5,854 research outputs found
Memory and mutualism in species sustainability: a time-fractional Lotka-Volterra model with harvesting
We first present a predator-prey model for two species and then extend the
model to three species where the two predator species engage in mutualistic
predation. Constant effort harvesting and the impact of by-catch issue are also
incorporated. Necessary sufficient conditions for the existence and stability
of positive equilibrium points are examined. It is shown that harvesting is
sustainable, and the memory concept of the fractional derivative damps out
oscillations in the population numbers so that the system as a whole settles on
an equilibrium quicker than it would with integer time derivatives. Finally,
some possible physical explanations are given for the obtained results. It is
shown that the stability requires the memory concept in the model
Kinematically Cold Populations at Large Radii in the Draco and Ursa Minor Dwarf Spheroidals
We present projected velocity dispersion profiles for the Draco and Ursa
Minor (UMi) dwarf spheroidal galaxies based on 207 and 162 discrete stellar
velocities, respectively. Both profiles show a sharp decline in the velocity
dispersion outside ~30 arcmin (Draco) and ~40 arcmin (UMi). New, deep
photometry of Draco reveals a break in the light profile at ~25 arcmin. These
data imply the existence of a kinematically cold population in the outer parts
of both galaxies. Possible explanations of both the photometric and kinematic
data in terms of both equilibrium and non-equilibrium models are discussed in
detail. We conclude that these data challenge the picture of dSphs as simple,
isolated stellar systems.Comment: 5 pages, accepted for publication in ApJ Letter
The Kinematics of the Ultra-Faint Milky Way Satellites: Solving the Missing Satellite Problem
We present Keck/DEIMOS spectroscopy of stars in 8 of the newly discovered
ultra-faint dwarf galaxies around the Milky Way. We measure the velocity
dispersions of Canes Venatici I and II, Ursa Major I and II, Coma Berenices,
Hercules, Leo IV and Leo T from the velocities of 18 - 214 stars in each galaxy
and find dispersions ranging from 3.3 to 7.6 km/s. The 6 galaxies with absolute
magnitudes M_V < -4 are highly dark matter-dominated, with mass-to-light ratios
approaching 1000. The measured velocity dispersions are inversely correlated
with their luminosities, indicating that a minimum mass for luminous galactic
systems may not yet have been reached. We also measure the metallicities of the
observed stars and find that the 6 brightest of the ultra-faint dwarfs extend
the luminosity-metallicity relationship followed by brighter dwarfs by 2 orders
of magnitude in luminosity; several of these objects have mean metallicities as
low as [Fe/H] = -2.3 and therefore represent some of the most metal-poor known
stellar systems. We detect metallicity spreads of up to 0.5 dex in several
objects, suggesting multiple star formation epochs. Having established the
masses of the ultra-faint dwarfs, we re-examine the missing satellite problem.
After correcting for the sky coverage of the SDSS, we find that the ultra-faint
dwarfs substantially alleviate the discrepancy between the predicted and
observed numbers of satellites around the Milky Way, but there are still a
factor of ~4 too few dwarf galaxies over a significant range of masses. We show
that if galaxy formation in low-mass dark matter halos is strongly suppressed
after reionization, the simulated circular velocity function of CDM subhalos
can be brought into approximate agreement with the observed circular velocity
function of Milky Way satellite galaxies. [slightly abridged]Comment: 22 pages, 15 figures (12 in color), 6 tables, minor revisions in
response to referee report. Accepted for publication in Ap
Effect of a thin AlO_x layer on transition-edge sensor properties
We have studied the physics of transition-edge sensor (TES) devices with an
insulating AlOx layer on top of the device to allow implementation of more
complex detector geometries. By comparing devices with and without the
insulating film, we have observed significant additional noise apparently
caused by the insulator layer. In addition, AlOx was found to be a relatively
good thermal conductor. This adds an unforeseen internal thermal feature to the
system.Comment: 6 pages, 5 figures, Low Temperature Detectors 14 conferenc
Effective surface motion on a reactive cylinder of particles that perform intermittent bulk diffusion
In many biological and small scale technological applications particles may
transiently bind to a cylindrical surface. In between two binding events the
particles diffuse in the bulk, thus producing an effective translation on the
cylinder surface. We here derive the effective motion on the surface, allowing
for additional diffusion on the cylinder surface itself. We find explicit
solutions for the number of adsorbed particles at one given instant, the
effective surface displacement, as well as the surface propagator. In
particular sub- and superdiffusive regimes are found, as well as an effective
stalling of diffusion visible as a plateau in the mean squared displacement. We
also investigate the corresponding first passage and first return problems.Comment: 26 pages, 5 figure
A Dynamical Fossil in the Ursa Minor Dwarf Spheroidal Galaxy
The nearby Ursa Minor dwarf spheroidal (UMi dSph) is one of the most dark
matter dominated galaxies known, with a central mass to light ratio roughly
equal to 70. Somewhat anomalously, it appears to contain morphological
substructure in the form of a second peak in the stellar number density. It is
often argued that this substructure must be transient because it could not
survive for the > 10 Gyr age of the system, given the crossing time implied by
UMi's 8.8 km/s internal velocity dispersion. In this paper, however, we present
evidence that the substructure has a cold kinematical signature, and argue that
UMi's clumpiness could indeed be a primordial artefact. Using numerical
simulations, we demonstrate that substructure is incompatible with the cusped
dark matter haloes predicted by the prevailing Cold Dark Matter (CDM) paradigm,
but is consistent with an unbound stellar cluster sloshing back and forth
within the nearly harmonic potential of a cored dark matter halo. Thus CDM
appears to disagree with observation at the least massive, most dark matter
dominated end of the galaxy mass spectrum.Comment: Astrophysical Journal (Letters), in pres
Galactic Archeology with 4MOST
4MOST is a new wide-field, high-multiplex spectroscopic survey facility for
the VISTA telescope of ESO. Starting in 2022, 4MOST will deploy more than 2400
fibres in a 4.1 square degree field-of-view using a positioner based on the
tilting spine principle. In this ontribution we give an outline of the major
science goals we wish to achieve with 4MOST in the area of Galactic Archeology.
The 4MOST Galactic Archeology surveys have been designed to address
long-standing and far-reaching problems in Galactic science. They are focused
on our major themes: 1) Near-field cosmology tests, 2) Chemo-dynamical
characterisation of the major Milky Way stellar components, 3) The Galactic
Halo and beyond, and 4) Discovery and characterisation of extremely metal-poor
stars. In addition to a top-level description of the Galactic surveys we
provide information about how the community will be able to join 4MOST via a
call for Public Spectroscopic Surveys that ESO will launch.Comment: To be published in "Rediscovering our Galaxy", IAU Symposium 334,
Eds. C. Chiappini, I. Minchev, E. Starkenburg, M. Valentin
Halo Cores and Phase Space Densities: Observational Constraints on Dark Matter Physics and Structure Formation
We explore observed dynamical trends in a wide range of dark matter dominated
systems (about seven orders of magnitude in mass) to constrain hypothetical
dark matter candidates and scenarios of structure formation. First, we argue
that neither generic warm dark matter (collisionless or collisional) nor
self-interacting dark matter can be responsible for the observed cores on all
scales. Both scenarios predict smaller cores for higher mass systems, in
conflict with observations; some cores must instead have a dynamical origin.
Second, we show that the core phase space densities of dwarf spheroidals,
rotating dwarf and low surface brightness galaxies, and clusters of galaxies
decrease with increasing velocity dispersion like Q ~ sigma^-3 ~ M^-1, as
predicted by a simple scaling argument based on merging equilibrium systems,
over a range of about eight orders of magnitude in Q. We discuss the processes
which set the overall normalization of the observed phase density hierarchy. As
an aside, we note that the observed phase-space scaling behavior and density
profiles of dark matter halos both resemble stellar components in elliptical
galaxies, likely reflecting a similar collisionless, hierarchical origin. Thus,
dark matter halos may suffer from the same systematic departures from homology
as seen in ellipticals, possibly explaining the shallower density profiles
observed in low mass halos. Finally, we use the maximum observed phase space
density in dwarf spheroidal galaxies to fix a minimum mass for relativistically
decoupled warm dark matter candidates of roughly 700 eV for thermal fermions,
and 300 eV for degenerate fermions.Comment: Submitted to the Astrophysical Journal, LaTeX, 26 pages including 4
pages of figure
Modelling the dynamical evolution of the Bootes dwarf spheroidal galaxy
We investigate a wide range of possible evolutionary histories for the
recently discovered Bootes dwarf spheroidal galaxy, a Milky Way satellite. By
means of N-body simulations we follow the evolution of possible progenitor
galaxies of Bootes for a variety of orbits in the gravitational potential of
the Milky Way. The progenitors considered cover the range from dark-matter-free
star clusters to massive, dark-matter dominated outcomes of cosmological
simulations. For each type of progenitor and orbit we compare the observable
properties of the remnant after 10 Gyr with those of Bootes observed today. Our
study suggests that the progenitor of Bootes must have been, and remains now,
dark matter dominated. In general our models are unable to reproduce the
observed high velocity dispersion in Bootes without dark matter. Our models do
not support time-dependent tidal effects as a mechanism able to inflate
significantly the internal velocity dispersion. As none of our initially
spherical models is able to reproduce the elongation of Bootes, our results
suggest that the progenitor of Bootes may have had some intrinsic flattening.
Although the focus of the present paper is the Bootes dwarf spheroidal, these
models may be of general relevance to understanding the structure, stability
and dark matter content of all dwarf spheroidal galaxies.Comment: 10 pages, 7 figures, accepted by MNRA
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