54,452 research outputs found
Cosmological Shock Waves in the Large Scale Structure of the Universe: Non-gravitational Effects
Cosmological shock waves result from supersonic flow motions induced by
hierarchical clustering of nonlinear structures in the universe. These shocks
govern the nature of cosmic plasma through thermalization of gas and
acceleration of nonthermal, cosmic-ray (CR) particles. We study the statistics
and energetics of shocks formed in cosmological simulations of a concordance
CDM universe, with a special emphasis on the effects of
non-gravitational processes such as radiative cooling, photoionization/heating,
and galactic superwind feedbacks. Adopting an improved model for gas
thermalization and CR acceleration efficiencies based on nonlinear diffusive
shock acceleration calculations, we then estimate the gas thermal energy and
the CR energy dissipated at shocks through the history of the universe. Since
shocks can serve as sites for generation of vorticity, we also examine the
vorticity that should have been generated mostly at curved shocks in
cosmological simulations. We find that the dynamics and energetics of shocks
are governed primarily by the gravity of matter, so other non-gravitational
processes do not affect significantly the global energy dissipation and
vorticity generation at cosmological shocks. Our results reinforce scenarios in
which the intracluster medium and warm-hot intergalactic medium contain
energetically significant populations of nonthermal particles and turbulent
flow motions.Comment: Submitted to ApJ. Pdf with full resolution figures can be downloaded
from http://canopus.cnu.ac.kr/ryu/krco.pd
Massive and Red Objects predicted by a semianalytical model of galaxy formation
We study whether hierarchical galaxy formation in a concordance CDM
universe can produce enough massive and red galaxies compared to the
observations. We implement a semi-analytical model in which the central black
holes gain their mass during major mergers of galaxies and the energy feedback
from active galaxy nuclei (AGN) suppresses the gas cooling in their host halos.
The energy feedback from AGN acts effectively only in massive galaxies when
supermassive black holes have been formed in the central bulges. Compared with
previous models without black hole formation, our model predicts more massive
and luminous galaxies at high redshift, agreeing with the observations of K20
up to . Also the predicted stellar mass density from massive galaxies
agrees with the observations of GDDS. Because of the energy feedback from AGN,
the formation of new stars is stopped in massive galaxies with the termination
of gas cooling and these galaxies soon become red with color 5 (Vega
magnitude), comparable to the Extremely Red Objects (EROs) observed at redshift
1-2. Still the predicted number density of very EROs is lower than
observed at , and it may be related to inadequate descriptions of dust
extinction, star formation history and AGN feedback in those luminous galaxies.Comment: Accepted for Publication in ApJ, added reference
Translated points and Rabinowitz Floer homology
We prove that if a contact manifold admits an exact filling then every local
contactomorphism isotopic to the identity admits a translated point in the
interior of its support, in the sense of Sandon [San11b]. In addition we prove
that if the Rabinowitz Floer homology of the filling is non-zero then every
contactomorphism isotopic to the identity admits a translated point, and if the
Rabinowitz Floer homology of the filling is infinite dimensional then every
contactmorphism isotopic to the identity has either infinitely many translated
points, or a translated point on a closed leaf. Moreover if the contact
manifold has dimension greater than or equal to 3, the latter option
generically doesn't happen. Finally, we prove that a generic contactomorphism
on has infinitely many geometrically distinct iterated
translated points all of which lie in the interior of its support.Comment: 13 pages, v2: numerous corrections, results unchange
State space collapse and diffusion approximation for a network operating under a fair bandwidth sharing policy
We consider a connection-level model of Internet congestion control,
introduced by Massouli\'{e} and Roberts [Telecommunication Systems 15 (2000)
185--201], that represents the randomly varying number of flows present in a
network. Here, bandwidth is shared fairly among elastic document transfers
according to a weighted -fair bandwidth sharing policy introduced by Mo
and Walrand [IEEE/ACM Transactions on Networking 8 (2000) 556--567] []. Assuming Poisson arrivals and exponentially distributed document
sizes, we focus on the heavy traffic regime in which the average load placed on
each resource is approximately equal to its capacity. A fluid model (or
functional law of large numbers approximation) for this stochastic model was
derived and analyzed in a prior work [Ann. Appl. Probab. 14 (2004) 1055--1083]
by two of the authors. Here, we use the long-time behavior of the solutions of
the fluid model established in that paper to derive a property called
multiplicative state space collapse, which, loosely speaking, shows that in
diffusion scale, the flow count process for the stochastic model can be
approximately recovered as a continuous lifting of the workload process.Comment: Published in at http://dx.doi.org/10.1214/08-AAP591 the Annals of
Applied Probability (http://www.imstat.org/aap/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Visualizing Quantum Well State Perturbations of Metallic Thin Films near Stacking Fault Defects
We demonstrate that quantum well states (QWS) of thin Pb films are highly
perturbed within the proximity of intrinsic film defects. Scanning Tunneling
Spectroscopy (STM/STS) measurements indicate that the energy of these states
have a strong distance dependence within 4 nm of the defect with the strongest
energetic fluctuations equaling up to 100 meV. These localized perturbations
show large spatially-dependent asymmetries in the LDOS around the defect site
for each corresponding quantum well state. These energetic fluctuations can be
described by a simple model which accounts for fluctuations in the confinement
potential induced by topographic changes.Comment: Updated Versio
Gauge/String-Gravity Duality and Froissart Bound
The gauge/string-gravity duality correspondence opened renewed hope and
possibility to address some of the fundamental and non-perturbative QCD
problems in particle physics, such as hadron spectrum and Regge behavior of the
scattering amplitude at high energies. One of the most fundamental and
long-standing problem is the high energy behavior of total cross-sections.
According to a series of exhaustive tests by the COMPETE group, (1). total
cross-sections have a universal Heisenberg behavior in energy corresponding to
the maximal energy behavior allowed by the Froissart bound, i.e., with and for all reactions,
and (2). the factorization relation among is well satisfied by experiments. I discuss the
recent interesting application of the gauge/string-gravity duality of
correspondence with a deformed background metric so as to break the conformal
symmetry that can lead to the Heisenberg behavior of rising total
cross-sections, and present some preliminary results on the high energy QCD
from Planckian scattering in and black-hole production.Comment: 10 pages, Presented to the Coral Gables Conference 2003, Launching of
BelleE\'poque in High Energy Physics and Cosmology, 17 - 21 December 2003,
Fort Lauderdale, Florid
Modelling spatially regulated B-catenin dynamics & invasion in intestinal crypts
Experimental data (e.g., genetic lineage and cell population studies) on intestinal crypts reveal that regulatory features of crypt behavior, such as control via morphogen gradients, are remarkably well conserved among numerous organisms (e.g., from mouse and rat to human) and throughout the different regions of the small and large intestines. In this article, we construct a partial differential equation model of a single colonic crypt that describes the spatial distribution of Wnt pathway proteins along the crypt axis. The novelty of our continuum model is that it is based upon assumptions that can be directly related to processes at the cellular and subcellular scales. We use the model to predict how the distributions of Wnt pathway proteins are affected by mutations. The model is then extended to investigate how mutant cell populations can invade neighboring crypts. The model simulations suggest that cell crowding caused by increased proliferation and decreased cell loss may be sufficient for a mutant cell population to colonize a neighboring healthy crypt
- …