Ana Acebron — Page 3

arXiv2017

Hubble Frontier Fields: systematic errors in strong lensing models of galaxy clusters - Implications for cosmography

Strong gravitational lensing by galaxy clusters is a fundamental tool to study dark matter and constrain the geometry of the Universe. Recently, the Hubble Space Telescope Frontier Fields programme has allowed a significant improvement of mass and magnification measurements but lensing models still have a residual root mean square between 0.2 arcsec and few arcsec- onds, not yet completely understood. Systematic errors have to be better understood and treated in order to use strong lensing clusters as reliable cosmological probes. We have analysed two simulated Hubble-Frontier-Fields-like clusters from the Hubble Frontier Fields Comparison Challenge, Ares and Hera. We use several estimators (relative bias on magnification, den- sity profiles, ellipticity and orientation) to quantify the goodness of our reconstructions by comparing our multiple models, optimized with the parametric software LENSTOOL , with the input models. We have quantified the impact of systematic errors arising, first, from the choice of different density profiles and configurations and, secondly, from the availability of con- straints (spectroscopic or photometric redshifts, redshift ranges of the background sources) in the parametric modelling of strong lensing galaxy clusters and therefore on the retrieval of cosmological parameters. We find that substructures in the outskirts have a significant im- pact on the position of the multiple images, yielding tighter cosmological contours. The need for wide-field imaging around massive clusters is thus reinforced. We show that competitive cosmological constraints can be obtained also with complex multimodal clusters and that photometric redshifts improve the constraints on cosmological parameters when considering a narrow range of (spectroscopic) redshifts for the sources.

astro-ph.CO↗

arXiv2016

Searching for filaments and large-scale structure around DAFT/FADA clusters

Clusters of galaxies are at the intersection of cosmic filaments and are still accreting galaxies and groups along these preferential directions, but, because of their relatively low contrast on the sky, they are difficult to detect (unless a large amount of spectroscopic data are available), and unambiguous detections have been limited until now to relatively low redshifts (z<0.3). We searched for extensions and filaments around the thirty clusters of the DAFT/FADA survey (redshift range 0.4<z<0.9) with deep wide field photometric data. For each cluster, based on a colour-magnitude diagram, we selected galaxies that were likely to belong to the red sequence, and hence to be at the cluster redshift, and built density maps. By computing the background for each map and drawing 3sigma contours, we estimated the elongations of the structures detected in this way. Whenever possible, we identified the other structures detected on the density maps with clusters listed in NED. We found clear elongations in twelve clusters, with sizes reaching up to 7.6 Mpc. Eleven other clusters have neighbouring structures, but the zones linking them are not detected in the density maps at a 3sigma level. Three clusters show no extended structure and no neighbours, and four clusters are of too low contrast to be clearly visible on our density maps. The simple method we have applied appears to work well to show the existence of filaments and/or extensions around a number of clusters in the redshift range 0.4<z<0.9. We plan to apply it to other large cluster samples such as the clusters detected in the CFHTLS and SDSS-Stripe 82 surveys in the near future.

astro-ph.CO↗