Constraining Omega using weak gravitational lensing by clusters

The morphology of galaxy clusters reflects the epoch at which they formed and hence depends on the value of the mean cosmological density, Omega. Recent studies have shown that the distribution of dark matter in clusters can be mapped from analysis of the small distortions in the shapes of background galaxies induced by weak gravitational lensing in the cluster potential. We construct new statistics to quantify the morphology of clusters which are insensitive to limitations in the mass reconstruction procedure. By simulating weak gravitational lensing in artificial clusters grown in numerical simulations of the formation of clusters in three different cosmologies, we obtain distributions of a quadrupole statistic which measures global deviations from spherical symmetry in a cluster. These distributions are very sensitive to the value of Omega_0 and, as a result, lensing observations of a small number of clusters should be sufficient to place broad constraints on Omega_{0} and certainly to distinguish between the extreme values of 0.2 and 1.Comment: Submitted to MNRAS. Compressed postscript also available at ftp://star-ftp.dur.ac.uk/pub/preprints/wcf2.ps.g

Is academic writing the most appropriate complement to art students' practice?

This thesis examines the relationship between art students' academic work and arts practice focusing on: the function of academic writing; the space for the author's identity; and, whether dichotomies in the production of academic text and development of creative visual work preclude academic writing from being the most effective complementary mode of communication alongside visual arts practice. Using semi-structured interviews as a primary data source, I explored the perspectives of nine undergraduate student volunteers. I used data from the interviews and multiple data sources emerging from them, such as examples of written and visual arts work, informal and email exchanges, to build qualitative case studies addressing three research questions: What is the function of academic writing in a higher education art and design context?; In what kinds of ways does the students' prior educational experience impact on their ability to engage with academic writing in their course?; and, Does academic writing enable art and design students to link theory and practice? During the study another question emerged: Are other means of evidencing art students' engagement with theory more appropriate and effective than academic writing? Here I looked at alternative models of assessment currently offered at five UK art and design institutions/faculties to find evidence of their efficacy in the context of my research questions. Findings suggest academic writing is often ineffective in engaging students with contextual studies and theory. They tend not to link this to creative practice and resent the time spent away from the art studio. Tensions in the power relations between students and tutors often force students to write what is expected, compromising their identities within, and ownership of, the work. Non-traditional students (those defined as: the first generation of their family to participate in higher education; living in areas of social deprivation, or belonging to lower socio-economic groups) sometimes lack the cultural capital and prior experience to engage fully in academic writing. They feel there is no place for their identity in academic discourse. This is fundamentally opposed to art and design practice, where identity is often central to the development of creative work. Different forms of writing may be more appropriate complementary modes of communication within art and design than traditional academic writing, and in particular those which enable the writer to explore, celebrate and situate their identities in relation to wider historical and contextual influences in art and design. In addition to this, the potential and significance of multi modality should be considered in the 21 st century, where images, film, typography and sensory information can enrich the communication of ideas and concepts, as well as being a familiar part of our everyday lives. These offer opportunities for alternative modes of assessment, which are more creative and equitable, particularly in art and design, but potentially in other disciplines

Gravitational lensing and galaxy clusters

Kaiser & Squires have proposed a technique for mapping the dark matter in galaxy clusters using the coherent weak distortion of background galaxy images caused by gravitational lensing. We investigate the effectiveness of this technique under controlled conditions by creating simulated CCD frames containing galaxies lensed by a model cluster, measuring the resulting galaxy shapes, and comparing the reconstructed mass distribution with the original. Typically, the reconstructed surface density is diminished in magnitude when compared to the original. The main cause of this reduced signal is the blurring of galaxy images by atmospheric seeing, but the overall factor by which the reconstructed surface density is reduced depends also on the signal-to-noise ratio in the CCD frame and on both the sizes of galaxy images and the magnitude limit of the sample that is analysed. We propose a method for estimating a multiplicative compensation factor, f, directly from a CCD frame which can then be used to correct the surface density estimates given by the Kaiser & Squires formalism. We test our technique using a lensing cluster drawn from a cosmological N-body simulation with a variety of realistic background galaxy populations and observing conditions. We conclude that weak lensing observations when calibrated using this method yield not only accurate maps of the cluster morphology but also quantitative estimates of the cluster mass distribution. We then show that weak lensing simulations by rich clusters can be used to constrain the likely range of values of the cosmological constant Ω(_0). Again employing the Kaiser & Squires mass density estimator, we model the lensing induced by a number of simulated clusters from three different cosmologies. We introduce new statistics which are independent of any uncertainties in the surface density mentioned above. We conclude that lensing observations of a small number of clusters should be sufficient to place broad constraints on Ω(_0) and certainly distinguish between the extreme values of 0.2 and 1.We also present deep two-colour photometry of two rich clusters at z = 0.18; A1689 and A665. We use the data to construct number counts as a function of magnitude. To the magnitude depth we were able to probe, we conclude that there is no strong evidence for a steep faint end slope to the galaxy luminosity functioning moderate-redshift clusters

Implementation of an attention training program with children with attention deficit hyperactivity disorder in South Africa

Attention Deficit/Hyperactivity Disorder (ADHD) is a group of behavioural symptoms that include inattentiveness, hyperactivity and impulsiveness and tends to be the most commonly diagnosed childhood behavioural disorder. The aim of this study was to determine the feasibility of the Pay Attention! Intervention with a small group of children diagnosed with ADHD as compared to matched controls. The intervention focused on sustained, selective, alternating and divided attention. After a baseline evaluation, five children aged 6 to 8 years were assigned to receive bi-weekly Pay Attention! sessions for 12 weeks and five matched controls were assigned to a Test-only group. Participants completed an outcome evaluation approximately 12 weeks after their baseline evaluation, both of which included neuropsychological and behavioural (both parent and teacher) assessments. Results show that the intervention is feasible to administer and acceptable to participants. Although no significant treatment effects were found on the neuropsychological outcomes and for the teacher ratings of ADHD, there were however significant effects found on parent ratings of ADHD symptoms on the Child Behaviour Checklist and the Vineland Adaptive Behaviour Scale-II. These preliminary findings add to the growing body of literature on attention training interventions for children with ADHD. However, a randomised controlled trial is warranted to further investigate the specific use of the Pay Attention! intervention with this population in the South African context