No abstract

In this paper, we describe a procedure for modelling strong lensing galaxy clusters with parametric methods, and to rank models quantitatively using the Bayesian evidence. We use a publicly-available Markov Chain Monte-Carlo (MCMC) sampler ("BayeSys"), allowing us to avoid local minima in the likelihood functions.To illustrate the power of the MCMC technique, we simulate three clusters of galaxies, each composed of a cluster-scale halo and a set of perturbing galaxyscale subhalos. We ray-trace three light beams through each model to produce a catalogue of multiple images, and then use the MCMC sampler to recover the model parameters in the three different lensing configurations.We find that, for typical HST-quality imaging data, the total mass in the Einstein radius is recovered with ∼ 1% to 5% error according to the considered lensing configuration. However, we find that the mass of the galaxies is strongly degenerate with the cluster mass when no multiple images appear in the cluster centre. The mass of the galaxies is generally recovered with a 20% error, due largely to the poorly constrained cut-off radius.Finally, we describe how to rank models quantitatively using the Bayesian evidence. We confirm the ability of strong lensing to constrain the mass profile in the central region of galaxy clusters in this way. Ultimately, such a method applied to strong lensing clusters with a very large number of multiple images may provide unique geometrical constraints on cosmology.The implementation of the MCMC sampler used in this paper has been done within the framework of the lenstool software package, which is publicly available. ‡ PACS numbers: 90 98.62.Sb 07.05.Kf 98.65.Cw 95.35.+d ‡

We present a reconstruction of the mass distribution of galaxy cluster Abell 1689 at z = 0.18 using detected strong lensing features from deep HST/ACS observations and extensive ground based spectroscopy. Earlier analyses have reported up to 32 multiply imaged systems in this cluster, of which only 3 were spectroscopically confirmed. In this work, we present a parametric strong lensing mass reconstruction using 24 multiply imaged systems with newly determined spectroscopic redshifts, which is a major step forward in building a robust mass model. In turn, the new spectroscopic data allows a more secure identification of multiply imaged systems. The resultant mass model enables us to reliably predict the redshifts of additional multiply imaged systems for which no spectra are currently available, and to use the location of these systems to further constrain the mass model. In particular, we have detected 5 strong galaxy-galaxy lensing systems just outside the Einstein ring region, further constraining the mass profile. Our strong lensing mass model is consistent with that inferred from our large scale weak lensing analysis derived using CFH12k wide field images. Thanks to a new method for reliably selecting a well defined background lensed galaxy population, we resolve the discrepancy found between the strong and weak lensing mass models reported in earlier work. [ABRIDGED]Comment: ApJ in press, 668, 643. Final article with figures and online data available at http://archive.dark-cosmology.dk

We report the clear detection of the 2175Å dust absorption feature in the optical afterglow spectrum of the gamma-ray burst (GRB) GRB 070802 at a redshift of z = 2.45. This is the highest redshift for a detected 2175Å dust bump to date, and it is the first clear detection of the 2175Å bump in a GRB host galaxy, while several tens of optical afterglow spectra without the bump have been recorded in the past decade. The derived extinction curve gives A V = 0.8-1.5 depending on the assumed intrinsic slope. Of the three local extinction laws, an LMC type extinction gives the best fit to the extinction curve of the host of GRB 070802. Besides the 2175Å bump we find that the spectrum of GRB 070802 is characterized by unusually strong low-ionization metal lines and possibly a high metallicity for a GRB sightline ([Si/H] = −0.46 ± 0.38, [Zn/H] = −0.50 ± 0.68). In particular, the spectrum of GRB 070802 is unique for a GRB spectrum in that it shows clear C I absorption features, leading us to propose a correlation between the presence of the bump and C I. The gas to dust ratio for the host galaxy is found to be significantly lower than that of other GRB hosts with N(H I)/A V = (2.4 ± 1.0)× 10 21 cm −2 mag −1 , which lies between typical MW and LMC values. Our results are in agreement with the tentative conclusion reached by Gordon et al. (2003) that the shape of the extinction curve, in particular the presence of the bump, is affected by the UV flux density in the environment of the dust.