2,758 research outputs found
The Mass Function and Average Mass Loss Rate of Dark Matter Subhaloes
We present a simple, semi-analytical model to compute the mass functions of
dark matter subhaloes. The masses of subhaloes at their time of accretion are
obtained from a standard merger tree. During the subsequent evolution, the
subhaloes experience mass loss due to the combined effect of dynamical
friction, tidal stripping, and tidal heating. Rather than integrating these
effects along individual subhalo orbits, we consider the average mass loss
rate, where the average is taken over all possible orbital configurations. This
allows us to write the average mass loss rate as a simple function that depends
only on redshift and on the instantaneous mass ratio of subhalo and parent
halo. After calibrating the model by matching the subhalo mass function (SHMF)
of cluster-sized dark matter haloes obtained from numerical simulations, we
investigate the predicted mass and redshift dependence of the SHMF.We find
that, contrary to previous claims, the subhalo mass function is not universal.
Instead, both the slope and the normalization depend on the ratio of the parent
halo mass, M, and the characteristic non-linear mass M*. This simply reflects a
halo formation time dependence; more massive parent haloes form later, thus
allowing less time for mass loss to operate. We analyze the halo-to-halo
scatter, and show that the subhalo mass fraction of individual haloes depends
most strongly on their accretion history in the last Gyr. Finally we provide a
simple fitting function for the average SHMF of a parent halo of any mass at
any redshift and for any cosmology, and briefly discuss several implications of
our findings.Comment: Replaced to match version accepted for publication in MNRAS. Small
section added that discusses higher-order moments of subhalo occupation
distribution (including a new figure). Otherwise, few small change
The dynamical distance and intrinsic structure of the globular cluster omega Centauri
We determine the dynamical distance D, inclination i, mass-to-light ratio M/L
and the intrinsic orbital structure of the globular cluster omega Cen, by
fitting axisymmetric dynamical models to the ground-based proper motions of van
Leeuwen et al. and line-of-sight velocities from four independent data-sets. We
correct the observed velocities for perspective rotation caused by the space
motion of the cluster, and show that the residual solid-body rotation component
in the proper motions can be taken out without any modelling other than
assuming axisymmetry. This also provides a tight constraint on D tan i.
Application of our axisymmetric implementation of Schwarzschild's orbit
superposition method to omega Cen reveals no dynamical evidence for a
significant radial dependence of M/L. The best-fit dynamical model has a
stellar V-band mass-to-light ratio M/L_V = 2.5 +/- 0.1 M_sun/L_sun and an
inclination i = 50 +/- 4 degrees, which corresponds to an average intrinsic
axial ratio of 0.78 +/- 0.03. The best-fit dynamical distance D = 4.8 +/- 0.3
kpc (distance modulus 13.75 +/- 0.13 mag) is significantly larger than obtained
by means of simple spherical or constant-anisotropy axisymmetric dynamical
models, and is consistent with the canonical value 5.0 +/- 0.2 kpc obtained by
photometric methods. The total mass of the cluster is (2.5 +/- 0.3) x 10^6
M_sun. The best-fit model is close to isotropic inside a radius of about 10
arcmin and becomes increasingly tangentially anisotropic in the outer region,
which displays significant mean rotation. This phase-space structure may well
be caused by the effects of the tidal field of the Milky Way. The cluster
contains a separate disk-like component in the radial range between 1 and 3
arcmin, contributing about 4% to the total mass.Comment: 37 pages (23 figures), accepted for publication in A&A, abstract
abridged, for PS and PDF file with full resolution figures, see
http://www.strw.leidenuniv.nl/~vdven/oc
The activity of supported vanadium oxide catalysts for the selective reduction of NO with ammonia
The activities of monolayer V2O5 catalysts for the selective reduction of NO with NH3 are compared with those of commercial available catalysts containing V and/or W. From steady state and pulse experiments it can be concluded that the reduction of surface sites proceeds either by NH3 + NO or by NH3 alone. The reoxidation of the reduced sites occurs by gaseous oxygen or NO. The experimental reaction stoichiometry can be explained in terms of suitable combinations of these four reactions
Schwarzschild models of the Sculptor dSph galaxy
We have developed a spherically symmetric dynamical model of a dwarf
spheroidal galaxy using the Schwarzschild method. This type of modelling yields
constraints both on the total mass distribution (e.g. enclosed mass and scale
radius) as well as on the orbital structure of the system modelled (e.g.
velocity anisotropy). Therefore not only can we derive the dark matter content
of these systems, but also explore possible formation scenarios. Here we
present preliminary results for the Sculptor dSph. We find that the mass of
Sculptor within 1kpc is 8.5\times10^(7\pm0.05) M\odot, its anisotropy profile
is tangentially biased and slightly more isotropic near the center. For an NFW
profile, the preferred concentration (~15) is compatible with cosmological
models. Very cuspy density profiles (steeper than NFW) are strongly disfavoured
for Sculptor.Comment: 2 pages, 4 figures, to appear in the proceedings of "Assembling the
Puzzle of the Milky Way", Le Grand Bornand (Apr. 17-22, 2011
Mass growth and mergers: direct observations of the luminosity function of LRG satellite galaxies out to z=0.7 from SDSS and BOSS images
We present a statistical study of the luminosity functions of galaxies
surrounding luminous red galaxies (LRGs) at average redshifts =0.34 and
=0.65. The luminosity functions are derived by extracting source photometry
around more than 40,000 LRGs and subtracting foreground and background
contamination using randomly selected control fields. We show that at both
studied redshifts the average luminosity functions of the LRGs and their
satellite galaxies are poorly fitted by a Schechter function due to a
luminosity gap between the centrals and their most luminous satellites. We
utilize a two-component fit of a Schechter function plus a log-normal
distribution to demonstrate that LRGs are typically brighter than their most
luminous satellite by roughly 1.3 magnitudes. This luminosity gap implies that
interactions within LRG environments are typically restricted to minor mergers
with mass ratios of 1:4 or lower. The luminosity functions further imply that
roughly 35% of the mass in the environment is locked in the LRG itself,
supporting the idea that mass growth through major mergers within the
environment is unlikely. Lastly, we show that the luminosity gap may be at
least partially explained by the selection of LRGs as the gap can be reproduced
by sparsely sampling a Schechter function. In that case LRGs may represent only
a small fraction of central galaxies in similar mass halos.Comment: ApJ accepted versio
Operando X-ray characterization of interfacial charge transfer and structural rearrangements
Key technologies in energy conversion and storage, sensing and chemical
synthesis rely on a detailed knowledge about charge transfer processes at
electrified solid-liquid interfaces. However, these interfaces continuously
evolve as a function of applied potentials, ionic concentrations and time. We
therefore need to characterize chemical composition, atomic arrangement and
electronic structure of both the liquid and the solid side of the interface
under operating conditions. In this chapter, we discuss the state-of-the-art
X-ray based spectroscopy and diffraction approaches for such 'operando'
characterization. We highlight recent examples from literature and demonstrate
how X-ray absorption spectroscopy, X-ray photoelectron spectroscopy and surface
X-ray diffraction can reveal the required interface-sensitive information
Orbit-based dynamical models of the Sculptor dSph galaxy
We have developed spherically symmetric dynamical models of dwarf spheroidal
galaxies using Schwarzschild's orbit superposition method. This type of
modelling yields constraints both on the total mass distribution (e.g. enclosed
mass and scale radius) as well as on the orbital structure of the system (e.g.
velocity anisotropy). This method is thus less prone to biases introduced by
assumptions in comparison to the more commonly used Jeans modelling, and it
allows us to derive the dark matter content in a robust way. Here we present
our results for the Sculptor dwarf spheroidal galaxy, after testing our methods
on mock data sets. We fit both the second and fourth velocity moment profile to
break the mass-anisotropy degeneracy. For an NFW dark matter halo profile, we
find that the mass of Sculptor within 1 kpc is M_1kpc = 1.03 \pm 0.07 x 10^8
Msol, and that its velocity anisotropy profile is tangentially biased and
nearly constant with radius. The preferred concentration (c ~ 15) is low for
its dark matter mass but consistent within the scatter found in N-body
cosmological simulations. When we let the value of the central logarithmic
slope \alpha{} vary, we find that the best-fit model has \alpha{} = 0, although
an NFW cusp or shallower is consistent at 1 \sigma{} confidence level. On the
other hand, very cuspy density profiles with logarithmic central slopes
\alpha{} < -1.5 are strongly disfavoured for Sculptor.Comment: 18 pages, 12 figures, accepted by MNRAS
http://mnras.oxfordjournals.org/content/433/4/317
New Constraints on the Efficiencies of Ram-Pressure Stripping and the Tidal Disruption of Satellite Galaxies
Using data from the Sloan Digital Sky Survey (SDSS) it has recently been
shown that the red fraction of satellite galaxies increases with stellar mass.
Semi-analytical models, however, predict red satellite fractions that are
independent of stellar mass, and much higher than observed. It has been argued
that this discrepancy owes to the fact that the models assume that satellite
galaxies are instantaneously stripped of their hot gas reservoirs at the moment
they are accreted into a bigger halo. In this letter we show that the fraction
of red satellites can be brought in better agreement with the data by simply
decreasing this stripping efficiency. However, this also results in a red
fraction of massive centrals that is much too low. This owes to the fact that
the massive centrals now accrete satellite galaxies that are bluer and more
gas-rich. However, if a significant fraction of low mass satellite galaxies is
tidally disrupted before being accreted by their central host galaxy, as
suggested by recent studies, the red fractions of both centrals and satellites
can be reproduced reasonably well. A problem remains with the red fraction of
centrals of intermediate mass, which is likely to reflect an oversimplified
treatment of AGN feedback.Comment: A few discussions added, updated to match the accepted version to ApJ
Letter
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Effects of plant pathogens on population dynamics and community composition in grassland ecosystems: two case studies
Grassland ecosystems comprise a major portion of the earth’s terrestrial surface, ranging from high-input cultivated monocultures or simple species mixtures to relatively unmanaged but dynamic systems. Plant pathogens are a component of these systems with their impact dependent on many interacting factors, including grassland species population dynamics and community composition, the topics covered in this paper. Plant pathogens are affected by these interactions and also act reciprocally by modifying their nature. We review these features of disease in grasslands and then introduce the 150-year long-term Park Grass Experiment (PGE) at Rothamsted Research in the UK. We then consider in detail two plant-pathogen systems present in the PGE, Tragopogon pratensis-Puccinia hysterium and Holcus lanata-Puccinia coronata. These two systems have very different life history characteristics: the first, a biennial member of the Asteraceae infected by its host-specific, systemic rust; the second, a perennial grass infected by a host-non-specific rust. We illustrate how observational, experimental and modelling studies can contribute to a better understanding of population dynamics, competitive interactions and evolutionary outcomes. With Tragopogon pratensis-Puccinia hysterium, characterised as an “outbreak” species in the PGE, we show that pathogen-induced mortality is unlikely to be involved in host population regulation; and that the presence of even a short-lived seed-bank can affect the qualitative outcomes of the host-pathogen dynamics. With Holcus lanata-Puccinia coronata, we show how nutrient conditions can affect adaptation in terms of host defence mechanisms, and that co-existence of competing species affected by a common generalist pathogen is unlikely
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