6,497 research outputs found
The tidal stripping of satellites
We present an improved analytic calculation for the tidal radius of
satellites and test our results against N-body simulations.
The tidal radius in general depends upon four factors: the potential of the
host galaxy, the potential of the satellite, the orbit of the satellite and
{\it the orbit of the star within the satellite}. We demonstrate that this last
point is critical and suggest using {\it three tidal radii} to cover the range
of orbits of stars within the satellite. In this way we show explicitly that
prograde star orbits will be more easily stripped than radial orbits; while
radial orbits are more easily stripped than retrograde ones. This result has
previously been established by several authors numerically, but can now be
understood analytically. For point mass, power-law (which includes the
isothermal sphere), and a restricted class of split power law potentials our
solution is fully analytic. For more general potentials, we provide an equation
which may be rapidly solved numerically. Over short times (\simlt 1-2 Gyrs
satellite orbit), we find excellent agreement between our analytic and
numerical models. Over longer times, star orbits within the satellite are
transformed by the tidal field of the host galaxy. In a Hubble time, this
causes a convergence of the three limiting tidal radii towards the prograde
stripping radius. Beyond the prograde stripping radius, the velocity dispersion
will be tangentially anisotropic.Comment: 10 pages, 5 figures. Final version accepted for publication in MNRAS.
Some new fully analytic tidal radii have been added for power law density
profiles (including the isothermal sphere) and some split power law
The mass of dwarf spheroidal galaxies and the missing satellite problem
We present the results from a suite of N-body simulations of the tidal
stripping of two-component dwarf galaxies comprising some stars and dark
matter. We show that recent kinematic data from the local group dwarf
spheroidal (dSph) galaxies suggests that dSph galaxies must be sufficiently
massive (M) that tidal stripping is of little
importance for the stars. We discuss the implications of these massive dSph
galaxies for cosmology and galaxy formation.Comment: 4 pages, 1 figure, to appear in the proceedings of the IAUC198
"Near-Field Cosmology with Dwarf Elliptical Galaxies", H. Jerjen & B.
Binggeli (eds.). Comments welcom
Alcohol: taking a population perspective
Alcohol consumption is a global phenomenon, as is the resultant health, social and economic harm. The nature of these harms varies with different drinking patterns and with the societal and political responses to the burden of harm; nevertheless, alcohol-related chronic diseases have a major effect on health. Strong evidence exists for the effectiveness of different strategies to minimize this damage and those policies that target price, availability and marketing of alcohol come out best, whereas those using education and information are much less effective. However, these policies can be portrayed as anti-libertarian and so viewing them in the context of alcohol-related harm to those other than the drinker, such as the most vulnerable in society, is important. When this strategy is successful, as in Scotland, it has been possible to pass strong and effective legislation, such as for a minimum unit price for alcohol
Further Evidence for a Merger Origin for the Thick Disk: Galactic Stars Along Lines-of-sight to Dwarf Spheroidal Galaxies
The history of the Milky Way Galaxy is written in the properties of its
stellar populations. Here we analyse stars observed as part of surveys of local
dwarf spheroidal galaxies, but which from their kinematics are highly probable
to be non-members. The selection function -- designed to target metal-poor
giants in the dwarf galaxies, at distances of ~100kpc -- includes F-M dwarfs in
the Milky Way, at distances of up to several kpc. Thestars whose motions are
analysed here lie in the cardinal directions of Galactic longitude l ~ 270 and
l ~ 90, where the radial velocity is sensitive to the orbital rotational
velocity. We demonstrate that the faint F/G stars contain a significant
population with V_phi ~ 100km/s, similar to that found by a targeted, but
limited in areal coverage, survey of thick-disk/halo stars by Gilmore, Wyse &
Norris (2002). This value of mean orbital rotation does not match either the
canonical thick disk or the stellar halo. We argue that this population,
detected at both l ~ 270 and l ~ 90, has the expected properties of `satellite
debris' in the thick-disk/halo interface, which we interpret as remnants of the
merger that heated a pre-existing thin disk to form the thick disk.Comment: Accepted, Astrophysical Journal Letter
Calculation of the unitary part of the Bures measure for N-level quantum systems
We use the canonical coset parameterization and provide a formula with the
unitary part of the Bures measure for non-degenerate systems in terms of the
product of even Euclidean balls. This formula is shown to be consistent with
the sampling of random states through the generation of random unitary
matrices
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 Tamm-Dancoff Approximation as the boson limit of the Richardson-Gaudin equations for pairing
A connection is made between the exact eigen states of the BCS Hamiltonian
and the predictions made by the Tamm-Dancoff Approximation. This connection is
made by means of a parametrised algebra, which gives the exact quasi-spin
algebra in one limit of the parameter and the Heisenberg-Weyl algebra in the
other. Using this algebra to construct the Bethe Ansatz solution of the BCS
Hamiltonian, we obtain parametrised Richardson-Gaudin equations, leading to the
secular equation of the Tamm-Dancoff Approximation in the bosonic limit. An
example is discussed in depth.Comment: Submitted to the proceedings of the Group28 conference
(Newcastle-upon-Tyne, UK). Journal of Physics: Conference Serie
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
First Clear Signature of an Extended Dark Matter Halo in the Draco Dwarf Spheroidal
We present the first clear evidence for an extended dark matter halo in the
Draco dwarf spheroidal galaxy based on a sample of new radial velocities for
159 giant stars out to large projected radii. Using a two parameter family of
halo models spanning a range of density profiles and velocity anisotropies, we
are able to rule out (at about the 2.5 sigma confidence level) haloes in which
mass follows light. The data strongly favor models in which the dark matter is
significantly more extended than the visible dwarf. However, haloes with
harmonic cores larger than the light distribution are also excluded. When
combined with existing measurements of the proper motion of Draco, our data
strongly suggest that Draco has not been tidally truncated within ~1 kpc. We
also show that the rising velocity dispersion at large radii represents a
serious problem for modified gravity (MOND).Comment: to be published in ApJL; 5 pages, 4 figure
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