10,344 research outputs found
The human superior colliculus: Neither necessary, nor sufficient for consciousness?
Non-invasive neuroimaging in humans permits direct investigation of the potential role for mesodiencephalic structures in consciousness. Activity in the superior colliculus can be correlated with the contents of consciousness, but it can be also identified for stimuli of which the subject is unaware; and consciousness of some types of visual stimuli may not require the superior colliculus
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Phenotypic and functional differences of dendritic cells generated under different in vitro conditions
A response factor approach to modelling long-term thermal behaviour of energy piles
Predicting the long-term thermal performance of an energy pile ground heat exchanger system requires a computationally efficient model that captures the three-dimensional features and interacting thermal boundary conditions found in such systems. We present a model that employs a response factor approach to predicting long-term thermal behaviour based on a Dynamic Thermal Network representation. This allows fully three-dimensional geometric representation of the problem and application of boundary conditions at the ground and building sub-surfaces in addition to the pipes. The model has been validated using data from long-term monitoring of a pile in Belgium excited by a combination of heating and cooling demands in a range of environmental conditions. We further demonstrate application of the model to simulated annual building energy demands and the significance of treatment of the upper surface boundary conditions. The model is shown to be capable of representing building system and environmental conditions effectively and furthermore computationally efficient enough for routine design and simulation tasks
Polarization in the prompt emission of gamma-ray bursts and their afterglows
Synchrotron is considered the dominant emission mechanism in the production
of gamma-ray burst photons in the prompt as well as in the afterglow phase.
Polarization is a characteristic feature of synchrotron and its study can
reveal a wealth of information on the properties of the magnetic field and of
the energy distribution in gamma-ray burst jets. In this paper I will review
the theory and observations of gamma-ray bursts polarization. While the theory
is well established, observations have prove difficult to perform, due to the
weakness of the signal. The discriminating power of polarization observations,
however, cannot be overestimated.Comment: 16 pages, 9 figures, accepted for publication in the New Journal of
Physics focus issue on Gamma Ray Burst
Cosmological Origin of the Stellar Velocity Dispersions in Massive Early-Type Galaxies
We show that the observed upper bound on the line-of-sight velocity
dispersion of the stars in an early-type galaxy, sigma<400km/s, may have a
simple dynamical origin within the LCDM cosmological model, under two main
hypotheses. The first is that most of the stars now in the luminous parts of a
giant elliptical formed at redshift z>6. Subsequently, the stars behaved
dynamically just as an additional component of the dark matter. The second
hypothesis is that the mass distribution characteristic of a newly formed dark
matter halo forgets such details of the initial conditions as the stellar
"collisionless matter" that was added to the dense parts of earlier generations
of halos. We also assume that the stellar velocity dispersion does not evolve
much at z<6, because a massive host halo grows mainly by the addition of
material at large radii well away from the stellar core of the galaxy. These
assumptions lead to a predicted number density of ellipticals as a function of
stellar velocity dispersion that is in promising agreement with the Sloan
Digital Sky Survey data.Comment: ApJ, in press (2003); matches published versio
Seventy years of sex education in Health Education Journal: a critical review
This paper examines key debates and perspectives on sex education in Health Education Journal (HEJ), from the date of the journalâs first publication in March 1943 to the present day. Matters relating to sexuality and sexual health are revealed to be integral to HEJâs history. First published as Health and Empire (1921 â 1942), a key purpose of the journal since its inception has been to share information on venereal disease and its prevention within the UK and across the former British Empire. From 1943 to the present day, discussions on sex education in the newly-christened HEJ both reflect and respond to evolving socio-cultural attitudes towards sexuality in the UK. Changing definitions of sex education across the decades are examined, from the prevention of venereal disease and moral decline in war-time Britain in the 1940s, to a range of responses to sexual liberation in the 1960s and 1970s; from a focus on preventing sexually-transmitted infections, teenage pregnancy and HIV in the 1980s, to the provision of sexual health services alongside sex education in the 2000s. Over the past 70 years, a shift from prevention of pre-marital sexual activity to the management of its outcomes is apparent; however, while these changes over time are notable, perhaps the most striking findings of this review are the continuities in arguments for and against the discussion of sexual issues. After more than 70 years of debate, it would seem that there is little consensus concerning motivations for and the content of sex education
Seeing statistics at the upgraded 3.8m UK infrared telescope (UKIRT)
From 1991 until 1997, the 3.8m UK Infrared Telescope (UKIRT) underwent a
programme of upgrades aimed at improving its intrinsic optical performance.
This resulted in images with a FWHM of 0."17 at 2.2 um in September 1998. To
understand and maintain the improvements to the delivered image quality since
the completion of the upgrades programme, we have regularly monitored the
overall atmospheric seeing, as measured by radial displacements of subaperture
images (i.e. seeing-generated focus fluctuations), and the delivered image
diameters. The latter have been measured and recorded automatically since the
beginning of 2001 whenever the facility imager UFTI (UKIRT Fast Track Imager)
has been in use.
In this paper we report the results of these measurements. We investigate the
relation between the delivered image diameter and the RMS atmospheric seeing
(as measured by focus fluctuations, mentioned above). We find that the best
seeing occurs in the second half of the night, generally after 2am HST and that
the best seeing occurs in the summer between the months of July and September.
We also find that the relationship between Zrms and delivered image diameter is
uncertain. As a result Zrms frequently predicts a larger FWHM than that
measured in the images.
Finally, we show that there is no correlation between near-infrared seeing
measured at UKIRT and sub-mm seeing measured at the Caltech Submillimetre
Observatory (CSO).Comment: 10 pages to appear in the SPIE proceeding vol. 4484 on Observatory
Operations to Maximize Scientific Retur
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
Constraining Alternate Models of Black Holes: Type I X-ray Bursts on Accreting Fermion-Fermion and Boson-Fermion Stars
The existence of black holes remains open to doubt until other conceivable
options are excluded. With this motivation, we consider a model of a compact
star in which most of the mass consists of dark particles of some kind, and a
small fraction of the mass is in the form of ordinary nucleonic gas. The gas
does not interact with the dark matter other than via gravity, but collects at
the center as a separate fermionic fluid component. Depending on whether the
dark mass is made of fermions or bosons, the objects may be called
fermion-fermion stars or boson-fermion stars, respectively. For appropriate
choices of the mass of the dark matter particles, these objects are viable
models of black hole candidates in X-ray binaries. We consider models with a
dark mass of 10 solar masses and a range of gas mass from 10^{-6} to nearly one
solar mass, and analyse the bursting properties of the models when they accrete
gas. We show that all the models would experience thermonuclear Type I X-ray
bursts at appropriate mass accretion rates. Since no Type I bursts have been
reported from black hole candidates, the models are ruled out. The case for
identifying black hole candidates in X-ray binaries as true black holes is thus
strengthened.Comment: 29 pages, 7 figures, to appear in The Astrophysical Journa
Is Double Reionization Physically Plausible?
Recent observations of z~6 quasars and the cosmic microwave background imply
a complex history to cosmic reionization. Such a history requires some form of
feedback to extend reionization over a long time interval, but the nature of
the feedback and how rapidly it operates remain highly uncertain. Here we focus
on one aspect of this complexity: which physical processes can cause the global
ionized fraction to evolve non-monotonically with cosmic time? We consider a
range of mechanisms and conclude that double reionization is much less likely
than a long, but still monotonic, ionization history. We first examine how
galactic winds affect the transition from metal-free to normal star formation.
Because the transition is actually spatially inhomogeneous and temporally
extended, this mechanism cannot be responsible for double reionization given
plausible parameters for the winds. We next consider photoheating, which causes
the cosmological Jeans mass to increase in ionized regions and hence suppresses
galaxy formation there. In this case, double reionization requires that small
halos form stars efficiently, that the suppression from photoheating is strong
relative to current expectations, and that ionizing photons are preferentially
produced outside of previously ionized regions. Finally, we consider H_2
photodissociation, in which the buildup of a soft ultraviolet background
suppresses star formation in small halos. This can in principle cause the
ionized fraction to temporarily decrease, but only during the earliest stages
of reionization. Finally, we briefly consider the effects of some of these
feedback mechanisms on the topology of reionization.Comment: 13 pages, 5 figures, in press at ApJ (reorganized significantly but
major conclusions unchanged
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