17,433 research outputs found
How to break the density-anisotropy degeneracy in spherical stellar systems
We present a new non-parametric Jeans code, GravSphere, that recovers the
density and velocity anisotropy of spherical stellar
systems, assuming only that they are in a steady-state. Using a large suite of
mock data, we confirm that with only line-of-sight velocity data, GravSphere
provides a good estimate of the density at the projected stellar half mass
radius, , but is not able to measure or ,
even with 10,000 tracer stars. We then test three popular methods for breaking
this degeneracy: using multiple populations with different
; using higher order `Virial Shape Parameters' (VSPs); and including
proper motion data.
We find that two populations provide an excellent recovery of
in-between their respective . However, even with a total of tracers, we are not able to well-constrain for either
population. By contrast, using 1000 tracers with higher order VSPs we are able
to measure over the range and broadly constrain
. Including proper motion data for all stars gives an even better
performance, with and well-measured over the range .
Finally, we test GravSphere on a triaxial mock galaxy that has axis ratios
typical of a merger remnant, . In this case, GravSphere can become
slightly biased. However, we find that when this occurs the data are poorly
fit, allowing us to detect when such departures from spherical symmetry become
problematic.Comment: 19 pages; 1 table; 11 Figures. Version accepted for publication in
MNRAS. (Minor changes from previously. Appendix B added showing decreasing
bias of VSP estimators with increasing sampling.
The case for a cold dark matter cusp in Draco
We use a new mass modelling method, GravSphere, to measure the central dark
matter density profile of the Draco dwarf spheroidal galaxy. Draco's star
formation shut down long ago, making it a prime candidate for hosting a
'pristine' dark matter cusp, unaffected by stellar feedback during galaxy
formation. We first test GravSphere on a suite of tidally stripped mock
'Draco'-like dwarfs. We show that we are able to correctly infer the dark
matter density profile of both cusped and cored mocks within our 95% confidence
intervals. While we obtain only a weak inference on the logarithmic slope of
these density profiles, we are able to obtain a robust inference of the
amplitude of the inner dark matter density at 150pc, . We show that, combined with constraints on the density profile at larger
radii, this is sufficient to distinguish a Cold Dark Matter
(CDM) cusp that has from alternative dark matter models
that have lower inner densities. We then apply GravSphere to the real Draco
data. We find that Draco has an inner dark matter density of , consistent with a CDM cusp. Using a velocity independent
SIDM model, calibrated on SIDM cosmological simulations, we show that
Draco's high central density gives an upper bound on the SIDM cross section of
at 99% confidence. We conclude that
the inner density of nearby dwarf galaxies like Draco provides a new and
competitive probe of dark matter models.Comment: 19 pages, 11 Figures. Final version accepted for publication in MNRA
Dark matter heats up in dwarf galaxies
Gravitational potential fluctuations driven by bursty star formation can
kinematically 'heat up' dark matter at the centres of dwarf galaxies. A key
prediction of such models is that, at a fixed dark matter halo mass, dwarfs
with a higher stellar mass will have a lower central dark matter density. We
use stellar kinematics and HI gas rotation curves to infer the inner dark
matter densities of eight dwarf spheroidal and eight dwarf irregular galaxies
with a wide range of star formation histories. For all galaxies, we estimate
the dark matter density at a common radius of 150pc, . We find that our sample of dwarfs falls into two
distinct classes. Those that stopped forming stars over 6Gyrs ago favour
central densities , consistent with cold dark matter cusps, while those with more
extended star formation favour , consistent with shallower dark matter cores. Using
abundance matching to infer pre-infall halo masses, , we show that
this dichotomy is in excellent agreement with models in which dark matter is
heated up by bursty star formation. In particular, we find that steadily decreases with increasing stellar mass-to-halo
mass ratio, . Our results suggest that, to leading order, dark
matter is a cold, collisionless, fluid that can be kinematically 'heated up'
and moved around.Comment: 22 pages, 10 Figures. Final version accepted for publication in MNRA
On the formation of dwarf galaxies and stellar halos
Using analytic arguments and a suite of very high resolution (10^3 Msun per
particle) cosmological hydro-dynamical simulations, we argue that high
redshift, z ~ 10, M ~ 10^8 Msun halos, form the smallest `baryonic building
block' (BBB) for galaxy formation. These halos are just massive enough to
efficiently form stars through atomic line cooling and to hold onto their gas
in the presence of supernovae winds and reionisation. These combined effects,
in particular that of the supernovae feedback, create a sharp transition: over
the mass range 3-10x10^7 Msun, the BBBs drop two orders ofmagnitude in stellar
mass. Below ~2x10^7 Msun, galaxies will be dark with almost no stars and no
gas. Above this scale is the smallest unit of galaxy formation: the BBB.
A small fraction (~100) of these gas rich BBBs fall in to a galaxy the size
of the Milky Way. Ten percent of these survive to become the observed LG dwarf
galaxies at the present epoch. Those in-falling halos on benign orbits which
keep them far away from the Milky Way or Andromeda manage to retain their gas
and slowly form stars - these become the smallest dwarf irregular galax ies;
those on more severe orbits lose their gas faster than they can form stars and
become the dwarf spheroidals. The remaining 90% of the BBBs will be accreted.
We show that this gives a metallicity and total stellar mass consistent with
the Milky Way old stellar halo (abridged).Comment: 15 pages, 7 figures, one figure added to match accepted version. Some
typos fixed. MNRAS in pres
Development and verification of design methods for ducts in a space nuclear shield
A practical method for computing the effectiveness of a space nuclear shield perforated by small tubing and cavities is reported. Performed calculations use solutions for a two dimensional transport code and evaluate perturbations of that solution using last flight estimates and other kernel integration techniques. In general, perturbations are viewed as a change in source strength of scattered radiation and a change in attenuation properties of the region
Interpreting Dark Matter Direct Detection Independently of the Local Velocity and Density Distribution
We demonstrate precisely what particle physics information can be extracted
from a single direct detection observation of dark matter while making
absolutely no assumptions about the local velocity distribution and local
density of dark matter. Our central conclusions follow from a very simple
observation: the velocity distribution of dark matter is positive definite,
f(v) >= 0. We demonstrate the utility of this result in several ways. First, we
show a falling deconvoluted recoil spectrum (deconvoluted of the nuclear form
factor), such as from ordinary elastic scattering, can be "mocked up" by any
mass of dark matter above a kinematic minimum. As an example, we show that dark
matter much heavier than previously considered can explain the CoGeNT excess.
Specifically, m_chi < m_Ge} can be in just as good agreement as light dark
matter, while m_\chi > m_Ge depends on understanding the sensitivity of Xenon
to dark matter at very low recoil energies, E_R ~ 6 keVnr. Second, we show that
any rise in the deconvoluted recoil spectrum represents distinct particle
physics information that cannot be faked by an arbitrary f(v). As examples of
resulting non-trivial particle physics, we show that inelastic dark matter and
dark matter with a form factor can both yield such a rise
Theoretical Model for the Semimetal Yb_4As_3
We present a model which can explain semiquantitatively a number of the
unusual properties of \mbox{YbAs}. The structural phase transition at
T_{\text{c}}\simeq300\,\mbox{K} is described by a band Jahn-Teller effect of
correlated electrons and is interpreted as a charge ordering of the Yb ions.
The low carrier concentration in the low-temperature phase follows from the
strong electron correlations of the 4f-holes on the Yb sites and can be viewed
as self-doping of charge-ordered chains. The observed heavy-fermion behaviour
is on a scale of T^\ast\simeq50\,\mbox{K} and is due to spinon-like
excitations in the Yb-chains. The appearance of a second low-energy
scale around 0.2\,K is due to the Fermi energy of the low-density carriers.Comment: 7 pages, REVTeX, 1 Postscript-figure separatel
Escape path complexity and its context dependency in Pacific blue-eyes (Pseudomugil signifer)
The escape trajectories animals take following a predatory attack appear to
show high degrees of apparent 'randomness' - a property that has been described
as 'protean behaviour'. Here we present a method of quantifying the escape
trajectories of individual animals using a path complexity approach. When fish
(Pseudomugil signifer) were attacked either on their own or in groups, we find
that an individual's path rapidly increases in entropy (our measure of
complexity) following the attack. For individuals on their own, this entropy
remains elevated (indicating a more random path) for a sustained period (10
seconds) after the attack, whilst it falls more quickly for individuals in
groups. The entropy of the path is context dependent. When attacks towards
single fish come from greater distances, a fish's path shows less complexity
compared to attacks that come from short range. This context dependency effect
did not exist, however, when individuals were in groups. Nor did the path
complexity of individuals in groups depend on a fish's local density of
neighbours. We separate out the components of speed and direction changes to
determine which of these components contributes to the overall increase in path
complexity following an attack. We found that both speed and direction measures
contribute similarly to an individual's path's complexity in absolute terms.
Our work highlights the adaptive behavioural tactics that animals use to avoid
predators and also provides a novel method for quantifying the escape
trajectories of animals.Comment: 9 page
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Midwinter suppression of baroclinic storm activity on Mars: observations and models
Baroclinic instability and intense traveling wave activity on Mars is well known to occur in “storm zones” (Hollingsworth et al. 1996) close to the edge of the advancing or retreating polar ice cap. Such activity usually sets in during Martian fall and continues until the onset of the summer season when large-scale instability mostly ceases as the atmosphere is no longer baroclinically unstable. The stormy season is typically characterized by large-scale, zonally-propagating waves with zonal wavenumbers m = 1-3, the lower wavenumber modes typically penetrating to considerable altitude though may also be surface-intensified.
As we show below, however, some observations suggest that this eddy activity does not persist uniformly throughout the autumn, winter and spring seasons, but appears to die down quite consistently within 10 sols or so either side of the winter solstice. This midwinter ‘solsticial pause’ appears to be a sufficiently consistent feature of each winter season in both hemispheres to be regarded as a significant feature of Martian climatology, and could affect a variety of aspects of Martian meteorology including global heat and momentum transport, occurrence of dust storms etc.
A somewhat similar phenomenon has also been documented for the Earth (e.g. Nakamura 1992; Penny et al. 2010), especially in relation to seasonal variations in the north Pacific storm tracks. The cause of this phenomenon is still not well established, though suggested mechanisms include the effects of enhanced barotropic shear (the so-called ‘barotropic governor’ (James & Gray 1986) and interactions with topography over central Asia.
In this presentation we examine evidence for this phenomenon in the assimilated record of Martian climate from the Thermal Emission Spectrometer on board the Mars Global Surveyor mission (MGSTES), in conjunction with the UK version of the LMD-Oxford-OU-IAA Mars GCM (Forget et al. 1999; Montabone et al. 2006; Lewis et al. 2007). This is further corroborated in other evidence from seasonal variations in the incidence of local and regional dust storms that owe their origin to circumpolar baroclinic storms. We also discuss the extent to which this ‘solsticial pause’ phenomenon is reproduced in stand-alone atmospheric models and present results of some simulations to test a number of hypotheses for its dynamical origin on Mars
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