2,799 research outputs found
Evidence for a Gradual Decline in the Universal Rest-Frame UV Luminosity Density for z < 1
We have utilized various magnitude-limited samples drawn from an extremely
deep and highly complete spectroscopic redshift survey of galaxies observed in
seven colors in the Hawaii Survey Fields and the Hubble Deep Field to
investigate the evolution of the universal rest-frame ultraviolet luminosity
density from z = 1 to the present. The multi-color data (U', B, V, R, I, J,
HK') enable the sample selection to be made in the rest-frame ultraviolet for
the entire redshift range. Due to the large sample size and depth (U_{AB} =
24.75, B_{AB} = 24.75, I_{AB} = 23.5), we are able to accurately determine the
luminosity density to z = 1. We do not confirm the very steep evolution
reported by Lilly et al. (1996) but instead find a shallower slope,
approximately (1+z)^{1.5} for q0 = 0.5, which would imply that galaxy formation
is continuing smoothly to the present time rather than peaking at z = 1. Much
of the present formation is taking place in smaller galaxies. Detailed
comparisons with other recent determinations of the evolution are presented.Comment: 37 pages including 18 figures. Also available at
http://www.ifa.hawaii.edu/~acowie/uvlum.html To be published in the August,
1999 Astronomical Journal (accepted April 22, 1999
Galaxy Halo Masses from Galaxy-Galaxy Lensing
We present measurements of the extended dark halo profiles of bright early
type galaxies at redshifts 0.1 to 0.9 obtained via galaxy-galaxy lensing
analysis of images taken at the CFHT using the UH8K CCD mosaic camera. Six half
degree fields were observed for a total of 2 hours each in I and V, resulting
in catalogs containing ~20 000 galaxies per field. We used V-I color and I
magnitude to select bright early type galaxies as the lens galaxies, yielding a
sample of massive lenses with fairly well determined redshifts and absolute
magnitudes M ~ M_* \pm 1. We paired these with faint galaxies lying at angular
distances 20" to 60", corresponding to physical radii of 26 to 77 kpc (z = 0.1)
and 105 to 315 kpc (z = 0.9), and computed the mean tangential shear of the
faint galaxies. The shear falls off with radius roughly as expected for flat
rotation curve halos. The shear values were weighted in proportion to the
square root of the luminosity of the lens galaxy. Our results give a value for
the average mean rotation velocity of an L_* galaxy halo at r~50-200 kpc of v_*
= 238^{+27}_{-30} km per sec for a flat lambda (Omega_m0 = 0.3, Omega_l0 = 0.7)
cosmology (v_* = 269^{+34}_{-39} km per sec for Einstein-de Sitter), and with
little evidence for evolution with redshift. We compare to halo masses measured
by other groups/techniques. We find a mass-to-light ratio of ~121\pm28h(r/100
kpc) and these halos constitute Omega ~0.04 \pm 0.01(r/100 kpc) of closure
density. (abridged)Comment: Accepted for publication in ApJ (minor modifications) - 32 pages, 11
figs, 5 table
A Bayesian analysis of neutron spin echo data on polymer coated gold nanoparticles in aqueous solutions
We present a neutron spin echo study (NSE) of the nanosecond dynamics of
polyethylene glycol (PEG) functionalised nanosized gold particles dissolved in
DO at two temperatures and two different PEG molecular weights. The
analysis of the NSE data was performed by applying a Bayesian approach to the
description of time correlation function decays in terms of exponential terms,
recently proved to be theoretically rigorous. This approach, which addresses in
a direct way the fundamental issue of model choice in any dynamical analysis,
provides here a guide to the most statistically supported way to follow the
decay of the Intermediate Scattering Functions I(Q, t) by basing on statistical
grounds the choice of the number of terms required for the description of the
nanosecond dynamics of the studied systems. Then, the presented analysis avoids
from the start resorting to a pre-selected framework and can be considered as
model free. By comparing the results of PEG coated nanoparticles with those
obtained in PEG2000 solutions, we were able to disentangle the translational
diffusion of the nanoparticles from the internal dynamics of the polymer
grafted to them, and to show that the polymer corona relaxation follows a pure
exponential decay in agreement with the behavior predicted by coarse grained
molecular dynamics simulations and theoretical models. This methodology has one
further advantage: in the presence of a complex dynamical scenario I(Q,t) is
often described in terms of the Kohlrausch-Williams-Watts function that can
implicitly represent a distribution of relaxation times. By choosing to
describe the I(Q,t) as a sum of exponential functions and with the support of
the Bayesian approach, we can explicitly determine when a finer-structure
analysis of the dynamical complexity of the system exists according to the
available data without the risk of overparametrisation
Bullying victimization and psychosis : the inter-dependence and independence of risk trajectories
In the last several years a number of studies have noted an association between bullying and psychotic symptoms. Our aim here is to offer an overview on the topic, focusing especially on a developmental perspective. First, we highlight the latest studies to date regarding psychosis across the continuum and its relationship with bullying. In the second section, we summarize the three main explanatory models investigated: developmental, biological and cognitive models. In the discussion section we affirm that the sharing of numerous risk factors put people at risk of both psychosis and of being bullied, and bullying itself may further enhance the development of psychosis. Moreover, bullying is a risk factor for several mental disorders and is non-specific for psychosis, but there is some particularity in the trajectory involved between victimization and the onset of psychosis. In conclusion we recommend that the study of bullying in psychosis requires careful study of the developmental trajectories involved and research should now focus on how personal, social and biological factors interact between them
Peer mentorship and positive effects on student mentor and mentee retention and academic success
This study examined how the introduction of peer mentorship in an undergraduate health and social welfare programme at a large northern university affected student learning. Using an ethnographic case study approach, the study draws upon data collected from a small group of mentors and their mentees over a period of one academic year using interviews, reflective journals, assessment and course evaluation data.
Analysis of the data collected identified a number of key findings: peer mentorship improves assessment performance for both mentee and mentor; reduces stress and anxiety, enhances participation and engagement in the academic community, and adds value to student outcomes
P-Values as analytical tools in probabilistic forecast assessments.
Much has been written about ?quality? of probabilistic forecasts. Often, providers and users of such forecasts are unclear about what ?quality? entails, leading to confusion and misinformation. Here we try to provide some guidance and suggest a general approach to communicate aspects of probabilistic forecast quality related to discriminatory ability (DA) and skill (S). In our opinion, these two components of forecast quality should be considered independently. DA represents the additional knowledge about future states arising from some forecast system (FS) over and above the total variability of the prognostic variable while S quantifies changes in the agreement between observed and predicted values when using a specific FS instead of a FS based on ?climatology? only. The major concerns are: generally poor distinction between DA and S; inappropriate use of significance testing to quantify DA and use of DA and S measures that do not account for the series lengths and/or number of classes of the FS. To address all of these issues, we propose the use of p-values derived from non-parametric tests as direct measures of DA and S. We illustrate this approach by quantifying DA and S of the Southern Oscillation Index applied to forecasting rainfall across Australia
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