The Minimum Description Length Principle and Model Selection in Spectropolarimetry

It is shown that the two-part Minimum Description Length Principle can be used to discriminate among different models that can explain a given observed dataset. The description length is chosen to be the sum of the lengths of the message needed to encode the model plus the message needed to encode the data when the model is applied to the dataset. It is verified that the proposed principle can efficiently distinguish the model that correctly fits the observations while avoiding over-fitting. The capabilities of this criterion are shown in two simple problems for the analysis of observed spectropolarimetric signals. The first is the de-noising of observations with the aid of the PCA technique. The second is the selection of the optimal number of parameters in LTE inversions. We propose this criterion as a quantitative approach for distinguising the most plausible model among a set of proposed models. This quantity is very easy to implement as an additional output on the existing inversion codes.Comment: Accepted for publication in the Astrophysical Journa

Can Virialization Shocks be Detected Around Galaxy Clusters Through the Sunyaev-Zel'dovich Effect?

In cosmological structure formation models, massive non-linear objects in the process of formation, such as galaxy clusters, are surrounded by large-scale shocks at or around the expected virial radius. Direct observational evidence for such virial shocks is currently lacking, but we show here that their presence can be inferred from future, high resolution, high-sensitivity observations of the Sunyaev-Zel'dovich (SZ) effect in galaxy clusters. We study the detectability of virial shocks in mock SZ maps, using simple models of cluster structure (gas density and temperature distributions) and noise (background and foreground galaxy clusters projected along the line of sight, as well as the cosmic microwave background anisotropies). We find that at an angular resolution of 2'' and sensitivity of 10 micro K, expected to be reached at ~ 100 GHz frequencies in a ~ 20 hr integration with the forthcoming ALMA instrument, virial shocks associated with massive M ~ 10^15 M_Sun clusters will stand out from the noise, and can be detected at high significance. More generally, our results imply that the projected SZ surface brightness profile in future, high-resolution experiments will provide sensitive constraints on the density profile of cluster gas.Comment: 15 pages, submitted to Ap

Temporal Variability of the X-ray Emission of the Crab Nebula Torus

We have analyzed five ROSAT HRI images of the Crab Nebula spanning the years 1991 to 1997 and have found significant changes in the emission structure of the X-ray torus surrounding the pulsar. Certain regions increase in brightness by about 20% over the six years, while others show decreases in surface brightness. The origin of these changes is unclear, but a possible explanation is that the bulk velocity of the synchrotron radiating electrons has decreased on the order of 20% as well.Comment: 15 pages plus 6 figures, figure 1 and figure 6 are in color, to appear in The Astrophysical Journal, Jan 1, 1999, Vol. 510, #

Prevalence of mental health disorders in adult minority ethnic populations in England: A systematic review

The aim of this project, commissioned by the Department of Health in England, was to undertake a systematic review of the survey literature that has estimated the prevalence of mental health disorders among adults from minority ethnic groups in England. Findings about the estimated prevalence of different mental health disorders, and about the extent and nature of the available survey evidence, are to be used to inform the design of a sampling strategy aimed at boosting the sample of people from minority ethnic groups in the Adult Psychiatric Morbidity Survey (APMS) due in 2021

Dark Matter: Introduction

This short review was prepared as an introduction to the Royal Society's 'Dark Matter' conference. It addresses the embarrassing fact that 95% of the universe is unaccounted for. Favoured dark matter candidates are axions or weakly-interacting particles that have survived from the very early universe, but more exotic options cannot be excluded. Experimental searches are being made for the 'dark' particles but we have indirect clues to their nature too. Comparisons of data (from, eg, gravitational lensing) with numerical simulations of galaxy formation can constrain (eg) the particle velocities and collision cross sections. The mean cosmic density of dark matter (plus baryons) is now pinned down to be only about 30% of the critical density However, other recent evidence -- microwave background anisotropies, complemented by data on distant supernovae -- reveals that our universe actually is 'flat', and that its dominant ingredient (about 70% of the total mass-energy) is something quite unexpected -- 'dark energy' pervading all space, with negative pressure. We now confront two mysteries: (i) Why does the universe have three quite distinct basic ingredients -- baryons, dark matter and dark energy -- in the proportions (roughly) 5%, 25% and 70%? (ii) What are the (almost certainly profound) implications of the 'dark energy' for fundamental physics?Comment: 10 pages, 1 figure. Late

Decoding neuronal ensembles in the human hippocampus

BACKGROUND: The hippocampus underpins our ability to navigate, to form and recollect memories, and to imagine future experiences. How activity across millions of hippocampal neurons supports these functions is a fundamental question in neuroscience, wherein the size, sparseness, and organization of the hippocampal neural code are debated. RESULTS: Here, by using multivariate pattern classification and high spatial resolution functional MRI, we decoded activity across the population of neurons in the human medial temporal lobe while participants navigated in a virtual reality environment. Remarkably, we could accurately predict the position of an individual within this environment solely from the pattern of activity in his hippocampus even when visual input and task were held constant. Moreover, we observed a dissociation between responses in the hippocampus and parahippocampal gyrus, suggesting that they play differing roles in navigation. CONCLUSIONS: These results show that highly abstracted representations of space are expressed in the human hippocampus. Furthermore, our findings have implications for understanding the hippocampal population code and suggest that, contrary to current consensus, neuronal ensembles representing place memories must be large and have an anisotropic structure

The Accretion of Lyman Alplha Clouds onto Gas-Rech Protogalaxies; A Scenario for the Formation of Globular Star Clusters

A satisfactory theory for the formation of globular star clusters (GCs) has long been elusive, perhaps because their true progenitors had not yet been guessed. In this paper I propose a causal relationship between the strongly decreasing densities of Lyman alpha (LyA) clouds at high redshift and the formation of GCs - namely that GCs were created by the accretion of LyA clouds onto protogalaxies. I describe a scenario which involves an inherently stable and orderly cycling of compression and cooling in the central cores of clouds during the extended period of dissipation in the outer regins of gas-rich proto galaxies, culminating in a burst of efficient star formation. I demonstrate that the comoving density of GCs is comparable to that of LyA clouds at high redshift, that the energetic requirements for compression to core GC densities can be met, and that the time-scale for cooling is within obvious limits imposed by dynamical stability. This dissipative process requires there to be a large column of dissipated gas about the attractor in order to form GCs. In addition, the energy requirements for compression requires attractor masses greater than that capable of sustaining circular velocities of ~40 km/s. If this scenario is supported by numerical simulations, then by implication, the GCs were formed at modest redshifts of z~1-3. This knowledge could help to break the degeneracy between lookback time and redshift. The model is consistent with a picture of hierarchical galaxy growth over time scales of many billions of years.Comment: 7 pages. Accepted, 10 June 1999 Astrophysical Journa

Asymmetric Supernovae, Pulsars, Magnetars, and Gamma-Ray Bursts

We outline the possible physical processes, associated timescales, and energetics that could lead to the production of pulsars, jets, asymmetric supernovae, and weak gamma-ray bursts in routine circumstances and to a magnetar and perhaps stronger gamma-ray burst in more extreme circumstances in the collapse of the bare core of a massive star. The production of a LeBlanc-Wilson MHD jet could provide an asymmetric supernova and result in a weak gamma-ray burst when the jet accelerates down the stellar density gradient of a hydrogen-poor photosphere. The matter-dominated jet would be formed promptly, but requires 5 to 10 s to reach the surface of the progenitor of a Type Ib/c supernova. During this time, the newly-born neutron star could contract, spin up, and wind up field lines or turn on an alpha-Omega dynamo. In addition, the light cylinder will contract from a radius large compared to the Alfven radius to a size comparable to that of the neutron star. This will disrupt the structure of any organized dipole field and promote the generation of ultrarelativistic MHD waves (UMHDW) at high density and Large Amplitude Electromagnetic Waves (LAEMW) at low density. The generation of the these waves would be delayed by the cooling time of the neutron star about 5 to 10 seconds, but the propagation time is short so the UMHDW could arrive at the surface at about the same time as the matter jet. In the density gradient of the star and the matter jet, the intense flux of UMHDW and LAEMW could drive shocks, generate pions by proton-proton collision, or create electron/positron pairs depending on the circumstances. The UMHDW and LAEMW could influence the dynamics of the explosion and might also tend to flow out the rotation axis to produce a collimated gamma-ray burst.Comment: 31 pages, LaTeX, revised for referee comments, accepted for ApJ, July 10 issu