33,774 research outputs found
Topology and Sizes of HII Regions during Cosmic Reionization
We use the results of large-scale simulations of reionization to explore methods for characterizing the topology and sizes of HII regions during reionization. We use four independent methods for characterizing the sizes of ionized regions. Three of them give us a full size distribution: the friends-of-friends (FOF) method, the spherical average method (SPA) and the power spectrum (PS) of the ionized fraction. These latter three methods are complementary: While the FOF method captures the size distribution of the small scale H~II regions, which contribute only a small amount to the total ionization fraction, the spherical average method provides a smoothed measure for the average size of the H~II regions constituting the main contribution to the ionized fraction, and the power spectrum does the same while retaining more details on the size distribution. Our fourth method for characterizing the sizes of the H II regions is the average size which results if we divide the total volume of the H II regions by their total surface area, (i.e. 3V/A), computed in terms of the ratio of the corresponding Minkowski functionals of the ionized fraction field. To characterize the topology of the ionized regions, we calculate the evolution of the Euler Characteristic. We find that the evolution of the topology during the first half of reionization is consistent with inside-out reionization of a Gaussian density field. We use these techniques to investigate the dependence of size and topology on some basic source properties, such as the halo mass-to-light ratio, susceptibility of haloes to negative feedback from reionization, and the minimum halo mass for sources to form. We find that suppression of ionizing sources within ionized regions slows the growth of H~II regions, and also changes their size distribution. Additionally, the topology of simulations including suppression is more complex. (abridged
Viscous heating effects in fluids with temperature-dependent viscosity: triggering of secondary flows
Viscous heating can play an important role in the dynamics of fluids with
strongly temperature-dependent viscosities because of the coupling between the
energy and momentum equations. The heat generated by viscous friction produces
a local temperature increase near the tube walls with a consequent decrease of
the viscosity and a strong stratification in the viscosity profile. The problem
of viscous heating in fluids was investigated and reviewed by Costa & Macedonio
(2003) because of its important implications in the study of magma flows.
Because of the strong coupling between viscosity and temperature, the
temperature rise due to the viscous heating may trigger instabilities in the
velocity field, which cannot be predicted by a simple isothermal Newtonian
model. When viscous heating produces a pronounced peak in the temperature
profile near the walls, a triggering of instabilities and a transition to
secondary flows can occur because of the stratification in the viscosity
profile. In this paper we focus on the thermal and mechanical effects caused by
viscous heating. We will present the linear stability equations and we will
show, as in certain regimes, these effects can trigger and sustain a particular
class of secondary rotational flows which appear organised in coherent
structures similar to roller vortices. This phenomenon can play a very
important role in the dynamics of magma flows in conduits and lava flows in
channels and, to our knowledge, it is the first time that it has been
investigated by a direct numerical simulation.Comment: 18 pages manuscript, 10 figures, to be published in Journal of Fluid
Mechanics (2005
A methodology for small scale rural land use mapping in semi-arid developing countries using orbital imagery. Part 4: Review of land use surveys using orbital imagery outside of the USA
The author has identified the following significant results. Outside the U.S., various attempts were made to investigate the feasibility of utilizing orbital MSS imagery in the production of small scale land use maps. Overall, these studies are not as elaborate or extensive in their scope as the U.S. ones, and generally the non-U.S. investigators have employed nonsophisticated and less expensive techniques. A representative range of studies is presented to demonstrate the approaches and trends dealing with reprocessing, interpretation, classification, sampling, and ground truth procedures
Investigating intra-host and intra-herd sequence diversity of foot-and-mouth disease virus
Due to the poor-fidelity of the enzymes involved in RNA genome replication, foot-and-mouth disease (FMD) virus samples comprise of unique polymorphic populations. In this study, deep sequencing was utilised to characterise the diversity of FMD virus (FMDV) populations in 6 infected cattle present on a single farm during the series of outbreaks in the UK in 2007. A novel RT–PCR method was developed to amplify a 7.6 kb nucleotide fragment encompassing the polyprotein coding region of the FMDV genome. Illumina sequencing of each sample identified the fine polymorphic structures at each nucleotide position, from consensus level changes to variants present at a 0.24% frequency. These data were used to investigate population dynamics of FMDV at both herd and host levels, evaluate the impact of host on the viral swarm structure and to identify transmission links with viruses recovered from other farms in the same series of outbreaks. In 7 samples, from 6 different animals, a total of 5 consensus level variants were identified, in addition to 104 sub-consensus variants of which 22 were shared between 2 or more animals. Further analysis revealed differences in swarm structures from samples derived from the same animal suggesting the presence of distinct viral populations evolving independently at different lesion sites within the same infected animal
Dwelling Quietly in the Rich Club: Brain Network Determinants of Slow Cortical Fluctuations
For more than a century, cerebral cartography has been driven by
investigations of structural and morphological properties of the brain across
spatial scales and the temporal/functional phenomena that emerge from these
underlying features. The next era of brain mapping will be driven by studies
that consider both of these components of brain organization simultaneously --
elucidating their interactions and dependencies. Using this guiding principle,
we explored the origin of slowly fluctuating patterns of synchronization within
the topological core of brain regions known as the rich club, implicated in the
regulation of mood and introspection. We find that a constellation of densely
interconnected regions that constitute the rich club (including the anterior
insula, amygdala, and precuneus) play a central role in promoting a stable,
dynamical core of spontaneous activity in the primate cortex. The slow time
scales are well matched to the regulation of internal visceral states,
corresponding to the somatic correlates of mood and anxiety. In contrast, the
topology of the surrounding "feeder" cortical regions show unstable, rapidly
fluctuating dynamics likely crucial for fast perceptual processes. We discuss
these findings in relation to psychiatric disorders and the future of
connectomics.Comment: 35 pages, 6 figure
DNA viewed as an out-of-equilibrium structure
The complexity of the primary structure of human DNA is explored using
methods from nonequilibrium statistical mechanics, dynamical systems theory and
information theory. The use of chi-square tests shows that DNA cannot be
described as a low order Markov chain of order up to . Although detailed
balance seems to hold at the level of purine-pyrimidine notation it fails when
all four basepairs are considered, suggesting spatial asymmetry and
irreversibility. Furthermore, the block entropy does not increase linearly with
the block size, reflecting the long range nature of the correlations in the
human genomic sequences. To probe locally the spatial structure of the chain we
study the exit distances from a specific symbol, the distribution of recurrence
distances and the Hurst exponent, all of which show power law tails and long
range characteristics. These results suggest that human DNA can be viewed as a
non-equilibrium structure maintained in its state through interactions with a
constantly changing environment. Based solely on the exit distance distribution
accounting for the nonequilibrium statistics and using the Monte Carlo
rejection sampling method we construct a model DNA sequence. This method allows
to keep all long range and short range statistical characteristics of the
original sequence. The model sequence presents the same characteristic
exponents as the natural DNA but fails to capture point-to-point details
AGN outflows trigger starbursts in gas-rich galaxies
Recent well resolved numerical simulations of AGN feedback have shown that
its effects on the host galaxy may be not only negative but also positive. In
the late gas poor phase, AGN feedback blows the gas away and terminates star
formation. However, in the gas-rich phase(s), AGN outflows trigger star
formation by over-compressing cold dense gas and thus provide positive feedback
on their hosts. In this paper we study this AGN-triggered starburst effect. We
show that star formation rate in the burst increases until the star formation
feedback counteracts locally the AGN outflow compression. Globally, this
predicts a strong nearly linear statistical correlation between the AGN and
starburst bolometric luminosities in disc galaxies, L_* \propto L_{AGN}^{5/6}.
The correlation is statistical only because AGN activity may fluctuate on short
time scales (as short as tens of years), and because AGN may turn off but its
effects on the host may continue to last until the AGN-driven outflow leaves
the host, which may be up to 10 times longer than the duration of the AGN
activity. The coefficient in front of this relation depends on the clumpiness
and morphology of the cold gas in the galaxy. A "maximum starburst" takes place
in am azimuthally uniform gas disc, for which we derive an upper limit of L_*
\sim 50 times larger than L_{AGN} for typical quasars. For more clumpy and/or
compact cold gas distributions, the starburst luminosity decreases. We also
suggest that similar AGN-triggerred starbursts are possible in hosts of all
geometries, including during galaxy mergers, provided the AGN is activated.
Finally, we note that due to the short duration of the AGN activity phase the
accelerating influence of AGN on starbursts may be much more common than
observations of simultaneous AGN and starbursts would suggest.Comment: 14 pages, 6 figures. Accepted for publication in MNRA
Sense of agency, associative learning, and schizotypy
Despite the fact that the role of learning is recognised in empirical and theoretical work on sense of agency (SoA), the nature of this learning has, rather surprisingly, received little attention. In the present study we consider the contribution of associative mechanisms to SoA. SoA can be measured quantitatively as a temporal linkage between voluntary actions and their external effects. Using an outcome blocking procedure, it was shown that training action-outcome associations under conditions of increased surprise augmented this temporal linkage. Moreover, these effects of surprise were correlated with schizotypy scores, suggesting that individual differences in higher level experiences are related to associative learning and to its impact on SoA. These results are discussed in terms of models of SoA, and our understanding of disrupted SoA in certain disorders
A novel method for subjective picture quality assessment and further studies of HDTV formats
This is the author's accepted manuscript. The final published article is available from the link below. Copyright @ IEEE 2008.This paper proposes a novel method for the assessment of picture quality, called triple stimulus continuous evaluation scale (TSCES), to allow the direct comparison of different HDTV formats. The method uses an upper picture quality anchor and a lower picture quality anchor with defined impairments. The HDTV format under test is evaluated in a subjective comparison with the upper and lower anchors. The method utilizes three displays in a particular vertical arrangement. In an initial series of tests with the novel method, the HDTV formats 1080p/50,1080i/25, and 720p/50 were compared at various bit-rates and with seven different content types on three identical 1920 times 1080 pixel displays. It was found that the new method provided stable and consistent results. The method was tested with 1080p/50,1080i/25, and 720p/50 HDTV images that had been coded with H.264/AVC High profile. The result of the assessment was that the progressive HDTV formats found higher appreciation by the assessors than the interlaced HDTV format. A system chain proposal is given for future media production and delivery to take advantage of this outcome. Recommendations for future research conclude the paper
Towards a standardized characterization of the potentially migrating silver European eel (Anguilla anguilla L.)
We defined a standardized method for discriminating candidate silver eels
that may undergo catadromous migration in the following season from the sedentary
fraction of a population. A combination of two qualitative criteria (state of differentiation
of the lateral line and colour contrast) and one quantitative criterion (Ocular Index
OI) was used to determine the development toward silvering. In the non-migratory
phase, we found a gradient of the three criteria between yellow (0 criterion), presilver
(1 to 2 criteria) and silver (3 criteria) eels. In the migrant phase, silver eels had ended
their metamorphosis process and were characterized at the same time by the presence
of the 3 silvering criteria. A mark-recapture survey using PIT-tags provided evidence
that only identified silver eels (3 silvering criteria present) in the catchment actually
emigrated the following season. Moreover, the use of a single criterion of silvering
among the three generated large variation in the estimated proportion of candidates for
emigration which varied between –22% and +63 %. Such a result confirmed that a
multicriteria approach is needed to characterize in a standard way the potentially migrating
silver eel
- …