8,073 research outputs found
MaGICC-WDM: the effects of warm dark matter in hydrodynamical simulations of disc galaxy formation
We study the effect of warm dark matter (WDM) on hydrodynamic simulations of
galaxy formation as part of the Making Galaxies in a Cosmological Context
(MaGICC) project. We simulate three different galaxies using three WDM
candidates of 1, 2 and 5 keV and compare results with pure cold dark matter
simulations. WDM slightly reduces star formation and produces less centrally
concentrated stellar profiles. These effects are most evident for the 1 keV
candidate but almost disappear for keV. All simulations
form similar stellar discs independent of WDM particle mass. In particular, the
disc scale length does not change when WDM is considered. The reduced amount of
star formation in the case of 1 keV particles is due to the effects of WDM on
merging satellites which are on average less concentrated and less gas rich.
The altered satellites cause a reduced starburst during mergers because they
trigger weaker disc instabilities in the main galaxy. Nevertheless we show that
disc galaxy evolution is much more sensitive to stellar feedback than it is to
WDM candidate mass. Overall we find that WDM, especially when restricted to
current observational constraints ( keV), has a minor
impact on disc galaxy formation.Comment: 13 pages, 9 figures, 2 tables; minor clarifications added in results
section, conclusions unchanged; accepted for publication in MNRA
Two-Stream Instability of Counter-Rotating Galaxies
The present study of the two-stream instability in stellar disks with
counter-rotating components of stars and/or gas is stimulated by recently
discovered counter-rotating spiral and S0 galaxies. Strong linear two-stream
instability of tightly-wrapped spiral waves is found for one and two-armed
waves with the pattern angular speed of the unstable waves always intermediate
between the angular speed of the co-rotating matter () and that of the
counter-rotating matter (). The instability arises from the
interaction of positive and negative energy modes in the co- and
counter-rotating components. The unstable waves are in general convective -
they move in radius and radial wavenumber space - with the result that
amplification of the advected wave is more important than the local growth
rate. For a galaxy of co-rotating stars and counter-rotating stars of
mass-fraction , or of counter-rotating gas of mass-fraction
, the largest amplification is usually for the one-armed
leading waves (with respect to the co-rotating stars). For the case of both
counter-rotating stars and gas, the largest amplifications are for , also for one-armed leading waves. The two-armed trailing
waves usually have smaller amplifications. The growth rates and amplifications
all decrease as the velocity spreads of the stars and/or gas increase. It is
suggested that the spiral waves can provide an effective viscosity for the gas
causing its accretion.Comment: 14 pages, submitted to ApJ. One table and 17 figures can be obtained
by sending address to R. Lovelace at [email protected]
A critical evaluation of the water supply and stormwater management performance of retrofittable domestic rainwater harvesting systems
Rainwater harvesting systems are often used as both an alternative water source and a stormwater management tool. Many studies have focused on the water-saving potential of these systems, but research into aspects that impact stormwater retention—such as demand patterns and climate change—is lacking. This paper investigates the short-term impact of demand on both water supply and stormwater management and examines future and potential performance over a longer time scale using climate change projections. To achieve this, data was collected from domestic rainwater harvesting systems in Broadhempston, UK, and used to create a yield-after-spillage model. The validation process showed that using constant demand as opposed to monitored data had little impact on accuracy. With regards to stormwater management, it was found that monitored households did not use all the non-potable available water, and that increasing their demand for this was the most effective way of increasing retention capacity based on the modelling study completed. Installing passive or active runoff control did not markedly improve performance. Passive systems reduced the outflow to greenfield runoff for the longest time, whereas active systems increased the outflow to a level substantially above roof runoff in the 30 largest events
The role of surface plasmons in the decay of image-potential states on silver surfaces
The combined effect of single-particle and collective surface excitations in
the decay of image-potential states on Ag surfaces is investigated, and the
origin of the long-standing discrepancy between experimental measurements and
previous theoretical predictions for the lifetime of these states is
elucidated. Although surface-plasmon excitation had been expected to reduce the
image-state lifetime, we demonstrate that the subtle combination of the spatial
variation of s-d polarization in Ag and the characteristic non-locality of
many-electron interactions near the surface yields surprisingly long
image-state lifetimes, in agreement with experiment.Comment: 4 pages, 2 figures, to appear in Phys. Rev. Let
Strategies to limit the impact of nematode pressure on sugarcane productivity in the Isis
Two trials were done in this project. One was a continuation of work started under a previous GRDC/SRDC-funded activity, 'Strategies to improve the integration of legumes into cane based farming systems'. This trial aimed to assess the impact of trash and tillage management options and nematicide application on nematodes and crop performance. Methods and results are contained in the following publication: Halpin NV, Stirling GR, Rehbein WE, Quinn B, Jakins A, Ginns SP. The impact of trash and tillage management options and nematicide application on crop performance and plant-parasitic nematode populations in a sugarcane/peanut farming system. Proc. Aust. Soc. Sugar Cane Technol. 37, 192-203. Nematicide application in the plant crop significantly reduced total numbers of plant parasitic nematodes (PPN) but there was no impact on yield. Application of nematicide to the ratoon crop significantly reduced sugar yield. The study confirmed other work demonstrating that implementation of strategies like reduced tillage reduced populations of total PPN, suggesting that the soil was more suppressive to PPN in those treatments. The second trial, a variety trial, demonstrated the limited value of nematicide application in sugarcane farming systems. This study has highlighted that growers shouldn’t view nematicides as a ‘cure all’ for paddocks that have historically had high PPN numbers. Nematicides have high mammalian toxicity, have the potential to contaminate ground water (Kookana et al. 1995) and are costly. The cost of nematicide used in R1 was approx. 350/ha, adding $3.50/t of cane in a 100 t/ha crop. Also, our study demonstrated that a single nematicide treatment at the application rate registered for sugarcane is not very effective in reducing populations of nematode pests. There appears to be some levels of resistance to nematodes within the current suite of varieties available to the southern canelands. For example the soil in plots that were growing Q183 had 560% more root knot nematodes / 200mL soil compared to plots that grew Q245. The authors see great value in investment into a nematode screening program that could rate varieties into groups of susceptibility to both major sugarcane nematode pests. Such a rating could then be built into a decision support ‘tree’ or tool to better enable producers to select varieties on a paddock by paddock basis
Vertical pairing of identical particles suspended in the plasma sheath
It is shown experimentally that vertical pairing of two identical
microspheres suspended in the sheath of a radio-frequency (rf) discharge at low
gas pressures (a few Pa), appears at a well defined instability threshold of
the rf power. The transition is reversible, but with significant hysteresis on
the second stage. A simple model, which uses measured microsphere resonance
frequencies and takes into account besides Coulomb interaction between
negatively charged microspheres also their interaction with positive ion wake
charges, seems to explain the instability threshold quite well.Comment: 4 pages, 6 figures. to appear in Phys. Rev. Lett. 86, May 14th (2001
VPLanet: The Virtual Planet Simulator
We describe a software package called VPLanet that simulates fundamental
aspects of planetary system evolution over Gyr timescales, with a focus on
investigating habitable worlds. In this initial release, eleven physics modules
are included that model internal, atmospheric, rotational, orbital, stellar,
and galactic processes. Many of these modules can be coupled simultaneously to
simulate the evolution of terrestrial planets, gaseous planets, and stars. The
code is validated by reproducing a selection of observations and past results.
VPLanet is written in C and designed so that the user can choose the physics
modules to apply to an individual object at runtime without recompiling, i.e.,
a single executable can simulate the diverse phenomena that are relevant to a
wide range of planetary and stellar systems. This feature is enabled by
matrices and vectors of function pointers that are dynamically allocated and
populated based on user input. The speed and modularity of VPLanet enables
large parameter sweeps and the versatility to add/remove physical phenomena to
assess their importance. VPLanet is publicly available from a repository that
contains extensive documentation, numerous examples, Python scripts for
plotting and data management, and infrastructure for community input and future
development.Comment: 75 pages, 34 figures, 10 tables, accepted to the Proceedings of the
Astronomical Society of the Pacific. Source code, documentation, and examples
available at https://github.com/VirtualPlanetaryLaboratory/vplane
Fractional Quantum Hall States of Clustered Composite Fermions
The energy spectra and wavefunctions of up to 14 interacting quasielectrons
(QE's) in the Laughlin nu=1/3 fractional quantum Hall (FQH) state are
investigated using exact numerical diagonalization. It is shown that at
sufficiently high density the QE's form pairs or larger clusters. This
behavior, opposite to Laughlin correlations, invalidates the (sometimes
invoked) reapplication of the composite fermion picture to the individual QE's.
The series of finite-size incompressible ground states are identified at the QE
filling factors nu_QE=1/2, 1/3, 2/3, corresponding to the electron fillings
nu=3/8, 4/11, 5/13. The equivalent quasihole (QH) states occur at nu_QH=1/4,
1/5, 2/7, corresponding to nu=3/10, 4/13, 5/17. All these six novel FQH states
were recently discovered experimentally. Detailed analysis indicates that QE or
QH correlations in these states are different from those of well-known FQH
electron states (e.g., Laughlin or Moore-Read states), leaving the origin of
their incompressibility uncertain. Halperin's idea of Laughlin states of QP
pairs is also explored, but is does not seem adequate.Comment: 14 pages, 9 figures; revision: 1 new figure, some new references,
some new data, title chang
Simulations of Damped Lyman-Alpha and Lyman Limit Absorbers in Different Cosmologies: Implications for Structure Formation at High Redshift
We use hydrodynamic cosmological simulations to study damped Lyman-alpha
(DLA) and Lyman limit (LL) absorption at redshifts z=2-4 in five variants of
the cold dark matter scenario. Our standard simulations resolve the formation
of dense concentrations of neutral gas in halos with circular velocity v_c
roughly 140 km/s for Omega_m=1 and 90 km/s for Omega_m=0.4, at z=2; an
additional LCDM simulation resolves halos down to v_c approximately 50 km/s at
z=3. We find a clear relation between HI column density and projected distance
to the center of the nearest galaxy, with DLA absorption usually confined to
galactocentric radii less than 10-15 kpc and LL absorption arising out to
projected separations of 30 kpc or more. Detailed examination provides evidence
of non-equilibrium effects on absorption cross-section. If we consider only
absorption in the halos resolved by our standard simulations, then all five
models fall short of reproducing the observed abundance of DLA and LL systems
at these redshifts. If we extrapolate to lower halo masses, we find all four
models are consistent with the observed abundance of DLA systems if the the
extrapolated behavior extends to circular velocities roughly 50-80 km/s, and
they may produce too much absorption if the relation continues to 40 km/s. Our
results suggest that LL absorption is closely akin to DLA absorption, arising
in less massive halos or at larger galactocentric radii but not caused by
processes acting on a radically different mass scale.Comment: 33 pages with 10 embedded EPS figures. Substantially revised and
updated from original version. Includes new high-resolution simulations.
Accepted for publication in the Ap
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