Adrienne Leonard

The behaviour of dark matter associated with four bright cluster galaxies in the 10 kpc core of Abell 3827

Galaxy cluster Abell 3827 hosts the stellar remnants of four almost equally bright elliptical galaxies within a core of radius 10 kpc. Such corrugation of the stellar distribution is very rare, and suggests recent formation by several simultaneous mergers. We map the distribution of associated dark matter, using new Hubble Space Telescope imaging and Very Large Telescope/Multi-Unit Spectroscopic Explorer integral field spectroscopy of a gravitationally lensed system threaded through the cluster core. We find that each of the central galaxies retains a dark matter halo, but that (at least) one of these is spatially offset from its stars. The best-constrained offset is 1.62(-0.49)(+0.47) kpc, where the 68 per cent confidence limit includes both statistical error and systematic biases in mass modelling. Such offsets are not seen in field galaxies, but are predicted during the long infall to a cluster, if dark matter self-interactions generate an extra drag force. With such a small physical separation, it is difficult to definitively rule out astrophysical effects operating exclusively in dense cluster core environments - but if interpreted solely as evidence for self-interacting dark matter, this offset implies a cross-section sigma(DM)/(m) similar to (1.7 +/- 0.7) x 10(-4) cm(2) g(-1) x (t(infall)/10(9) yr)(-2), where t(infall) is the infall duration.

Galaxy Alignments: An Overview

The alignments between galaxies, their underlying matter structures, and the cosmic web constitute vital ingredients for a comprehensive understanding of gravity, the nature of matter, and structure formation in the Universe. We provide an overview on the state of the art in the study of these alignment processes and their observational signatures, aimed at a non-specialist audience. The development of the field over the past one hundred years is briefly reviewed. We also discuss the impact of galaxy alignments on measurements of weak gravitational lensing, and discuss avenues for making theoretical and observational progress over the coming decade.

Gravitational Shear, Flexion, and Strong Lensing in Abell 1689

We present a gravitational lensing analysis of the galaxy cluster Abell 1689, incorporating measurements of the weak shear, flexion, and strong lensing induced in background galaxies. This is the first time that a shapelet technique has been used to reconstruct the distribution of mass in this cluster and the first time that a flexion signal has been measured using cluster members as lenses. From weak shear measurements alone, we generate a nonparametric mass reconstruction, which shows significant substructure corresponding to groups of galaxies within the cluster. In addition, our galaxy-galaxy flexion signal demonstrates that the cluster galaxies can be well fit by a singular isothermal sphere model with a characteristic velocity dispersion of σv = (295 ± 40) km s^(-1). We identify a major, distinct dark matter clump, offset by 40 h^(-1) kpc from the central cluster members, which was not apparent from shear measurements alone. This secondary clump is present in a parametric mass reconstruction using flexion data alone, and its existence is suggested in a nonparametric reconstruction of the cluster using a combination of strong and weak lensing. As found in previous studies, the mass profile obtained by combining weak and strong lensing data shows a much steeper profile than that obtained from only weak lensing data.

The impact of AGN feedback and baryonic cooling on galaxy clusters as gravitational lenses

We investigate the impact of AGN feedback on the gravitational lensing properties of a sample of galaxy clusters with masses in the range 10 14 - 10 15 M , using state-ofthe-art simulations. Adopting a ray-tracing algorithm, we compute the cross-section of giant arcs from clusters simulated with dark-matter only physics (DM), dark matter plus gas with cooling and star formation (CSF), and dark matter plus gas with cooling, star formation and AGN feedback (CSFBH). Once AGN feedback is included, baryonic physics boosts the strong lensing cross-section by much less than previously estimated using clusters simulated with only cooling and star formation. For a cluster with a virial mass of 7:4 10 14 M , inclusion of baryonic physics without feedback can boost the cross-section by as much as a factor of 3, in agreement with previous studies, whereas once AGN feedback is included this maximal gure falls to a factor of 2 at most. Typically, clusters simulated with DM and CSFBH physics have similar crosssections for the production of giant arcs. We also investigate how baryonic physics aects the weak lensing properties of the simulated clusters by tting NFW proles to synthetic weak lensing data sets using a Markov Chain Monte Carlo approach, and by performing non-parametric mass reconstructions. Without the inclusion of AGN feedback, measured concentration parameters can be much larger than those obtained with AGN feedback, which are similar to the dark-matter only case.

Galaxy alignments: Observations and impact on cosmology

Galaxy shapes are not randomly oriented, rather they are statistically aligned in a way that can depend on formation environment, history and galaxy type. Studying the alignment of galaxies can therefore deliver important information about the physics of galaxy formation and evolution as well as the growth of structure in the Universe. In this review paper we summarise key measurements of galaxy alignments, divided by galaxy type, scale and environment. We also cover the statistics and formalism necessary to understand the observations in the literature. With the emergence of weak gravitational lensing as a precision probe of cosmology, galaxy alignments have taken on an added importance because they can mimic cosmic shear, the effect of gravitational lensing by large-scale structure on observed galaxy shapes. This makes galaxy alignments, commonly referred to as intrinsic alignments, an important systematic effect in weak lensing studies. We quantify the impact of intrinsic alignments on cosmic shear surveys and finish by reviewing practical mitigation techniques which attempt to remove contamination by intrinsic alignments.

Galaxy Alignments: Theory, Modelling & Simulations

The shapes of galaxies are not randomly oriented on the sky. During the galaxy formation and evolution process, environment has a strong influence, as tidal gravitational fields in the large-scale structure tend to align nearby galaxies. Additionally, events such as galaxy mergers affect the relative alignments of both the shapes and angular momenta of galaxies throughout their history. These “intrinsic galaxy alignments” are known to exist, but are still poorly understood. This review will offer a pedagogical introduction to the current theories that describe intrinsic galaxy alignments, including the apparent difference in intrinsic alignment between early- and late-type galaxies and the latest efforts to model them analytically. It will then describe the ongoing efforts to simulate intrinsic alignments using both N

Detecting mass substructure in galaxy clusters: an aperture mass statistic for gravitational flexion

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Cosmological constraints from the capture of non-Gaussianity in weak lensing data

-body and hydrodynamic simulations. Due to the relative youth of this field, there is still much to be done to understand intrinsic galaxy alignments and this review summarises the current state of the field, providing a solid basis for future work.

A Compressed Sensing Approach to 3D Weak Lensing

Gravitational flexion has been introduced as a technique by which one can map out and study substructure in clusters of galaxies. Previous analyses involving flexion have measured the individual galaxy–galaxy flexion signal, or used either parametric techniques or a Kaiser, Squires and Broadhurst (KSB)-type inversion to reconstruct the mass distribution in Abell 1689. In this paper, we present an aperture mass statistic for flexion, and apply it to the lensed images of background galaxies obtained by ray-tracing simulations through a simple analytic mass distribution and through a galaxy cluster from the Millennium Simulation. We show that this method is effective at detecting and accurately tracing structure within clusters of galaxies on subarcminute scales with high signal to noise even using a moderate background source number density and image resolution. In addition, the method provides much more information about both the overall shape and the small-scale structure of a cluster of galaxies than can be achieved through a weak lensing mass reconstruction using gravitational shear data. Lastly, we discuss how the zero-points of the aperture mass might be used to infer the masses of structures identified using this method.

GLIMPSE: accurate 3D weak lensing reconstructions using sparsity

Weak gravitational lensing has become a common tool to constrain the cosmological model. The majority of the methods to derive constraints on cosmological parameters use second-order statistics of the cosmic shear. Despite their success, second-order statistics are not optimal and degeneracies between some parameters remain. Tighter constraints can be obtained if second-order statistics are combined with a statistic that is efficient to capture non-Gaussianity. In this paper, we search for such a statistical tool and we show that there is additional information to be extracted from statistical analysis of the convergence maps beyond what can be obtained from statistical analysis of the shear field. For this purpose, we have carried out a large number of cosmological simulations along the σ8–Ωm degeneracy, and we have considered three different statistics commonly used for characterization of non-Gaussian features: skewness, kurtosis and peak count. To be able to investigate non-Gaussianity directly in the shear field, we have used the aperture mass definition of these three statistics for different scales. Then, the results have been compared with the results obtained with the same statistics estimated in the convergence maps at the same scales. First, we show that shear statistics give similar constraints to those given by convergence statistics, if the same scale is considered. In addition, we find that the peak count statistic is the best to capture non-Gaussianities in the weak lensing field and to break the σ8–Ωm degeneracy. We show that this statistical analysis should be conducted in the convergence maps: first, because there exist fast algorithms to compute the convergence map for different scales and secondly because it offers the opportunity to denoise the reconstructed convergence map, which improves the extraction of non-Gaussian features.