Comparing Projections and Outcomes of IMF-Supported Programs

"Program numbers" from a sample of IMF-supported programs are studied as if they were forecasts, through statistical analyses of the relationship between projections and outcomes for growth, inflation, and three balance of payments concepts. Statistical bias is found only for projections of inflation and official reserves. Statistical efficiency can be rejected for all variables except growth, suggesting that some program projections were less accurate than they might have been. Nevertheless, most projections are found to have some predictive value. Since several findings are shown to be sample dependent, the full-sample results should be interpreted cautiously. Copyright 2002, International Monetary Fund

Highly-Ionized Oxygen Absorbers in the Intergalactic Medium

Recent ultraviolet and X-ray observations of intergalactic OVI and OVII absorption systems along lines of sight to bright quasars have opened a new window onto the ``warm-hot intergalactic medium'' (WHIM). These systems appear to provide a significant reservoir for baryons in the local universe, and comparison to cosmological simulations suggests that their abundance roughly matches theoretical predictions. Here we use analytic arguments to elucidate the physical properties of the absorbers and their role in structure formation. We first show that if the absorbers result from structure-formation shocks, the observed column densities naturally follow from postshock-cooling models, if we include fast-cooling shocks as well as those that cannot cool within a Hubble time. In this case, the known OVI absorbers should show stronger OVII absorption than expected from collisional-ionization equilibrium (and much more than expected for photoionized systems). We then argue that higher-temperature shocks will be spatially associated with more massive virialized objects even well outside the virial radius. Thus the different oxygen ions will trace different structures; OVII absorbers are the most common because that ion dominates over a wide temperature range (corresponding to a large range in halo mass). If each dark-matter halo is surrounded by a network of shocks with total cross section a few times the size of the virialized systems, then we can reproduce the observed number densities of absorbers with plausible parameters. A simple comparison with simulations shows that these assumptions are reasonable, although the actual distribution of shocked gas is too complex for analytic models to describe fully. Our models suggest that these absorbers cannot be explained as a single-temperature phase.Comment: 13 pages, 12 figures, in press at MNRAS (minor modifications, conclusions unchanged

Correlation analyses of deep galaxy samples

Methods are set out for the study of galaxy correlations using deep Schmidt plates and an automatic measuring procedure. The covariance function is defined and shown to be an objective measure of galaxy clustering. The work of previous authors on the angular covariance function and the conversion of this to the spatial covariance function is reviewed. The clustering of clusters is discussed. The form of the data used is described and the procedure for calculating the angular covariance function is set out. The form of the angular covariance function is found to be consistent with a power law of index –u.8, in agreement with previous estimates. In order to compare the amplitude of the angular covariance function for the very deep samples with that obtained from the earlier shallow samples, a cosmological generalization of Limber's formula is derived. In order to evaluate this, the distribution in distance of the galaxies in the sample is required. This is obtained via the distribution of angular diameters of the galaxies. A detailed model of the galaxy population is used to determine the expected distribution of angular diameters and the best fit to the observed counts gives the most likely parameters for the model. This in turn gives the distance distribution of the visible galaxies. It is found that the amplitude is lower than expected and it is judged that this is due to the use of very small areas which may not truly reflect the overall clustering pattern. Nonetheless, it is felt that the methods described will prove a valuable and powerful means of exploring the large scale distribution of galaxies