Weak Lensing Analysis of Galaxy Groups

Weak gravitational lensing is an important technique to study galaxy associations such as clusters and groups. Based on the coherent distortion imprint on the shape of background galaxies, this technique is capable of measuring the total mass content of the systems under investigation without making any assumption on their dynamical or evolutionary state. Mass estimates of galaxy groups are very relevant in the field of cosmology. In contrast to galaxy clusters, groups can be found in abundance at the redshift range where one can discriminate between cosmological models. However, the shape distortion of background galaxies is not only affected by the gravitational field of the main halo but by all the mass content along the line-of-sight. Given the low mass range of galaxy groups, the line-of-sight contamination becomes very important when compared to the weak lensing shear induced by these systems. The aim of this thesis is to study galaxy groups using weak gravitational lensing as a tool. We particularly focus on the contamination introduced by line-of-sight objects. Using COSMOS shear mock data we analyze the shear profile around 165 groups and investigate the level at which the neighboring groups can enhance or suppress the shear signal from the main halo. The mock data are based on CFHT and Subaru observations, which are used to obtain the photometric redshifts of galaxies in the field, and a realistic galaxy density, given by the weak lensing distortion analysis of the observed data. We further use the X-ray information taken from the COSMOS X-ray catalog of extended sources to trace the galaxy group distribution. Our group sample is characterized by a median mass of M200 = 3.1 × 1013 M⊙ and median redshift of z = 0.68. The expected gravitational shear field of these groups is calculated assuming that the halos are described by Navarro-Frenk-White (NFW) density profiles. The total shear is then computed by summing the shear over all the lenses. We conclude that, on average, the signal-to-noise for a detection of the main halo is affected by ≈ 15%× √ ngal/30 with respect to the signal-to-noise the same halo would have if it was isolated in the sky, where ngal is the observed galaxy density. Groups with neighbors that are close in projected distance (. 1′) are the most strongly affected, but halos located at larger angular distances also cause a measurable shear signal. These (angular) distant groups can be interpreted as uncorrelated large-scale structure. The average bias in the mass excess estimate of individual groups that is introduced by the external halos is zero with an rms value of ∼ 6 − 72%, depending on the aperture size used. One way to eliminate this bias is by stacking the density contrast profiles of several groups. The shear signal introduced by large-scale structure acts as an external source of noise. The averaged uncertainty introduced is σLSS γt ∼ 0.006 per component for an aperture size of θ ∼ 5′, which corresponds to ∼ 1.8% of the one-component intrinsic ellipticity value. This large-scale structure noise error becomes equal to intrinsic ellipticity noise if there are measurements for ∼ 3000 galaxies within a certain aperture, a number that is already achieved by current deep surveys such as COSMOS and, therefore, that should not be ignored.