581 research outputs found
Audiovisual integration of stimulus transients
AbstractA change in sound intensity can facilitate luminance change detection. We found that this effect did not depend on whether sound intensity and luminance increased or decreased. In contrast, luminance identification was strongly influenced by the congruence of luminance and sound intensity change leaving only unsigned stimulus transients as the basis for audiovisual integration. Facilitation of luminance detection occurred even with varying audiovisual stimulus onset asynchrony and even when the sound lagged behind the luminance change by 75ms supporting the interpretation that perceptual integration rather than a reduction of temporal uncertainty or effects of attention caused the effect
The evolution of faint AGN between z~1 and z~5 from the COMBO-17 survey
We present a determination of the optical/UV AGN luminosity function and its
evolution, based on a large sample of faint (R<24) QSOs identified in the
COMBO-17 survey. Using multi-band photometry in 17 filters within 350nm <
lambda_obs < 930nm, we could simultaneously determine photometric redshifts
with an accuracy of sigma_z<0.03 and obtain spectral energy distributions. The
redshift range covered by the sample is 1.2<z<4.8, which implies that even at
z~3, the sample reaches below luminosities corresponding to M_B = -23,
conventionally employed to distinguish between Seyfert galaxies and quasars. We
clearly detect a broad plateau-like maximum of quasar activity around z~2 and
map out the smooth turnover between z~1 and z~4. The shape of the LF is
characterised by some mild curvature, but no sharp `break' is present within
the range of luminosities covered. Using only the COMBO-17 data, the evolving
LF can be adequately described by either a pure density evolution (PDE) or a
pure luminosity evolution (PLE) model. However, the absence of a strong L*-like
feature in the shape of the LF inhibits a robust distinction between these
modes. We present a robust estimate for the integrated UV luminosity generation
by AGN as a function of redshift. We find that the LF continues to rise even at
the lowest luminosities probed by our survey, but that the slope is
sufficiently shallow that the contribution of low-luminosity AGN to the UV
luminosity density is negligible. Although our sample reaches much fainter flux
levels than previous data sets, our results on space densities and LF slopes
are completely consistent with extrapolations from recent major surveys such as
SDSS and 2QZ.Comment: 17 pages, 14 figures, Astronomy & Astrophysics, in print, revised
versio
Strategies for prompt searches for GRB afterglows: the discovery of the GRB 001011 optical/near-infrared counterpart using colour-colour selection
We report the discovery of the optical and near-infrared counterparts to GRB
001011. The GRB 001011 error box determined by Beppo-SAX was simultaneously
imaged in the near-infrared by the 3.58-m New Technology Telescope and in the
optical by the 1.54-m Danish Telescope ~8 hr after the gamma-ray event. Here we
implement the colour-colour discrimination technique proposed by Rhoads (2001)
and extend it using near-IR data as well. We present the results provided by an
automatic colour-colour discrimination pipe-line developed to discern the
different populations of objects present in the GRB 001011 error box. Our
software revealed three candidates based on single-epoch images. Second-epoch
observations carried out ~3.2 days after the burst revealed that the most
likely candidate had faded, thus identifying it with the counterpart to the
GRB. In deep R-band images obtained 7 months after the burst a faint
(R=25.38+/-0.25) elongated object, presumably the host galaxy of GRB 001011,
was detected at the position of the afterglow. The GRB 001011 afterglow is the
first discovered with the assistance of colour-colour diagram techniques. We
discuss the advantages of using this method and its application to error boxes
determined by future missions.Comment: Accepted for publication in Astronomy and Astrophysics, 13 pages, 16
figure
Deep Ly-alpha imaging of two z=2.04 GRB host galaxy fields
We report on the results of deep narrow-band Ly-alpha, U and I imaging of the
fields of two GRBs at z=2.04 (GRB 000301C and GRB 000926). We find that the
host galaxy of GRB 000926 is an extended, strong Ly-alpha emitter with a
rest-frame equivalent width of 71+20-15 AA. The galaxy consists of two main
components and several fainter knots. GRB 000926 occurred in the western
component, whereas most of the Ly-alpha luminosity (about 65%) originates in
the eastern component. Using archival HST images of the host galaxy we measure
the spectral slopes (f_lambda prop. to lambda^beta) of the two components to
beta = -2.4+-0.3 (east) and -1.4+-0.2 (west). This implies that both components
contain at most small amounts of dust, consistent with the observed strong
Ly-alpha emission. The western component has a slightly redder V-I colour than
the eastern component, suggesting the presence of at least some dust. We do not
detect the host galaxy of GRB 000301C in neither Ly-alpha emission nor in U and
I-band images. We infer a limit of U(AB)>27.7 (2-sigma limit per arcsec^2). The
upper limits on the Ly-alpha flux implies a Ly-alpha equivalent width upper
limit of 150AA. We find 11 and 8 other galaxies with excess emission in the
narrow filter in the fields of GRB 000301C and GRB 000926 respectively. Based
on these detections we conclude that GRB 000926 occurred in one of the
strongest centres of star formation within several Mpc, whereas GRB 000301C
occurred in an intrinsically very faint galaxy far from being the strongest
centre of star formation in its galactic environment. Under the hypothesis that
GRBs trace star formation, the wide range of GRB host galaxy luminosities
implies a very steep faint end slope of the high redshift galaxy luminosity
function.Comment: 13 pages, 7 figures. Accepted for publication in A&
BCS to Bose Crossover in Anisotropic Superconductors
In this work we use functional integral techniques to examine the nearest
neighbour attractive Hubbard model on a quasi-2D lattice. It is a simple
phenomenological model for the high-Tc cuprates that allows both extended
(non-local) s- and d-wave singlet superconductivity as well as mixed symmetry
states. The Hartree-Gor'kov mean field theory of the model has a finite
temperature phase diagram which shows a transition from pure s-wave to pure
d-wave superconductivity, via a mixed symmetry s+id state, as a function of
doping. Including Gaussian fluctuations we examine the crossover from
weak-coupling BCS superconductivity to the strong-coupling Bose-Einstein
condensation of composite s- or d-wave bosons and comment on the origin and
symmetry of the pseudogap.Comment: 20 pages inc. 13 figure
The role of visual spatial attention in audiovisual speech perception
International audienc
Relative energetics and structural properties of zirconia using a self-consistent tight-binding model
We describe an empirical, self-consistent, orthogonal tight-binding model for
zirconia, which allows for the polarizability of the anions at dipole and
quadrupole levels and for crystal field splitting of the cation d orbitals.
This is achieved by mixing the orbitals of different symmetry on a site with
coupling coefficients driven by the Coulomb potentials up to octapole level.
The additional forces on atoms due to the self-consistency and polarizabilities
are exactly obtained by straightforward electrostatics, by analogy with the
Hellmann-Feynman theorem as applied in first-principles calculations. The model
correctly orders the zero temperature energies of all zirconia polymorphs. The
Zr-O matrix elements of the Hamiltonian, which measure covalency, make a
greater contribution than the polarizability to the energy differences between
phases. Results for elastic constants of the cubic and tetragonal phases and
phonon frequencies of the cubic phase are also presented and compared with some
experimental data and first-principles calculations. We suggest that the model
will be useful for studying finite temperature effects by means of molecular
dynamics.Comment: to be published in Physical Review B (1 march 2000
Seasonal melting and the formation of sedimentary rocks on Mars, with predictions for the Gale Crater mound
A model for the formation and distribution of sedimentary rocks on Mars is
proposed. The rate-limiting step is supply of liquid water from seasonal
melting of snow or ice. The model is run for a O(10^2) mbar pure CO2
atmosphere, dusty snow, and solar luminosity reduced by 23%. For these
conditions snow only melts near the equator, and only when obliquity >40
degrees, eccentricity >0.12, and perihelion occurs near equinox. These
requirements for melting are satisfied by 0.01-20% of the probability
distribution of Mars' past spin-orbit parameters. Total melt production is
sufficient to account for aqueous alteration of the sedimentary rocks. The
pattern of seasonal snowmelt is integrated over all spin-orbit parameters and
compared to the observed distribution of sedimentary rocks. The global
distribution of snowmelt has maxima in Valles Marineris, Meridiani Planum and
Gale Crater. These correspond to maxima in the sedimentary-rock distribution.
Higher pressures and especially higher temperatures lead to melting over a
broader range of spin-orbit parameters. The pattern of sedimentary rocks on
Mars is most consistent with a Mars paleoclimate that only rarely produced
enough meltwater to precipitate aqueous cements and indurate sediment. The
results suggest intermittency of snowmelt and long globally-dry intervals,
unfavorable for past life on Mars. This model makes testable predictions for
the Mars Science Laboratory rover at Gale Crater. Gale Crater is predicted to
be a hemispheric maximum for snowmelt on Mars.Comment: Submitted to Icarus. Minor changes from submitted versio
A Magnified View of Circumnuclear Star Formation and Feedback around an Active Galactic Nucleus at z=2.6
© 2018. The American Astronomical Society. All rights reserved.We present Atacama Large Millimeter/submillimeter Array observations of an intrinsically radio-bright (L 1.4 GHz = (1.7 ± 0.1) Ă10 25 W Hz -1) and infrared luminous (L IR â 10 13 L o) galaxy at z = 2.6. The infrared-to-radio luminosity ratio, q = 1.8, indicates the presence of a radio-loud active galactic nucleus (AGN). Gravitational lensing by two foreground galaxies at z â 0.2 provides access to physical scales of approximately 360 pc, and we resolve a 2.5 kpc radius ring of star-forming molecular gas, traced by atomic carbon C i (1 â 0) and carbon monoxide CO (J = 4 â 3). We also detect emission from the cyanide radical, CN (N = 4 â 3). With a velocity width of 680 km s -1, this traces dense molecular gas traveling at velocities nearly a factor of two larger than the rotation speed of the molecular ring. While this could indicate the presence of a dynamical and photochemical interaction between the AGN and molecular interstellar medium on scales of a few 100 pc, ongoing feedback is unlikely to have a significant impact on the assembly of stellar mass in the molecular ring, given the âŒ10 s Myr depletion timescale due to star formation.Peer reviewedFinal Published versio
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The coupling of vision with locomotion in cortical blindness
© 2014 Elsevier B.V.. Maintaining or modifying the speed and direction of locomotion requires the coupling of the locomotion with the retinal optic flow that it generates. It is shown that this essential behavioral capability, which requires on-line neural control, is preserved in the cortically blind hemifield of a hemianope. In experiments, optic flow stimuli were presented to either the normal or blind hemifield while the patient was walking on a treadmill. Little difference was found between the hemifields with respect to the coupling (i.e. co-dependency) of optic flow detection with locomotion. Even in the cortically blind hemifield, faster walking resulted in the perceptual slowing of detected optic flow, and self-selected locomotion speeds demonstrated behavioral discrimination between different optic flow speeds. The results indicate that the processing of optic flow, and thereby on-line visuo-locomotor coupling, can take place along neural pathways that function without processing in Area V1, and thus in the absence of conscious intervention. These and earlier findings suggest that optic flow and object motion are processed in parallel along with correlated non-visual locomotion signals. Extrastriate interactions may be responsible for discounting the optical effects of locomotion on the perceived direction of object motion, and maintaining visually guided self-motion
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