The Radiative Feedback of the First Cosmological Objects

In hierarchical models of structure formation, an early cosmic UV background (UVB) is produced by the small (T_vir < 10^4 K) halos that collapse before reionization. The UVB at energies below 13.6eV suppresses the formation of stars or black holes inside small halos, by photo-dissociating their only cooling agent, molecular H2. We self-consistently compute the buildup of the early UVB in Press-Schechter models, coupled with H2 photo-dissociation both in the intergalactic medium (IGM), and inside virialized halos. We find that the intergalactic H2 has a negligible effect on the UVB, both because its initial optical depth is small (tau<0.1), and because it is photo-dissociated at an early stage. If the UV sources in the first collapsed halos are stars, then their UV flux suppresses further star-formation inside small halos. This results in a pause in the buildup of the UVB, and reionization is delayed until larger halos (T_vir> 10^4 K) collapse. If the small halos host mini-quasars with hard spectra extending to approximately 1 keV, then their X-rays balance the effects of the UVB, the negative feedback does not occur, and reionization can be caused by the small halos.Comment: 16 pages, 16 figures included, uses emulateapj.sty. Submitted to Ap

Comparing Internal Migration between Countries: Who Collects What?

This paper derives from a program of research which aims to develop a robust framework for cross-national comparisons of internal migration. Stage one examined the obstacles to such comparisons and made proposals for a battery of 15 migration indicators covering four broad dimensions of population mobility which were then tested using British and Australian data. Wider implementation requires assembly of databases for countries around the world. Stage two takes the first steps towards this goal by establishing a worldwide inventory of contemporary practice with respect to collection of internal migration data, based on published sources and a comprehensive survey of national statistical agencies. Information collected includes the source and type of data, migration intervals and zonal system. This paper summarises the conclusions from Stage 1, reports results from the Stage 2 inventory and sets out proposals for a collaborative network to implement the cross-national indicators worldwid

The surveillance of racing cyclists in training: a Bourdieusian perspective

Research into the complexities of social identity construction and maintenance within racing cycling cultures has been neglected in sport sociology and studies of cycling group interactions are lacking. In this paper, preliminary findings from an on-going ethnographic research study on understanding the social world of a group (n= 73) of male racing cyclists aged between 17 and 56 years in the north east of England are discussed using the work of Pierre Bourdieu to interpret and explain what we term ‘lateral participatory social surveillance’ which pervades the subculture of racing cyclists in training. Extracts from field notes and photographs from group training rides are used to reveal how social order is governed via this type of surveillance. Social order occurs through a commitment to the shared values of the training group under study, and through acceptance of initiatory rituals where newcomers are tested through acts of what Bourdieu terms symbolic violence. These preliminary findings suggest that whilst the exclusiveness of this subcultural field provides much of the attraction to established group members, it may also represent a barrier to participation for newcomers and a better understanding may inform debates related to building broader, more inclusive cycling cultures

Metal Cooling in Simulations of Cosmic Structure Formation

The addition of metals to any gas can significantly alter its evolution by increasing the rate of radiative cooling. In star-forming environments, enhanced cooling can potentially lead to fragmentation and the formation of low-mass stars, where metal-free gas-clouds have been shown not to fragment. Adding metal cooling to numerical simulations has traditionally required a choice between speed and accuracy. We introduce a method that uses the sophisticated chemical network of the photoionization software, Cloudy, to include radiative cooling from a complete set of metals up to atomic number 30 (Zn) that can be used with large-scale three-dimensional hydrodynamic simulations. Our method is valid over an extremely large temperature range (10 K < T < 10^8 K), up to hydrogen number densities of 10^12 cm^-3. At this density, a sphere of 1 Msun has a radius of roughly 40 AU. We implement our method in the adaptive mesh refinement (AMR) hydrodynamic/N-body code, Enzo. Using cooling rates generated with this method, we study the physical conditions that led to the transition from Population III to Population II star formation. While C, O, Fe, and Si have been previously shown to make the strongest contribution to the cooling in low-metallicity gas, we find that up to 40% of the metal cooling comes from fine-structure emission by S, when solar abundance patterns are present. At metallicities, Z > 10^-4 Zsun, regions of density and temperature exist where gas is both thermally unstable and has a cooling time less than its dynamical time. We identify these doubly unstable regions as the most inducive to fragmentation. At high redshifts, the CMB inhibits efficient cooling at low temperatures and, thus, reduces the size of the doubly unstable regions, making fragmentation more difficult.Comment: 19 pages, 12 figures, significant revision, including new figure

Continuity of care for carers of people with severe mental illness: results of a longitudinal study

Introduction: Continuity of care has been demonstrated to be important for service users and carer groups have voiced major concerns over disruptions of care. We aimed to assess the experienced continuity of care in carers of patients with both psychotic and non-psychotic disorders and explore its association with carer characteristics and psychological well-being. Methods: Friends and relatives caring for two groups of service users in the care of community mental health teams (CMHTs), 69 with psychotic and 38 with non-psychotic disorders, were assessed annually at three and two time points, respectively. CONTINUES, a measure specifically designed to assess continuity of care for carers themselves, was utilized along with assessments of psychological well-being and caregiving. Results: One hundred and seven carers participated. They reported moderately low continuity of care. Only 22 had had a carer’s assessment and just under a third recorded psychological distress on the GHQ. For those caring for people with psychotic disorders, reported continuity was higher if the carer was male, employed, lived with the user and had had a carer’s assessment; for those caring for people with non-psychotic disorders, it was higher if the carer was from the service user’s immediate family, lived with them and had had a carer’s assessment. Conclusion: The vast majority of the carers had not had a carer’s assessment provided by the CMHT despite this being a clear national priority and being an intervention with obvious potential to increase carers’ reported low levels of continuity of care. Improving continuity of contact with carers may have an important part to play in the overall improvement of care in this patient group and deserves greater attention

A Minihalo Model for the Lyman Limit Absorption Systems at High Redshift

We propose that a large fraction of the QSO Lyman limit absorption systems (LLS) observed at high redshift (z > 3) originate from gas trapped in small objects, such as minihalos, that form prior to reionization. In the absence of a strong UV flux, the gas is predominantly neutral and may form clouds with HI column density NHI > 10^18 cm^-2. Due to their high densities and high HI column densities, these clouds are not destroyed by the onset of the UV background at a later time. Thus, if not disrupted by other processes, such as mergers into larger systems or `blow away' by supernovae, they will produce LLS. We show that the observed number density of LLS at high redshifts can well be reproduced by the survived `minihalos' in hierarchical clustering models such as the standard cold dark matter model. The number density of LLS in such a population increases with z even beyond the redshifts accessible to current observations and dies off quickly at z < 2. This population is distinct from other populations because the absorbing systems have small velocity widths and a close to primordial chemical composition. The existence of such a population requires that the reionization of the universe occurs late, at z < 20.Comment: 4 pages, 1 PostScript figure, accepted for publication in ApJ Letters, 8 Dec 199