Accretion Disc Particle Accretion in Major Merger Simulations

A recent approach to simulating localized feedback from active galactic nuclei by Power et al. (2011) uses an accretion disc particle to represent both the black hole and its accretion disc. We have extrapolated and adapted this approach to simulations of Milky Way-sized galaxy mergers containing black holes and explored the impact of the various parameters in this model as well as its resolution dependence. The two key parameters in the model are an effective accretion radius, which determines the radius within which gas particles are added to the accretion disc, and a viscous time-scale which determines how long it takes for material in the accretion disc to accrete on to the black hole itself. We find that there is a limited range of permitted accretion radii and viscous time-scales, with unphysical results produced outside this range. For permitted model parameters, the nuclear regions of simulations with the same resolution follow similar evolutionary paths, producing final black hole masses that are consistent within a factor of two. When comparing the resolution dependence of the model, there is a trend towards higher resolution producing slightly lower mass black holes, but values for the two resolutions studied again agree within a factor of two. We also compare these results to two other AGN feedback algorithms found in the literature. While the evolution of the systems vary, most notably the intermediate total black hole mass, the final black hole masses differ by less than a factor of five amongst all of our models, and the remnants exhibit similar structural parameters. The implication of this accretion model is that, unlike most accretion algorithms, a decoupling of the accretion rate on to the black hole and the local gas properties is permitted and obtained; this allows for black hole growth even after feedback has prevented additional accretion events on to the disc.Comment: 17 pages, accepted to MNRA

Formal-informal economy linkages and unemployment in South Africa:

South Africa's high involuntary unemployment and small informal sector are attributed to an underperforming formal sector and barriers to entry in the informal sector. This paper examines the economywide linkages between the formal and informal economies while accounting for different types of informal activities. A multiregion empirically calibrated general equilibrium model is developed capturing both product and labor markets. Three policy options are considered. First, results indicate that trade liberalization reduces national employment. At the same time, it increases formal employment, hurts informal producers, and favors informal traders, who benefit from lower import prices. Past liberalization may, therefore, partly explain South Africa's small informal sector and its concentration among traders rather than producers. Second, wage subsidies on low-skilled formal workers increase national employment but hurt informal producers by heightening competition in domestic product markets. This suggests that it is insufficient to examine unemployment policies by focusing only on labor markets. Third, unconditional cash transfers stimulate demand for informally produced products, thereby raising informal employment without undermining formal producers. The transfer does, however, place a large fiscal burden on the state and is less effective at reducing national unemployment than a wage subsidy. Overall, these findings underline the importance of distinguishing between the formal and informal sector implications of socioeconomic policies.informal economy, involuntary unemployment, formal economy, labor markets, trade liberalization, national employment, Cash transfers, wage subsidy, Computable general equilibrium (CGE) modeling, Development strategies,

Milnor Invariants and Twisted Whitney Towers

This paper describes the relationship between the first non-vanishing Milnor invariants of a classical link and the intersection invariant of a twisted Whitney tower. This is a certain 2-complex in the 4-ball, built from immersed disks bounded by the given link in the 3-sphere together with finitely many `layers' of Whitney disks. The intersection invariant is a higher-order generalization of the intersection number between two immersed disks in the 4-ball, well known to give the linking number of the link on the boundary, which measures intersections among the Whitney disks and the disks bounding the given link, together with information that measures the twists (framing obstructions) of the Whitney disks. This interpretation of Milnor invariants as higher-order intersection invariants plays a key role in the classifications of both the framed and twisted Whitney tower filtrations on link concordance (as sketched in this paper). Here we show how to realize the higher-order Arf invariants, which also play a role in the classifications, and derive new geometric characterizations of links with vanishing Milnor invariants of length less than or equal to 2k.Comment: Typo corrected in statement of Theorem 16; no change to proof needed. Otherwise, this revision conforms with the version published in the Journal of Topology. 36 pages, 23 figure

Representing the family: how does the state 'think family'?

Over the last decade the family and family-centred policies and practices have received increasing attention within the public service agenda, culminating in the emphatic instruction to ‘think family’ individually, collectively and institutionally. This has occurred at a time when the sociology of the family has increasingly emphasised the difficulties of thinking family in a coherent way. In this article we explore this agenda through an examination of the representational tools with which public service professionals and managers have been recently equipped. We conclude by questioning the adequacy of these tools for effectively representing family relations

Partnership as conversation: why partnerships are condemned to talk and what they need to talk about

Participants and observers regularly complain that multi-agency partnerships are “talking shops,” engaged in constant discussion which gets in the way of “doing” the work of partnership. In this paper we engage with and criticize this characterization. Drawing on ideas from the Cultural Theory of Mary Douglas, we argue that true multi-agency partnerships are structurally condemned to talk. Instead of criticizing this talk and contrasting it with “doing” we should see it as a critical part of the doing of partnership. We should therefore concentrate on organizing and structuring partnership talk in order to move things forward rather than trying to minimize it. In the second half of the paper we therefore put forward a proposal for how partnership talk should be organized into five “conversations” concerning the principles, policies, processes, practices and politics of partnership. While we can make no predictions for the outcome of these conversations in any given case, we can, we believe, establish some necessary preconditions for effective interaction. We illustrate our arguments drawing on a range of empirical work in education and wider public services reform

If you build it, will they come? How researchers perceive and use web 2.0

Over the past 15 years, the web has transformed the way we seek and use information. In the last 5 years in particular a set of innovative techniques – collectively termed ‘web 2.0’ – have enabled people to become producers as well as consumers of information. It has been suggested that these relatively easy-to-use tools, and the behaviours which underpin their use, have enormous potential for scholarly researchers, enabling them to communicate their research and its findings more rapidly, broadly and effectively than ever before. This report is based on a study commissioned by the Research Information Network to investigate whether such aspirations are being realised. It seeks to improve our currently limited understanding of whether, and if so how, researchers are making use of various web 2.0 tools in the course of their work, the factors that encourage or inhibit adoption, and researchers’ attitudes towards web 2.0 and other forms of communication. Context: How researchers communicate their work and their findings varies in different subjects or disciplines, and in different institutional settings. Such differences have a strong influence on how researchers approach the adoption – or not – of new information and communications technologies. It is also important to stress that ‘web 2.0’ encompasses a wide range of interactions between technologies and social practices which allow web users to generate, repurpose and share content with each other. We focus in this study on a range of generic tools – wikis, blogs and some social networking systems – as well as those designed specifically by and for people within the scholarly community. Method: Our study was designed not only to capture current attitudes and patterns of adoption but also to identify researchers’ needs and aspirations, and problems that they encounter. We began with an online survey, which collected information about researchers’ information gathering and dissemination habits and their attitudes towards web 2.0. This was followed by in-depth, semi-structured interviews with a stratified sample of survey respondents to explore in more depth their experience of web 2.0, including perceived barriers as well as drivers to adoption. Finally, we undertook five case studies of web 2.0 services to investigate their development and adoption across different communities and business models. Key findings: Our study indicates that a majority of researchers are making at least occasional use of one or more web 2.0 tools or services for purposes related to their research: for communicating their work; for developing and sustaining networks and collaborations; or for finding out about what others are doing. But frequent or intensive use is rare, and some researchers regard blogs, wikis and other novel forms of communication as a waste of time or even dangerous. In deciding if they will make web 2.0 tools and services part of their everyday practice, the key questions for researchers are the benefits they may secure from doing so, and how it fits with their use of established services. Researchers who use web 2.0 tools and services do not see them as comparable to or substitutes for other channels and means of communication, but as having their own distinctive role for specific purposes and at particular stages of research. And frequent use of one kind of tool does not imply frequent use of others as well