382 research outputs found
Can solubilizing excipients be used with the rat BATA model to assess the taste of poorly water-soluble drugs?
Measuring and Modelling the Redshift Evolution of Clustering: the Hubble Deep Field North
(abridged) The evolution of galaxy clustering from z=0 to z=4.5 is analyzed
using the angular correlation function and the photometric redshift
distribution of galaxies brighter than I_{AB}\le 28.5 in the HDF North. The
reliability of the photometric redshift estimates is discussed on the basis of
the available spectroscopic redshifts, comparing different codes and
investigating the effects of photometric errors. The redshift bins in which the
clustering properties are measured are then optimized to take into account the
uncertainties of the photometric redshifts. The results show that the comoving
correlation length has a small decrease in the range 0<z<1 followed by an
increase at higher z. We compare these results with the theoretical predictions
of a variety of cosmological models belonging to the general class of CDM. The
comparison with the expected mass clustering evolution indicates that the
observed high-redshift galaxies are biased tracers of the dark matter with an
effective bias b strongly increasing with redshift. Assuming an Einstein-de
Sitter universe, we obtain b\simeq 2 at z=2 and b\simeq 5 at z=4. A comparison
of the clustering amplitudes that we measured at z=3 with those reported for
LBG suggests that the clustering depends on the abundance of the objects: more
abundant objects are less clustered, as expected in the paradigm of
hierarchical galaxy formation. The strong clustering and high bias measured at
z=3 are consistent with the expected density of massive haloes predicted for
the various cosmologies here considered. At z=4, the strong clustering observed
in the HDF requires a significant fraction of massive haloes to be already
formed by that epoch. This feature could be a discriminant test for the
cosmological parameters if confirmed by future observations.Comment: 23 pages, Latex using MN style, figures enclosed. Version accepted
for publication in MNRA
Quasars Probing Quasars II: The Anisotropic Clustering of Optically Thick Absorbers around Quasars
With close pairs of quasars at different redshifts, a background quasar
sightline can be used to study a foreground quasar's environment in absorption.
We used a sample of 17 Lyman limit systems with column density N_HI > 10^19
cm^-2 selected from 149 projected quasar pair sightlines, to investigate the
clustering pattern of optically thick absorbers around luminous quasars at z ~
2.5. Specifically, we measured the quasar-absorber correlation function in the
transverse direction, and found a comoving correlation length of
r_0=9.2_{+1.5}_{-1.7} Mpc/h (comoving) assuming a power law correlation
function, with gamma=1.6. Applying this transverse clustering strength to the
line-of-sight, would predict that ~ 15-50% of all quasars should show a N_HI >
10^19 cm^-2 absorber within a velocity window of v < 3000 km/s. This
overpredicts the number of absorbers along the line-of-sight by a large factor,
providing compelling evidence that the clustering pattern of optically thick
absorbers around quasars is highly anisotropic. The most plausible
explanationfor the anisotropy is that the transverse direction is less likely
to be illuminated by ionizing photons than the line-of-sight, and that
absorbers along the line-of-sight are being photoevaporated. A simple model for
the photoevaporation of absorbers subject to the ionizing flux of a quasar is
presented, and it is shown that absorbers with volume densities n_H < 0.1 cm^-3
will be photoevaporated if they lie within ~ 1 Mpc (proper) of a luminous
quasar. Using this simple model, we illustrate how comparisons of the
transverse and line-of-sight clustering around quasars can ultimately be used
to constrain the distribution of gas in optically thick absorption line
systems.Comment: 14 pages of emulateapj, 7 figures, submitted to Ap
Interaction of perceptual grouping and crossmodal temporal capture in tactile apparent-motion
Previous studies have shown that in tasks requiring participants to report the direction of apparent motion, task-irrelevant mono-beeps can "capture'' visual motion perception when the beeps occur temporally close to the visual stimuli. However, the contributions of the relative timing of multimodal events and the event structure, modulating uni- and/or crossmodal perceptual grouping, remain unclear. To examine this question and extend the investigation to the tactile modality, the current experiments presented tactile two-tap apparent-motion streams, with an SOA of 400 ms between successive, left-/right-hand middle-finger taps, accompanied by task-irrelevant, non-spatial auditory stimuli. The streams were shown for 90 seconds, and participants' task was to continuously report the perceived (left-or rightward) direction of tactile motion. In Experiment 1, each tactile stimulus was paired with an auditory beep, though odd-numbered taps were paired with an asynchronous beep, with audiotactile SOAs ranging from -75 ms to 75 ms. Perceived direction of tactile motion varied systematically with audiotactile SOA, indicative of a temporal-capture effect. In Experiment 2, two audiotactile SOAs-one short (75 ms), one long (325 ms)-were compared. The long-SOA condition preserved the crossmodal event structure (so the temporal-capture dynamics should have been similar to that in Experiment 1), but both beeps now occurred temporally close to the taps on one side (even-numbered taps). The two SOAs were found to produce opposite modulations of apparent motion, indicative of an influence of crossmodal grouping. In Experiment 3, only odd-numbered, but not even-numbered, taps were paired with auditory beeps. This abolished the temporal-capture effect and, instead, a dominant percept of apparent motion from the audiotactile side to the tactile-only side was observed independently of the SOA variation. These findings suggest that asymmetric crossmodal grouping leads to an attentional modulation of apparent motion, which inhibits crossmodal temporal-capture effects
Sound-contingent visual motion aftereffect
<p>Abstract</p> <p>Background</p> <p>After a prolonged exposure to a paired presentation of different types of signals (e.g., color and motion), one of the signals (color) becomes a driver for the other signal (motion). This phenomenon, which is known as contingent motion aftereffect, indicates that the brain can establish new neural representations even in the adult's brain. However, contingent motion aftereffect has been reported only in visual or auditory domain. Here, we demonstrate that a visual motion aftereffect can be contingent on a specific sound.</p> <p>Results</p> <p>Dynamic random dots moving in an alternating right or left direction were presented to the participants. Each direction of motion was accompanied by an auditory tone of a unique and specific frequency. After a 3-minutes exposure, the tones began to exert marked influence on the visual motion perception, and the percentage of dots required to trigger motion perception systematically changed depending on the tones. Furthermore, this effect lasted for at least 2 days.</p> <p>Conclusions</p> <p>These results indicate that a new neural representation can be rapidly established between auditory and visual modalities.</p
Probing reionization with Lyman-alpha emission lines
Lyman-alpha emission from high-redshift galaxies may be a powerful probe of
the ionization history of the IGM at z>6: the observed Lyman-alpha emission
line is sensitive to the neutral fraction of IGM hydrogen in the range 0.1-1.
We present calculations of observed Lyman-alpha emission lines from z>6
galaxies, illustrating the effect of varying the many free parameters
associated with the emitting galaxy, its halo, and the IGM around the galaxy.
In particular, we use a dynamic model of the IGM that includes the effect of
IGM infall toward the emitting galaxy. Galactic winds may play a crucial role
in determining observed Lyman-alpha line fluxes. We compare our model
predictions with observations of two z=6.5 galaxies and conclude that, if
galactic winds are allowed for, existing observations place no constraint on
the neutral fraction of the IGM at z=6.5. Future space-based observations will
constrain the importance of galactic winds; if winds are unimportant for the
observed z=6.5 galaxies, our models suggest that the IGM neutral fraction at
z=6.5 is <~0.1.Comment: 17 pages, 25 figures, submitted to MNRA
Auditory Motion Information Drives Visual Motion Perception
BACKGROUND: Vision provides the most salient information with regard to the stimulus motion. However, it has recently been demonstrated that static visual stimuli are perceived as moving laterally by alternating left-right sound sources. The underlying mechanism of this phenomenon remains unclear; it has not yet been determined whether auditory motion signals, rather than auditory positional signals, can directly contribute to visual motion perception. METHODOLOGY/PRINCIPAL FINDINGS: Static visual flashes were presented at retinal locations outside the fovea together with a lateral auditory motion provided by a virtual stereo noise source smoothly shifting in the horizontal plane. The flash appeared to move by means of the auditory motion when the spatiotemporal position of the flashes was in the middle of the auditory motion trajectory. Furthermore, the lateral auditory motion altered visual motion perception in a global motion display where different localized motion signals of multiple visual stimuli were combined to produce a coherent visual motion perception. CONCLUSIONS/SIGNIFICANCE: These findings suggest there exist direct interactions between auditory and visual motion signals, and that there might be common neural substrates for auditory and visual motion processing
A suppressed contribution of low mass galaxies to reionization due to supernova feedback
Motivated by recent observations of the star formation rate density function
out to z~7, we describe a simple model for the star formation rate density
function at high redshift based on the extended Press-Schechter formalism. This
model postulates a starburst following each major merger, lasting for a time
t_SF and converting at most f_star of galactic gas into stars. We include a
simple physical prescription for supernovae feedback that suppresses star
formation in low mass galaxies. Constraining t_SF and f_star to describe the
observed star formation rate density at high redshifts, we find that individual
starbursts were terminated after a time t_SF~10^7 years. This is comparable to
the main-sequence lifetimes of supernova progenitors, indicating that high
redshift starbursts are quenched once supernovae feedback had time to develop.
High redshift galaxies convert ~10% of their mass into stars for galaxies with
star formation rates above ~1 solar mass per year, but a smaller fraction for
lower luminosity galaxies. Our best fit model successfully predicts the
observed relation between star formation rate and stellar mass at z>~4, while
our deduced relation between stellar mass and halo mass is also consistent with
data on the dwarf satellites of the Milky Way. We find that supernovae feedback
lowers the efficiency of star formation in the lowest mass galaxies and makes
their contribution to reionization small. As a result, photo-ionization
feedback on low mass galaxy formation does not significantly affect the
reionization history. Using a semi-analytic model for the reionization history,
we infer that approximately half of the ionizing photons needed to complete
reionization have already been observed in star-forming galaxies.Comment: 16 pages, 10 figures. Submitted to MNRA
The escape of ionising radiation from high-redshift dwarf galaxies
The UV escape fraction from high-redshift galaxies plays a key role in models
of cosmic reionisation. Because it is currently not possible to deduce the
escape fractions during the epoch of reionisation from observations, we have to
rely on numerical simulations. Our aim is to better constrain the escape
fraction from high-redshift dwarf galaxies, as these are the most likely
sources responsible for reionising the Universe. We employ a N-body/SPH method
that includes realistic prescriptions for the physical processes that are
important for the evolution of dwarf galaxies. These models are post-processed
with radiative transfer to determine the escape fraction of ionising radiation.
We perform a parameter study to assess the influence of the spin parameter, gas
fraction and formation redshift of the galaxy and study the importance of
numerical parameters as resolution, source distribution and local gas clearing.
We find that the UV escape fraction from high-redshift dwarf galaxies that have
formed a rotationally supported disc lie between 1e-5 and 0.1. The mass and
angular momentum of the galaxy are the most important parameters that determine
the escape fraction. We compare our results to previous work and discuss the
uncertainties of our models. The low escape fraction we find for high-redshift
dwarf galaxies is balanced by their high stellar content, resulting in an
efficiency parameter for stars that is only marginally lower than the values
found by semi-analytic models of reionisation. We therefore conclude that dwarf
galaxies play an important role in cosmic reionisation also after the initial
starburst phase, when the gas has settled into a disc.Comment: 19 pages, 14 figures. Accepted for publication in A&
Large-scale structure in the Lyman-alpha forest II: analysis of a group of ten QSOs
The spatial distribution of Ly-alpha forest absorption systems towards ten
QSOs has been analysed to search for large-scale structure over the redshift
range 2.2 < z < 3.4. The QSOs form a closely spaced group on the sky and are
concentrated within a 1 deg^2 field. We have employed a technique based on the
first and second moments of the transmission probability density function which
is capable of identifying and assessing the significance of regions of over- or
underdense Ly-alpha absorption. We find evidence for large-scale structure in
the distribution of Ly-alpha forest absorption at the > 99 per cent confidence
level. In individual spectra we find overdense Ly-alpha absorption on scales of
up to 1200 km s^-1. There is also strong evidence for correlated absorption
across line of sight pairs separated by < 3 h^-1 proper Mpc (q_0 = 0.5). For
larger separations the cross-correlation signal becomes progressively less
significant.Comment: 15 pages, LaTeX, 6 Postscript figures, accepted for publication in
MNRA
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