MERLIN/VLA imaging of the gravitational lens system B0218+357

Gravitational lenses offer the possibility of accurately determining the Hubble parameter (H_0) over cosmological distances, and B0218+357 is one of the most promising systems for an application of this technique. In particular this system has an accurately measured time delay (10.5+/-0.4 d; Biggs et al. 1999) and preliminary mass modelling has given a value for H_0 of 69 +13/-19 km/s/Mpc. The error on this estimate is now dominated by the uncertainty in the mass modelling. As this system contains an Einstein ring it should be possible to constrain the model better by imaging the ring at high resolution. To achieve this we have combined data from MERLIN and the VLA at a frequency of 5 GHz. In particular MERLIN has been used in multi-frequency mode in order to improve substantially the aperture coverage of the combined data set. The resulting map is the best that has been made of the ring and contains many new and interesting features. Efforts are currently underway to exploit the new data for lensing constraints using the LensClean algorithm (Kochanek & Narayan 1992).Comment: Accepted for publication in MNRAS. 6 pages, 4 included PostScript figure

High resolution observations and mass modelling of the CLASS gravitational lens B1152+199

We present a series of high resolution radio and optical observations of the CLASS gravitational lens system B1152+199 obtained with the Multi-Element Radio-Linked Interferometer Network (MERLIN), Very Long Baseline Array (VLBA) and Hubble Space Telescope (HST). Based on the milliarcsecond-scale substructure of the lensed radio components and precise optical astrometry for the lensing galaxy, we construct models for the system and place constraints on the galaxy mass profile. For a single galaxy model with surface mass density Sigma(r) propto r^-beta, we find that 0.95 < beta < 1.21 at 2-sigma confidence. Including a second deflector to represent a possible satellite galaxy of the primary lens leads to slightly steeper mass profiles.Comment: 7 pages, post-referee revision for MNRA

Redshifts of the Gravitational Lenses MG0414+0534 and MG0751+2716

We report redshifts in two gravitational lens systems, MG0414+0534 and MG0751+2716. The lens galaxy in MG0414+0534 lies at z_l=0.9584+/-0.0002. The luminosity and extreme red color of the lens are then typical of a passively evolving, early-type, ~2L* galaxy. The galaxy cannot have a significant global mean extinction without being anomalously luminous. The lens galaxy in MG0751+2716 has a redshift of z_l=0.3502+/-0.0003 and it is a member of a small group. The group includes the nearby, bright companion galaxy whose redshift we confirmed to be z=0.3501+/-0.0001 and a nearby emission line galaxy with z=0.3505+/-0.0003. A second emission line galaxy with z=0.5216+/-0.0001 was found nearly superposed on the first emission line galaxy. The source in MG0751+2716 is a z_s=3.200+/-0.001 radio quasar. For flat universes with Omega_0=1.0 (0.3), 96% (87%) of lenses like MG0414+0534 and 7% (3%) of lenses like MG0751+2716 are expected to have lower lens redshifts than observed.Comment: 9 pages, AASTeX Latex, including 5 Postscript figures, submitted to Astronomical Journa

Multi wavelength study of the gravitational lens system RXS J1131-1231: II Lens model and source reconstruction

High angular resolution images of the complex gravitational lens system RXS J1131-1231 (a quadruply imaged AGN with a bright Einstein ring) obtained with the Advanced Camera for Surveys and NICMOS instruments onboard the Hubble Space Telescope are analysed to determine the lens model and to reconstruct the host galaxy. Results: 1- Precise astrometry and photometry of the four QSO lensed images (A-D) and of the lensing galaxy (G) are obtained. They are found in agreement with an independent study presented in a companion paper. The position and colours of the X object seen in projection close to the lens are found to be only compatible with a satellite galaxy associated with the lens. 2- The Singular Isothermal Ellipsoid plus external shear provides a good fit of the astrometry of images A-D. The positions of extended substructures are also well reproduced. However an octupole (m=4) must be added to the lens potential in order to reproduce the observed lens position, as well as the IB/IC point-like image flux ratio. The ellipticity and orientation of the mass quadrupole are found similar to those of the light distribution, fitted by a Sersic profile. The lens (z=0.295) is found to be a massive elliptical in a rich environment and showing possible evolution with respect to z=0. 3- The host galaxy (z=0.658) is found to be a substantially magnified (M ~ 9) luminous Seyfert 1 spiral galaxy. The angular resolution is sufficient to see regions where stars are intensively forming. Interaction with a closeby companion is also observed. 4- Finally, in the case of RXS J1131-1231, extended lensed structures do not help much in constraining the lens model.Comment: 17 pages, 6 figures, accepted in Astronomy and Astrophysics; improved Latex processing. Version with full resolution figures available at http://www.astro.ulg.ac.be/~claesken/lens1131_II.pd

The Baryon Fractions and Mass-to-Light Ratios of Early-Type Galaxies

We jointly model 22 early-type gravitational lens galaxies with stellar dynamical measurements using standard CDM halo models. The sample is inhomogeneous in both its mass distributions and the evolution of its stellar populations unless the true uncertainties are significantly larger than the reported measurement errors. In general, the individual systems cannot constrain halo models, in the sense that the data poorly constrains the stellar mass fraction of the halo. The ensemble of systems, however, strongly constrains the average stellar mass represented by the visible galaxies to $0.026\pm0.006$ of the halo mass if we neglect adiabatic compression, rising to $0.056\pm0.011$ of the halo mass if we include adiabatic compression. Both estimates are significantly smaller than the global baryon fraction, corresponding to a star formation efficiency for early-type galaxies of $10$. In the adiabatically compressed models, we find an average local B-band stellar mass-to-light ratio of (M/L)_0 = (7.2\pm0.5)(M_{\sun}/L_{\sun}) that evolves by $d\log(M/L)/dz = -0.72\pm0.08$ per unit redshift. Adjusting the isotropy of the stellar orbits has little effect on the results. The adiabatically compressed models are strongly favored if we impose either local estimates of the mass-to-light ratios of early-type galaxies or the weak lensing measurements for the lens galaxies on 100 kpc scales as model constraints.Comment: 9 figure

Gravitational Lensing by Power-Law Mass Distributions: A Fast and Exact Series Approach

We present an analytical formulation of gravitational lensing using familiar triaxial power-law mass distributions, where the 3-dimensional mass density is given by $\rho(X,Y,Z) = \rho_0 [1 + (\frac{X}{a})^2 + (\frac{Y}{b})^2 + (\frac{Z}{c})^2]^{-\nu/2}$. The deflection angle and magnification factor are obtained analytically as Fourier series. We give the exact expressions for the deflection angle and magnification factor. The formulae for the deflection angle and magnification factor given in this paper will be useful for numerical studies of observed lens systems. An application of our results to the Einstein Cross can be found in Chae, Turnshek, & Khersonsky (1998). Our series approach can be viewed as a user-friendly and efficient method to calculate lensing properties that is better than the more conventional approaches, e.g., numerical integrations, multipole expansions.Comment: 24 pages, 3 Postscript figures, ApJ in press (October 10th

COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses VIII. Deconvolution of high resolution near-IR images and simple mass models for 7 gravitationally lensed quasars

We apply the iterative MCS deconvolution method (ISMCS) to near-IR HST archives data of seven gravitationally lensed quasars currently monitored by the COSMOGRAIL collaboration: HE 0047-1756, RX J1131-1231, SDSS J1138+0314, SDSS J1155+6346, SDSS J1226-0006, WFI J2026-4536 and HS 2209+1914. In doing so, we obtain relative positions for the lensed images and shape parameters for the light distribution of the lensing galaxy in each system. The lensed image positions are derived with 1-2 mas accuracy. To predict time delays and to test the ability of simple mass models to reproduce the observed configuration, isothermal and de Vaucouleurs mass models are calculated for the whole sample using state-of-the-art modeling techniques. The effect of the lens environment on the lens mass models is taken into account with a shear term. Doubly imaged quasars are equally well fitted by each of these models. A large amount of shear is necessary to reproduce SDSS J1155+6346 and SDSS J1226-006. In the latter case, we identify a nearby galaxy as the dominant source of shear. The quadruply imaged quasar SDSS J1138+0314 is well reproduced by simple lens models, which is not the case for the two other quads, RX J1131-1231 and WFI J2026-4536. This might be the signature of astrometric perturbations due to massive substructures in the lensing galaxy unaccounted for by the models. Other possible explanations are also presented.Comment: 14 pages, 1 figure (with 28 frames), 5 tables, accepted for publication in Astronomy & Astrophysics (08/07/2010

Completeness in Photometric and Spectroscopic Searches for Clusters

We investigate, using simulated galaxy catalogues, the completeness of searches for massive clusters of galaxies in redshift surveys or imaging surveys with photometric redshift estimates, i.e. what fraction of clusters (M>10^14/h Msun) are found in such surveys. We demonstrate that the matched filter method provides an efficient and reliable means of identifying massive clusters even when the redshift estimates are crude. In true redshift surveys the method works extremely well. We demonstrate that it is possible to construct catalogues with high completeness, low contamination and both varying little with redshift.Comment: ApJ in press, 15 pages, 10 figure

A New Shear Estimator for Weak Lensing Observations

We present a new shear estimator for weak lensing observations which properly accounts for the effects of a realistic point spread function (PSF). Images of faint galaxies are subject to gravitational shearing followed by smearing with the instrumental and/or atmospheric PSF. We construct a `finite resolution shear operator' which when applied to an observed image has the same effect as a gravitational shear applied prior to smearing. This operator allows one to calibrate essentially any shear estimator. We then specialize to the case of weighted second moment shear estimators. We compute the shear polarizability which gives the response of an individual galaxy's polarization to a gravitational shear. We then compute the response of the population of galaxies, and thereby construct an optimal weighting scheme for combining shear estimates from galaxies of various shapes, luminosities and sizes. We define a figure of merit --- an inverse shear variance per unit solid angle --- which characterizes the quality of image data for shear measurement. The new method is tested with simulated image data. We discuss the correction for anisotropy of the PSF and propose a new technique involving measuring shapes from images which have been convolved with a re-circularizing PSF. We draw attention to a hitherto ignored noise related bias and show how this can be analyzed and corrected for. The analysis here draws heavily on the properties of real PSF's and we include as an appendix a brief review, highlighting those aspects which are relevant for weak lensing.Comment: 39 pages, 9 figure