4,976 research outputs found
Signal strength determines the nature of the relationship between perception and working memory
Neurophysiological and behavioral studies have shown that perception and memory share neural substrates and functional properties. But are perception and the active working memory of a stimulus one and the same? To address this question in the spatial domain, we compared the percept and the working memory of the position of a target stimulus embedded within a surround of moving dots. Motion in a particular direction after the target's offset biased the memory of target location in the same direction. However, motion simultaneous with a high-contrast, perceptually strong target biased the percept of target location in the opposite direction. Thus, perception and working memory can be modified by motion in qualitatively different ways. Manipulations to strengthen the memory trace had no effect on the direction of the memory bias, indicating that memory signal strength can never equal that of the percept of a strong stimulus. However, the percept of a weak stimulus was biased in the direction of motion. Thus, although perception and working memory are not inherently different, they can differ behaviorally depending on the strength of the perceptual signal. Understanding how a changing surround biases neural representations in general, and postsensory processes in particular, can help one understand past reports of spatial mislocalization
Small Scale Anisotropies of UHECRs from Super-Heavy Halo Dark Matter
The decay of very heavy metastable relics of the Early Universe can produce
ultra-high energy cosmic rays (UHECRs) in the halo of our own Galaxy. In this
model, no Greisen-Zatsepin-Kuzmin cutoff is expected because of the short
propagation distances. We show here that, as a consequence of the hierarchical
build up of the halo, this scenario predicts the existence of small scale
anisotropies in the arrival directions of UHECRs, in addition to a large scale
anisotropy, known from previous studies. We also suggest some other observable
consequences of this scenario which will be testable with upcoming experiments,
as Auger, EUSO and OWL.Comment: Contribution given at ICRC 2001 - August 7-15, 2001 - Hambur
The Mass Function of Dark Halos in Superclusters and Voids
A modification of the Press-Schechter theory allowing for presence of a
background large-scale structure (LSS) - a supercluster or a void, is proposed.
The LSS is accounted as the statistical constraints in form of linear
functionals of the random overdensity field. The deviation of the background
density within the LSS is interpreted in a pseudo-cosmological sense. Using the
constraints formalism may help us to probe non-trivial spatial statistics of
haloes, e.g. edge and shape effects on boundaries of the superclusters and
voids. Parameters of the constraints are connected to features of the LSS: its
mean overdensity, a spatial scale and a shape, and spatial momenta of higher
orders. It is shown that presence of a non-virialized LSS can lead to an
observable deviation of the mass function. This effect is exploited to build a
procedure to recover parameters of the background perturbation from the
observationally estimated mass function.Comment: 23 pages, 6 figures; to be appeared in Astronomy Reports, 2014, Vol.
58, No. 6, pp. 386-39
On the Distribution of Haloes, Galaxies and Mass
The stochasticity in the distribution of dark haloes in the cosmic density
field is reflected in the distribution function which gives
the probability of finding haloes in a volume with mass density
contrast . We study the properties of this function using
high-resolution -body simulations, and find that is
significantly non-Poisson. The ratio between the variance and the mean goes
from (Poisson) at to (sub-Poisson) at
to (super-Poisson) at . The mean bias
relation is found to be well described by halo bias models based on the
Press-Schechter formalism. The sub-Poisson variance can be explained as a
result of halo-exclusion while the super-Poisson variance at high
may be explained as a result of halo clustering. A simple phenomenological
model is proposed to describe the behavior of the variance as a function of
. Galaxy distribution in the cosmic density field predicted by
semi-analytic models of galaxy formation shows similar stochastic behavior. We
discuss the implications of the stochasticity in halo bias to the modelling of
higher-order moments of dark haloes and of galaxies.Comment: 10 pages, 6 figures, Latex using MN2e style. Minor changes. Accepted
for publication in MNRA
Selection bias in dynamically-measured super-massive black hole samples: consequences for pulsar timing arrays
Supermassive black hole -- host galaxy relations are key to the computation
of the expected gravitational wave background (GWB) in the pulsar timing array
(PTA) frequency band. It has been recently pointed out that standard relations
adopted in GWB computations are in fact biased-high. We show that when this
selection bias is taken into account, the expected GWB in the PTA band is a
factor of about three smaller than previously estimated. Compared to other
scaling relations recently published in the literature, the median amplitude of
the signal at yr drops from to
. Although this solves any potential tension between
theoretical predictions and recent PTA limits without invoking other dynamical
effects (such as stalling, eccentricity or strong coupling with the galactic
environment), it also makes the GWB detection more challenging.Comment: 6 pages 4 figures, submitted to MNRAS letter
Large scale bias and the inaccuracy of the peak-background split
The peak-background split argument is commonly used to relate the abundance
of dark matter halos to their spatial clustering. Testing this argument
requires an accurate determination of the halo mass function. We present a
Maximum Likelihood method for fitting parametric functional forms to halo
abundances which differs from previous work because it does not require binned
counts. Our conclusions do not depend on whether we use our method or more
conventional ones. In addition, halo abundances depend on how halos are
defined. Our conclusions do not depend on the choice of link length associated
with the friends-of-friends halo-finder, nor do they change if we identify
halos using a spherical overdensity algorithm instead. The large scale halo
bias measured from the matter-halo cross spectrum b_x and the halo
autocorrelation function b_xi (on scales k~0.03h/Mpc and r ~50 Mpc/h) can
differ by as much as 5% for halos that are significantly more massive than the
characteristic mass M*. At these large masses, the peak background split
estimate of the linear bias factor b1 is 3-5% smaller than b_xi, which is 5%
smaller than b_x. We discuss the origin of these discrepancies: deterministic
nonlinear local bias, with parameters determined by the peak-background split
argument, is unable to account for the discrepancies we see. A simple linear
but nonlocal bias model, motivated by peaks theory, may also be difficult to
reconcile with our measurements. More work on such nonlocal bias models may be
needed to understand the nature of halo bias at this level of precision.Comment: MNRAS accepted. New section with Spherical Overdensity identified
halos included. Appendix enlarge
Self-consistency of the Excursion Set Approach
The excursion set approach provides a framework for predicting how the
abundance of dark matter halos depends on the initial conditions. A key
ingredient of this formalism comes from the physics of halo formation: the
specification of a critical overdensity threshold (barrier) which protohalos
must exceed if they are to form bound virialized halos at a later time. Another
ingredient is statistical, as it requires the specification of the appropriate
statistical ensemble over which to average when making predictions. The
excursion set approach explicitly averages over all initial positions, thus
implicitly assuming that the appropriate ensemble is that associated with
randomly chosen positions in space, rather than special positions such as peaks
of the initial density field. Since halos are known to collapse around special
positions, it is not clear that the physical and statistical assumptions which
underlie the excursion set approach are self-consistent. We argue that they are
at least for low mass halos, and illustrate by comparing our excursion set
predictions with numerical data from the DEUS simulations.Comment: 5 pages, 2 figure
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