 is associated with the quasar rather than with the lensing potential.
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    An optical time-delay for the lensed BAL quasar HE 2149-2745
    (2002) Burud, I; Courbin, F; Magain, P; Lidman, C; Hutsemékers, D; Kneib, JP; Hjorth, J; Brewer, J; Pompei, E; Germany, L; Pritchard, J; Jaunsen, AO; Letawe, G; Meylan, G
    We present optical V and i-band light curves of the gravitationally lensed BAL quasar HE 2149-2745. The data, obtained with the 1.5 m Danish Telescope (ESO-La Silla) between October 1998 and December 2000, are the first from a long-term project aimed at monitoring selected lensed quasars in the Southern Hemisphere. A time delay of 103+/-12 days is determined from the light curves. In addition, VLT/FORS1 spectra of HE 2149 2745 are deconvolved in order to obtain the spectrum of the faint lensing galaxy, free of any contamination by the bright nearby two quasar images. By cross-correlating the spectrum with galaxy-templates we obtain a tentative redshift estimate of z = 0.495+/-0:01. Adopting this redshift, a Omega = 0.3, Lambda = 0.7 cosmology, and a chosen analytical lens model, our time-delay measurement yields a Hubble constant of H-0 = 66+/-8 km s(-1) Mpc(-1) (1sigma error) with an estimated systematic error of +/-3 km s(-1) Mpc(-1). Using non-parametric models yields H-0 = 65+/-8 km s(-1) Mpc(-1) (1sigma error) and confirms that the lens exhibits a very dense/concentrated mass profile. Finally, we note, as in other cases, that the flux ratio between the two quasar components is wavelength dependent. While the flux ratio in the broad emission lines-equal to 3.7-remains constant with wavelength, the continuum of the brighter component is bluer. Although the data do not rule out extinction of one quasar image relative to the other as a possible explanation, the effect could also be produced by differential microlensing by stars in the lensing galaxy.
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    Constraining the mass distribution of galaxies using galaxy-galaxy lensing in clusters and in the field
    (2005) Limousin, M; Kneib, JP; Natarajan, P
    We present a maximum-likelihood analysis of galaxy-galaxy lensing effects in galaxy clusters and in the field. The aim is to determine the accuracy and robustness of constraints that can be obtained on galaxy halo properties in both environments: the high-density cluster and the low-density field. This paper is theoretically motivated, therefore we work exclusively with simulated data (nevertheless defined to match observations) to study the accuracy with which input parameters for mass distributions for galaxies can be extracted. We model galaxies in the cluster and the field using a wide range of mass profiles: the truncated pseudo-isothermal elliptical mass distribution, the Navarro-Frenk-White profile, and a power-law model with a core radius. We find that independent of the choice of profile the mean mass of galaxies (of the order of 10(12) M-circle dot) can be estimated to within 15 per cent from ground-based data and with an error of less than 10 per cent with space observations. Additionally, robust constraints can be obtained on the mean slope of the mass profile. The two standard parameters that characterize galaxy halo models, the central velocity dispersion and the truncation radius, can also be retrieved reliably from the maximum-likelihood analysis. We find that there is an optimal scale R-max which marks the boundary between lenses that effectively contribute to the measured shear. Lenses beyond R-max in fact dilute the shear signal. Furthermore, going beyond the usual formulation, we propose a reparametrization of the mass models that allows us to put yet stronger constraints on the aperture mass of a galaxy halo (with less than 10 per cent error). The gain in signal-to-noise using space observations, expected for instance with the proposed SuperNova/Acceleration Probe (SNAP) satellite compared to ground-based data in terms of accuracy of retrieving input parameters, is highly significant.
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    Cosmic alignment toward the radio Einstein ring PKS 1830-211?
    (2002) Courbin, F; Meylan, G; Kneib, JP
    Optical and near-IR Hubble Space Telescope and Gemini North adaptive optics images, further improved through deconvolution, are used to explore the gravitationally lensed radio source PKS 1830-211. The line of sight to the quasar at z = 2.507 appears to be very busy, with the presence, within 0."5 from the source, of (1) a possible galactic main-sequence star, (2) a faint red lensing galaxy visible only in H band, and (3) a new object whose colors and morphology match those of an almost face-on spiral. The V-I color and faint I magnitude of the latter suggest that it is associated with the molecular absorber seen toward PKS 1830-211, at z = 0.89 rather than with the z = 0.19 H I absorber previously reported in the spectrum of PKS 1830-211. While this discovery might ease the interpretation of the observed absorption lines, it also complicates the modeling of the lensing potential well, hence decreasing the interest in using this system as a means to measure H-0 through the time delay between the lensed images. This is the first case of a quasar lensed by an almost face-on spiral galaxy.
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    Exploring the gravitationally lensed system HE 1104-1805: near-IR spectroscopy
    (2000) Courbin, F; Lidman, C; Meylan, G; Kneib, JP; Magain, P
    A new technique for the spatial deconvolution of spectra is applied to near-IR (0.95-2.50 mu m) NTT/SOFI spec tra of the lensed, radio-quiet quasar HE 1104-1805. The continuum of the lensing galaxy is revealed between 1.5 mu m and 2.5 mu m Although the spectrum does not show strong emission features, it is used in combination with previous optical and IR photometry to infer a plausible redshift in the range 0.8 < z < 1.2. Modeling of the system shows that the lens is complex, probably composed of the red galaxy seen between the quasar images and a more extended component associated with a galaxy cluster with fairly low velocity dispersion (575 km s(-1)). Unless more constrains can be put on the mass distribution of the cluster, e.g. from deep X-ray observations, HE 1104-1805 will not be a good system to determine H-0. We stress that multiply imaged quasars with known time delays may prove more useful as tools for detecting dark mass in distant lenses than for determining cosmological parameters.
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    Exploring the gravitationally lensed system HE 1104-1805: VLT spectroscopy of the lens at z=0.729
    (2000) Lidman, C; Courbin, F; Kneib, JP; Golse, G; Castander, F; Soucail, G
    Using FORS2, mounted on Kueyen CUT2 of the VLT), we have obtained the redshift of the lensing galaxy in the gravitational lens system HE 1104-1805. We measure z = 0.729 +/- 0.001, in good agreement with previous estimates based on the time delay and the position of the lens on the fundamental plane. It also coincides with one of the metal line absorption systems that are seen in high resolution spectra of HE 1104-1805.
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    The lensing system towards the doubly imaged quasar SBS 1520+530
    (2002) Faure, C; Courbin, F; Kneib, JP; Alloin, D; Bolzonella, M; Burud, I
    The gravitational potential responsible for the lensing effect in SBS 1520+530 is studied over length scales from a few arc-seconds to a few arc-minutes. For this purpose, we use sharply deconvolved Hubble Space Telescope images in the optical and near-IR, in combination with ground based optical data obtained over a wider field-of-view. In particular, we have carried out a multi-color analysis in order to identify groups or clusters of galaxies along the line of sight. Photometric redshifts are measured for 139 galaxies unveiling significant excesses of galaxies 1.0' NW and 1.7' SW of the main lensing galaxy. The photometric redshift inferred both for the main lensing galaxy and for the galaxy concentrations is z = 0.9(-0.25)(+0.10). This is in rough agreement with the measured spectroscopic redshift of the main lensing galaxy, z = 0.71 (Burud et al. 2002), suggesting that it is part of a larger group or cluster. We investigate the impact of including the galaxy cluster, first on the modelling of the lensing system, and second on the expected time-delay between the two quasar images.