2,423 research outputs found
Anisotropic Galactic Outflows and Enrichment of the Intergalactic Medium. I: Monte Carlo Simulations
We have developed an analytical model to describe the evolution of
anisotropic galactic outflows. With it, we investigate the impact of varying
opening angle on galaxy formation and the evolution of the IGM. We have
implemented this model in a Monte Carlo algorithm to simulate galaxy formation
and outflows in a cosmological context. Using this algorithm, we have simulated
the evolution of a comoving volume of size [12h^(-1)Mpc]^3 in the LCDM
universe. Starting from a Gaussian density field at redshift z=24, we follow
the formation of ~20,000 galaxies, and simulate the galactic outflows produced
by these galaxies. When these outflows collide with density peaks, ram pressure
stripping of the gas inside the peak may result. This occurs in around half the
cases and prevents the formation of galaxies. Anisotropic outflows follow the
path of least resistance, and thus travel preferentially into low-density
regions, away from cosmological structures (filaments and pancakes) where
galaxies form. As a result, the number of collisions is reduced, leading to the
formation of a larger number of galaxies. Anisotropic outflows can
significantly enrich low-density systems with metals. Conversely, the
cross-pollution in metals of objects located in a common cosmological
structure, like a filament, is significantly reduced. Highly anisotropic
outflows can travel across cosmological voids and deposit metals in other,
unrelated cosmological structures.Comment: 32 pages, 9 figures (2 color). Revised version accepted in Ap
Gravitational perturbations of the Schwarzschild spacetime: A practical covariant and gauge-invariant formalism
We present a formalism to study the metric perturbations of the Schwarzschild
spacetime. The formalism is gauge invariant, and it is also covariant under
two-dimensional coordinate transformations that leave the angular coordinates
unchanged. The formalism is applied to the typical problem of calculating the
gravitational waves produced by material sources moving in the Schwarzschild
spacetime. We examine the radiation escaping to future null infinity as well as
the radiation crossing the event horizon. The waveforms, the energy radiated,
and the angular-momentum radiated can all be expressed in terms of two
gauge-invariant scalar functions that satisfy one-dimensional wave equations.
The first is the Zerilli-Moncrief function, which satisfies the Zerilli
equation, and which represents the even-parity sector of the perturbation. The
second is the Cunningham-Price-Moncrief function, which satisfies the
Regge-Wheeler equation, and which represents the odd-parity sector of the
perturbation. The covariant forms of these wave equations are presented here,
complete with covariant source terms that are derived from the stress-energy
tensor of the matter responsible for the perturbation. Our presentation of the
formalism is concluded with a separate examination of the monopole and dipole
components of the metric perturbation.Comment: 21 page
Exploring the Moon: A Teacher's Guide with Activities for Earth and Space Sciences
The "Teacher's Guide" tells the story of the Moon's geological history and how scientists try to decipher the story. This background information may be useful reading for students as well. Key facts about the Moon appear on the "Moon ABCs" and "Rock ABCs" pages. These pages were named to emphasize the basic nature of the information. The "Progress in Lunar Science Chart" summarizes our knowledge about the Moon from 1959 to 1997
Structure and spacing of cellulose microfibrils in woody cell walls of dicots
The structure of cellulose microfibrils in situ in wood from the dicotyledonous (hardwood) species cherry and birch, and the vascular tissue from sunflower stems, was examined by wide-angle X-ray and neutron scattering (WAXS and WANS) and small-angle neutron scattering (SANS). Deuteration of accessible cellulose chains followed by WANS showed that these chains were packed at similar spacings to crystalline cellulose, consistent with their inclusion in the microfibril dimensions and with a location at the surface of the microfibrils. Using the Scherrer equation and correcting for considerable lateral disorder, the microfibril dimensions of cherry, birch and sunflower microfibrils perpendicular to the [200] crystal plane were estimated as 3.0, 3.4 and 3.3Â nm respectively. The lateral dimensions in other directions were more difficult to correct for disorder but appeared to be 3Â nm or less. However for cherry and sunflower, the microfibril spacing estimated by SANS was about 4Â nm and was insensitive to the presence of moisture. If the microfibril width was 3Â nm as estimated by WAXS, the SANS spacing suggests that a non-cellulosic polymer segment might in places separate the aggregated cellulose microfibrils
PGI6 Drug Utilization Review of Acid Suppressants (Durable) â an Audit to Assess the Utilization of Proton Pump Inhibitors and Histamine H2-Receptor Antagonists in Canadian Hospitals
Non-volatile molecular memory elements based on ambipolar nanotube field effect transistors
We have fabricated air-stable n-type, ambipolar carbon nanotube field effect
transistors (CNFETs), and used them in nanoscale memory cells. N-type
transistors are achieved by annealing of nanotubes in hydrogen gas and
contacting them by cobalt electrodes. Scanning gate microscopy reveals that the
bulk response of these devices is similar to gold-contacted p-CNFETs,
confirming that Schottky barrier formation at the contact interface determines
accessibility of electron and hole transport regimes. The transfer
characteristics and Coulomb Blockade (CB) spectroscopy in ambipolar devices
show strongly enhanced gate coupling, most likely due to reduction of defect
density at the silicon/silicon-dioxide interface during hydrogen anneal. The CB
data in the ``on''-state indicates that these CNFETs are nearly ballistic
conductors at high electrostatic doping. Due to their nanoscale capacitance,
CNFETs are extremely sensitive to presence of individual charge around the
channel. We demonstrate that this property can be harnessed to construct data
storage elements that operate at the few-electron level.Comment: 6 pages text, 3 figures and 1 table of content graphic; available as
NanoLetters ASAP article on the we
Carbon Nanotubes as Schottky Barrier Transistors
We show that carbon nanotube transistors operate as unconventional "Schottky
barrier transistors", in which transistor action occurs primarily by varying
the contact resistance rather than the channel conductance. Transistor
characteristics are calculated for both idealized and realistic geometries, and
scaling behavior is demonstrated. Our results explain a variety of experimental
observations, including the quite different effects of doping and adsorbed
gases. The electrode geometry is shown to be crucial for good device
performance.Comment: 4 pages, 5 figures, appears in Physical Review Letter
Note on flat foliations of spherically symmetric spacetimes
It is known that spherically symmetric spacetimes admit flat spacelike
foliations. We point out a simple method of seeing this result via the
Hamiltonian constraints of general relativity. The method yields explicit
formulas for the extrinsic curvatures of the slicings.Comment: 4 pages, to appear in PRD, reference added, typos correcte
Comment on `Hawking radiation from fluctuating black holes'
Takahashi & Soda (2010 Class. Quantum Grav. v27 p175008, arXiv:1005.0286)
have recently considered the effect (at lowest non-trivial order) of dynamical,
quantized gravitational fluctuations on the spectrum of scalar Hawking
radiation from a collapsing Schwarzschild black hole. However, due to an
unfortunate choice of gauge, the dominant (even divergent) contribution to the
coefficient of the spectrum correction that they identify is a pure gauge
artifact. I summarize the logic of their calculation, comment on the
divergences encountered in its course and comment on how they could be
eliminated, and thus the calculation be completed.Comment: 12 pages, 1 fig; feynmp, amsref
Instability of two interacting, quasi-monochromatic waves in shallow water
We study the nonlinear interactions of waves with a doubled-peaked power
spectrum in shallow water. The starting point is the prototypical equation for
nonlinear uni-directional waves in shallow water, i.e. the Korteweg de Vries
equation. Using a multiple-scale technique two defocusing coupled Nonlinear
Schr\"odinger equations are derived. We show analytically that plane wave
solutions of such a system can be unstable to small perturbations. This
surprising result suggests the existence of a new energy exchange mechanism
which could influence the behaviour of ocean waves in shallow water.Comment: 4 pages, 2 figure
- âŠ