J. M. Miralles

Cosmic shear analysis of archival HST/ACS data. I. Comparison of early ACS pure parallel data to the HST/GEMS survey

Context. This is the first paper of a series describing our measurement of weak lensing by large-scale structure, also termed “cosmic shear”, using archival observations from the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). Aims. In this work we present results from a pilot study testing the capabilities of the ACS for cosmic shear measurements with early parallel observations and presenting a re-analysis of HST/ACS data from the GEMS survey and the GOODS observations of the Chandra Deep Field South (CDFS). Methods. We describe the data reduction and, in particular, a new correction scheme for the time-dependent ACS point-spread-function (PSF) based on observations of stellar fields. This is currently th e only technique which takes the full time variation of the PSF between individual ACS exposures into account. We estimate that our PSF correction scheme reduces the systematic contribution to the shear correlation functions due to PSF distortions to< 2× 10 −6 for galaxy fields containing at least 10 stars, which corresp onds to. 5% of the cosmological signal expected on scales of a single ACS field. Results. We perform a number of diagnostic tests indicating that the remaining level of systematics is consistent with zero for the GEMS and GOODS data confirming the success of our PSF correction sc heme. For the parallel data we detect a low level of remaining systematics which we interpret to be caused by a lack of suffi cient dithering of the data. Combining the shear estimate of the GEMS and GOODS observations using 96 galaxies arcmin −2 with the photometric redshift catalogue of the GOODS-MUSIC sample, we determine a local single field estimate for the mass power spectrum normalisationσ8,CDFS = 0.52 +0.11 −0.15 (stat)± 0.07(sys) (68% confidence assuming Gaussian ! ,

Strong and weak lensing united: II. The cluster mass distribution of the most X-ray luminous cluster RX J1347.5-1145

We have shown that the cluster-mass reconstruction method which combines strong and weak gravitational lensing data, developed in the first paper in the series, successfully reconstructs the mass distribution of a simulated cluster. In this paper we apply the method to the ground-based high-quality multi-colour data of RX J1347.5-1145 , the most X-ray luminous cluster to date. A new analysis of the cluster core on very deep, multi-colour data analysis of VLT/FORS data reveals many more arc candidates than previously known for this cluster. The combined strong and weak lensing reconstruction confirms that the cluster is indeed very massive. If the redshift and identification of the multiple-image system as well as the redshift estimates of the source galaxies used for weak lensing are correct, we determine the enclosed cluster mass in a cylinder to M(<360 h -1 kpc)= (1.2± 0.3) x 1015 M⊙. In addition the reconstructed mass distribution follows the distribution found with independent methods (X-ray measurements, SZ). With higher resolution (e.g. HST imaging data) more reliable multiple imaging information can be obtained and the reconstruction can be improved to accuracies greater than what is currently possible with weak and strong lensing techniques.

A conspicuous tangential alignment of galaxies in a STIS Parallel Shear Survey field: A new dark-lens candidate?

We report the serendipituous discovery of a conspicuous alignment of galaxies in a eld obtained through the STIS Parallel Shear Survey. This project collects randomly distributed 50 00 50 00 elds to investigate the cosmic shear eect on this scale. Analyzing the parallel observations having the Seyfert galaxy NGC 625 as primary target, we recognized over the whole eld of view a strong apparent tangential alignment of galaxy ellipticities towards the image center. The eld shows several arclet-like features typical for images of massive galaxy clusters, but no obvious over-density of bright foreground galaxies. We also nd a multiple image candidate. On the basis of the possible strong and weak lensing eect within the data, we discuss whether this could be compatible with a massive halo with no clear optical counterpart.

A weak lensing analysis of a STIS dark-lens candidate

We perform a weak lensing analysis on a previously reported dark-lens candidate on STIS Parallel data (Miralles et al. 2002). New VLT-data indicate that the reported signal originates from a small number of galaxies tangentially aligned towards the center of the STIS field but no signature for an extended mass distribution is found. We argue that we should be able to detect a massive cluster (M > 3.2 x 10 1 4 M O .) through its lensing signal up to a redshift of z 0.6 with our data. Also the double image hypothesis of two galaxies with very similar morphologies in the STIS data is ruled out with colour information.

Cosmic Shear from STIS pure parallels III. Analysis of Cycle 9 pure parallels

Following the detection of a cosmic shear signal at the 30 �� scale using archival parallel data from the STIS CCD camera onboard HST in the previous paper of this series, we analyzed a larger data set obtained from an HST GO pure parallel program. Although this data set is considerably larger than the one analyzed previously, we do not obtain a significant detection of the cosmic shear signal. The most likely cause of this null result is that the signal is comparable to (or less than) the random measurement of the noise. We also show that multiple systematics plague the STIS CCD data for its use in cosmic shear programs, and in particular the continuous degradation of the CCD charge transfer efficiency after 4 years in space, which is a source of additional noise that is still difficult to correct for in the absence of physical models.

Cosmic Shear with ACS

Distant galaxies are continuously deflected while travelling through the gravitational potential of the large-scale matter distribution in the Universe. The resulting distortion of the galaxy shapes can be measured statistically. Such a Cosmic Shear analysis provides a powerful tool to probe the growth of structure on different scales, and thus to distinguish between different cosmological models. We have accomplished a Cosmic Shear analysis of HST/ACS pure parallel observations. Due to its relatively large field-of-view in combination with space-based resolution, ACS provides excellent conditions to measure Cosmic Shear on small and intermediate angular scales. 54 deep ACS/WFC images with an average galaxy number density of ~ 85 per square arcminute and a median galaxy i-band AB magnitude of 25.8 mag were analysed. We thoroughly investigated the effect of the HST/ACS point spread function (psf) on the shear measurement. To account for the low number of stars present in many fields at high galactic latitudes, we developed a new method for the psf anisotropy correction. In this talk we will discuss identified systematic effects and present the results of the Cosmic Shear analysis.