289 research outputs found
Cores and Cusps in the Dwarf Spheroidals
We consider the problem of determining the structure of the dark halo of
nearby dwarf spheroidal galaxies (dSphs) from the spherical Jeans equations.
Whether the dark halos are cusped or cored at the centre is an important
strategic problem in modern astronomy. The observational data comprise the
line-of-sight velocity dispersion of a luminous tracer population. We show that
when such data are analysed to find the dark matter density with the spherical
Poisson and Jeans equations, then the generic solution is a dark halo density
that is cusped like an isothermal. Although milder cusps (like the
Navarro-Frenk-White 1/r cusp and even cores are possible, they are not generic.
Such solutions exist only if the anisotropy parameter beta and the logarithmic
slope of the stellar density gamma satisfy the constraint gamma = 2 x beta at
the centre or if the radial velocity dispersion falls to zero at the centre.
This surprisingly strong statement is really a consequence of the assumption of
spherical symmetry, and the consequent coordinate singularity at the origin.
So, for example, a dSph with an exponential light profile can exist in
Navarro-Frenk- White halo and have a flat velocity dispersion, but anisotropy
in general drives the dark halo solution to an isothermal cusp. The identified
cusp or core is therefore a consequence of the assumptions (particularly of
spherical symmetry and isotropy), and not the data.Comment: MNRAS, in pres
Observed Properties of Dark Matter: dynamical studies of dSph galaxies
The Milky Way satellite dwarf spheroidal (dSph) galaxies are the smallest
dark matter dominated systems in the universe. We have underway dynamical
studies of the dSph to quantify the shortest scale lengths on which Dark Matter
is distributed, the range of Dark Matter central densities, and the density
profile(s) of DM on small scales. Current results suggest some surprises: the
central DM density profile is typically cored, not cusped, with scale sizes
never less than a few hundred pc; the central densities are typically 10-20
GeV/cc; no galaxy is found with a dark mass halo less massive than ~5.10^7
M_sun. We are discovering many more dSphs, which we are analysing to test the
generality of these results.Comment: Presented at UCLA Dark Matter 2006 conference, March 2006. In press
in Nuclear Physics
An inefficient dwarf: Chemical abundances and the evolution of the Ursa Minor dwarf spheroidal galaxy
We present detailed chemical element abundance ratios of 17 elements in three
metal poor stars in the Ursa Minor dwarf spheroidal galaxy, which we combine
with extant data from the literature to assess the predictions of a novel suite
of galaxy chemical evolution models. The spectroscopic data were obtained with
the Keck/HIRES instrument and revealed low metallicities of [Fe/H]=-2.12, -2.13
and -2.67 dex. While the most metal poor star in our sample shows an
overabundance of [Mn/Fe] and other Fe-peak elements, our overall findings are
in agreement with previous studies of this galaxy: elevated values of the
[alpha/Fe] ratios that are similar to, or only slightly lower than, the halo
values but with SN Ia enrichment at very low metallicity, as well as an
enhancement of the ratio of first to second peak neutron capture elements
[Y/Ba] with decreasing metallicity. The chemical evolution models which were
tailored to reproduce the metallicity distribution function of the dSph,
indicate that UMi had an extended star formation which lasted nearly 5 Gyr with
low efficiency and are able to explain the [Y/Ba] enhancement at low
metallicity for the first time. In particular, we show that the present day
lack of gas is probably due to continuous loss of gas from the system, which we
model as winds.Comment: 10 pages, 7 figures, table
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
Two-body relaxation in modified Newtonian dynamics
A naive extension to MOND of the standard computation of the two-body
relaxation time Tb implies that Tb is comparable to the crossing time
regardless of the number N of stars in the system. This computation is
questionable in view of the non-linearity of MOND's field equation. A
non-standard approach to the calculation of Tb is developed that can be
extended to MOND whenever discreteness noise generates force fluctuations that
are small compared to the mean-field force. It is shown that this approach
yields standard Newtonian results for systems in which the mean density profile
is either plane-parallel or spherical. In the plane-parallel case we find that
in the deep-MOND regime Tbb scales with N as in the Newtonian case, but is
shorter by the square of the factor by which MOND enhances the gravitational
force over its Newtonian value for the same system. Application of these
results to dwarf galaxies and groups and clusters of galaxies reveals that in
MOND luminosity segregation should be far advanced in groups and clusters of
galaxies, two body relaxation should have substantially modified the density
profiles of galaxy groups, while objects with masses in excess of ~10M_sun
should have spiralled to the centres of dwarf galaxies.(Abridged)Comment: 8 pages, accepted by MNRA
A Survey for Outer Satellites of Mars: Limits to Completeness
We surveyed the Hill sphere of Mars for irregular satellites. Our search
covered nearly the entire Hill Sphere, but scattered light from Mars excluded
the inner few arcminutes where the satellites Phobos and Deimos reside. No new
satellites were found to an apparent limiting red magnitude of 23.5, which
corresponds to radii of about 0.09 km using an albedo of 0.07.Comment: 5 figures (1 color), 2 Tables, to appear in AJ Nov. 200
- âŠ