1,219 research outputs found
Past permafrost dynamics can inform future permafrost carbon-climate feedbacks
Climate warming threatens to destabilize vast northern permafrost areas, potentially releasing large quantities of organic carbon that could further disrupt the climate. Here we synthesize paleorecords of past permafrost-carbon dynamics to contextualize future permafrost stability and carbon feedbacks. We identify key landscape differences between the last deglaciation and today that influence the response of permafrost to atmospheric warming, as well as landscape-level differences that limit subsequent carbon uptake. We show that the current magnitude of thaw has not yet exceeded that of previous deglaciations, but that permafrost carbon release has the potential to exert a strong feedback on future Arctic climate as temperatures exceed those of the Pleistocene. Better constraints on the extent of subsea permafrost and its carbon pool, and on carbon dynamics from a range of permafrost thaw processes, including blowout craters and megaslumps, are needed to help quantify the future permafrost-carbon-climate feedbacks
Towards optimal softening in 3D N-body codes: I. Minimizing the force error
In N-body simulations of collisionless stellar systems, the forces are
softened to reduce the shot noise. Softening modifies gravity at r=|x-y|
smaller than softening length epsilon and the softened forces are increasingly
biased for ever larger epsilon. There is, thus, some optimum between reducing
the fluctuations and introducing a bias. Here, analytical relations are derived
for the amplitudes of the bias and the fluctuations in the limit of small
epsilon and large N. It is shown that the fluctuations of the force are
generated locally, in contrast to the variations of the potential, which
originate from noise in the whole system. Based on the asymptotic relations and
using numerical experiments, I study the dependence of the resulting force
error on N, epsilon, and on the functional form by which Newtonian gravity is
replaced. The Plummer softening, where each body is replaced by a Plummer
sphere of scale radius epsilon, yields significantly larger force errors than
do methods in which the bodies are replaced by density kernels of finite
extent. I also give special kernels, which reduce the errors even further.
These kernels largely compensate the errors made with too small inter-particle
forces at r<epsilon by exceeding Newtonian forces at r epsilon. Additionally,
the possibilities of locally adapting epsilon and of using unequal weights for
the bodies are investigated. These various techniques allow, without increasing
N, to reduce the rms force error by a factor 2 compared to Plummer softening
with constant epsilon. The results of this study are directly relevant to
N-body simulations using direct summation techniques or the tree code.
(abridged)Comment: 20 pages, 8 figs, accepted for publication in MNRA
A Magellan-IMACS-IFU Search for Dynamical Drivers of Nuclear Activity. I. Reduction Pipeline and Galaxy Catalog
Using the Inamori Magellan Areal Camera and Spectrograph (IMACS)
integral-field unit (IFU) on the 6.5m Magellan telescope, we have designed the
first statistically significant investigation of the two-dimensional
distribution and kinematics of ionized gas and stars in the central kiloparsec
regions of a well-matched sample of Seyfert and inactive control galaxies
selected from the Sloan Digital Sky Survey. The goals of the project are to use
the fine spatial sampling (0.2 arcsec/pixel) and large wavelength coverage
(4000-7000A) of the IMACS-IFU to search for dynamical triggers of nuclear
activity in the central region where active galactic nucleus (AGN) activity and
dynamical timescales become comparable, to identify and assess the impact of
AGN-driven outflows on the host galaxy and to provide a definitive sample of
local galaxy kinematics for comparison with future three-dimensional kinematic
studies of high-redshift systems. In this paper, we provide the first detailed
description of the procedure to reduce and calibrate data from the IMACS-IFU in
`long mode' to obtain two-dimensional maps of the distribution and kinematics
of ionized gas and stars. The sample selection criteria are presented,
observing strategy described and resulting maps of the sample galaxies
presented along with a description of the observed properties of each galaxy
and the overall observed properties of the sample.Comment: 62 pages. 41 figures. 5 tables. Accepted for publication in ApJS.
High-resolution version available at:
http://www.astro.ljmu.ac.uk/~pbw/IMACS-IFU/IMACS-1-highRes.pd
Black holes and Galactic density cusps -- I. Radial orbit cusps and bulges
In this paper, we study the distribution functions that arise naturally
during self-similar radial infall of collisionless matter. Such matter may be
thought of either as stars or as dark matter particles. If a rigorous steady
state is assumed, then the system is infinite and is described by a universal
distribution function given the self-similar index. The steady logarithmic
potential case is exceptional and yields the familiar Gaussian for an infinite
system with an inverse-square density profile. We show subsequently that for
time-dependent radial self-similar infall, the logarithmic case is accurately
described by the Fridmann and Polyachenko distribution function. The system in
this case is finite but growing. We are able to embed a central mass in the
universal steady distribution only by iteration, except in the case of massless
particles. The iteration yields logarithmic corrections to the massless
particle case and requires a `renormalization' of the central mass. A central
spherical mass may be accurately embedded in the Fridmann and Polyachenko
growing distribution however. Some speculation is given concerning the
importance of radial collisionless infall in actual galaxy formation.Comment: 10 pp, 3 fig
Improved Modeling of the Mass Distribution of Disk Galaxies by the Einasto Halo Model
(Abridged) The analysis of the rotation curves (RCs) of spiral galaxies
provides an efficient diagnostic for studying the properties of dark matter
halos and their relations with the baryonic material. We have modeled the RCs
of galaxies from The HI Nearby Galaxy Survey (THINGS) with the Einasto halo
model, which has emerged as the best-fitting model of the halos arising in
dissipationless cosmological N-body simulations. We find that the RCs are
significantly better fit with the Einasto halo than with either a
pseudo-isothermal sphere (Iso) or Navarro-Frenk-White (NFW) halo models. In our
best-fit models, the radius of density slope -2 and the density at this radius
are highly correlated. The Einasto index, which controls the overall shape of
the density profile, is near unity on average for intermediate and low mass
halos. This is not in agreement with the predictions from LCDM simulations. The
indices of the most massive halos are in rough agreement with those of
cosmological simulations and appear correlated with the halo virial mass. We
find that a typical Einasto density profile declines more strongly in its
outermost parts than any of the Iso or NFW models whereas it is relatively
shallow in its innermost regions. The core nature of those regions of halos
thus extends the cusp-core controversy found for the NFW model with low surface
density galaxies to the Einasto halo with more massive galaxies like those of
THINGS. We thus find that the Einasto halo model provides, so far, the best
match to the observed RCs, and can therefore be considered as a new standard
model for dark matter halos.Comment: 15 pages, 14 figures, The Astronomical Journal, in press, Volume 4,
2011 Octobe
DEVELOPMENT OF CORROSION-RESISTANT ALLOYS FOR USE AS CONTAINER MATERIALS FOR DECLADDING SOLUTIONS OR AS WELDING ALLOYS
Twenty-four experimental alloys were developed and evaluated as container materials or welding alloys for use with Sulfex and Niflex decladding solutions. Niflex solutions which were more corrosive than Sulfex solutions to most of the experimental alloys, produced severe localized attack on weldments made on vacuum-melted Hastelloy F with the experimental alloys. However, several of the alloys, when self-welded, were not selectively attacked. Some of these showed a substantial improvement in resistance to the decladding solutions. The most promising alloys were based on either 45 wt.% nickel--22 wt.% chromium or 50 wt.% nickel--25 wt.% chromium, with at least 6 wt.% molybdenum, and 1 wt.% titanium, 0.6 wt.% manganese, 0.4 wt. % silicon, 0.02 wt.% carbon, and the balance, iron. The alloy most resistant to both solutions contained 6 wt.% molybdenum and 1 wt.% copper in the 50 wt.% nickel--25 wt.% chromium base. Its corrosion rate of 22 mils per month in Niflex, with no selective attack, was significantly lower than the 105 mils per month recorded for Hastelloy F. Even lower rates would be expected under the less stringent conditions of actual process operation. Indications are that more resistance might be obtained by increasing the chromium and nickel contents. (auth
An ATP-binding cassette-type cysteine transporter in Campylobacter jejuni inferred from the structure of an extracytoplasmic solute receptor protein
Campylobacter jejuni is a Gram-negative food-borne pathogen associated with gastroenteritis in humans as well as cases of the autoimmune disease Guillain Barre syndrome. C. jejuni is asaccharolytic because it lacks an active glycolytic pathway for the use of sugars as a carbon source. This suggests an increased reliance on amino acids as nutrients and indeed the genome sequence of this organism indicates the presence of a number of amino acid uptake systems. Cj0982, also known as CjaA, is a putative extracytoplasmic solute receptor for one such uptake system as well as a major surface antigen and vaccine candidate. The crystal structure of Cj0982 reveals a two-domain protein with density in the enclosed cavity between the domains that clearly defines the presence of a bound cysteine ligand. Fluorescence titration experiments were used to demonstrate that Cj0982 binds cysteine tightly and specifically with a K-d of similar to 10(-7) M consistent with a role as a receptor for a high- affinity transporter. These data imply that Cj0982 is the binding protein component of an ABC-type cysteine transporter system and that cysteine uptake is important in the physiology of C. jejuni
Statistical properties of the dark matter haloes of dwarf galaxies and correlations with the environment
According to the now strongly supported concordance CDM model,
galaxies may be grossly described as a luminous component embedded in a dark
matter halo. The density profile of these mass dominating haloes may be
determined by N - body simulations which mimic the evolution of the tiny
initial density perturbations during the process leading to the structures we
observe today. Unfortunately, when the effect of baryons is taken into account,
the situation gets much more complicated due to the difficulties in simulating
their physics. As a consequence, a definitive prediction of how dark matter
haloes should presently look like is still missing. We revisit here this issue
from an observational point of view devoting our attention to dwarf galaxies.
Being likely dark matter dominated, these systems are ideal candidates to
investigate the present day halo density profiles and check whether dark matter
related quantities correlate with the stellar ones or the environment. By
fitting a large sample of well measured rotation curves, we infer constraints
on both halo structural parameters (such as the logarithmic slope of the
density profile and its concentration) and derived quantities (e.g., the mass
fraction and the Newtonian acceleration) which could then be used to constrain
galaxy formation scenarios. Moreover, we investigate whether the halo
properties correlates with the environment the galaxy lives in thus offering a
new tool to deepen our understanding of galaxy formation.Comment: 14 pages, 8 tables, 5 figures, accepted for publication on MNRA
High redshift quasars and the supermassive black hole mass budget: constraints on quasar formation models
We investigate the constraints on models of supermassive black hole (SMBH)
and quasar formation obtainable from two recent observational developments: the
discovery of luminous quasars at z~6, and estimates of the local mass density
of SMBHs. If ~90 per cent of this mass was accreted at redshifts z<3, as
suggested by the observed quasar luminosity functions, these joint constraints
pose a challenge for models, which must account for the observed luminous
quasar population at z~6 within a very limited `mass budget'. We investigate a
class of models based within the hierarchical structure formation scenario, in
which major mergers lead to black hole formation and fuelling, and the
resulting quasars shine at their Eddington-limited rate until their fuel is
exhausted. We show that the simplest such model, in which a constant fraction
of the gas within the halo is accreted in each major merger, cannot satisfy
both constraints simultaneously. When this model is normalized to reproduce the
number density of luminous quasars at z~6, the mass budget is grossly exceeded
due to an overabundance of lower mass SMBHs. We explore a range of
modifications to the simple model designed to overcome this problem. We show
that both constraints can be satisfied if the gas accretion fraction scales as
a function of the halo virial velocity. Similar scalings have been proposed in
order to reproduce the local M-sigma relation. Successful models can also be
constructed by restricting the formation of seed black holes to redshifts above
z~11.5 or to haloes above a velocity threshold ~55 km/s, or assuming that only
a fraction of major mergers result in formation of a seed SMBH. (abridged)Comment: 19 pages, 6 figures, 1 table. v2: Corrected references. v3: Extended
Section 5.1, corrected Section 3.2, various other corrections and additions
suggested by referee. Accepted by MNRAS in this for
Cosmology Using Cluster Internal Velocity Dispersions
We compare the distribution of internal velocity dispersions of galaxy
clusters for an observational sample to those obtained from a set of N-body
simulations of seven COBE-normalised cosmological scenarios: the standard CDM
(SCDM) and a tilted (n=0.85) CDM (TCDM) model, a CHDM model with 25% of massive
neutrinos, two low-density LCDM models with Omega_0=0.3 and 0.5, two open OCDM
models with Omega_0=0.4 and 0.6. Simulated clusters are observed in projection
so as to reproduce the main observational biases and are analysed by applying
the same algorithm for interlopers removal and velocity dispersion estimate as
for the reference observational sample. Velocity dispersions for individual
clusters can be largely affected by observational biases in a model-dependent
way: models in which clusters had less time to virialize show larger
discrepancies between 3D and projected velocity dispersions. From the
comparison with real clusters we find that both SCDM and TCDM largely
overproduce clusters. The CHDM model marginally overproduces clusters and
requires a somewhat larger sigma_8 than a purely CDM model in order to produce
the same cluster abundance. The LCDM model with Omega_0=0.3 agrees with data,
while the open model with Omega_0=0.4 and 0.6 underproduces and marginally
overproduces clusters, respectively.Comment: 28 pages, LaTeX uses Elsevier style file, 7 postscript figures (3
bitmapped to lower res.) included. Submitted to New Astronom
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