18 research outputs found
Tidal stirring of Milky Way satellites: a simple picture with the integrated tidal force
Most of dwarf spheroidal galaxies in the Local Group were probably formed via
environmental processes like the tidal interaction with the Milky Way. We study
this process via N-body simulations of dwarf galaxies evolving on seven
different orbits around the Galaxy. The dwarf galaxy is initially composed of a
rotating stellar disk and a dark matter halo. Due to the action of tidal forces
it loses mass and the disk gradually transforms into a spheroid while stellar
motions become increasingly random. We measure the characteristic scale-length
of the dwarf, its maximum circular velocity, mass, shape and kinematics as a
function of the integrated tidal force along the orbit. The final properties of
the evolved dwarfs are remarkably similar if the total tidal force they
experienced was the same, independently of the actual size and eccentricity of
the orbit.Comment: 5 pages, 2 figures, contribution to the proceedings of JENAM 2010 in
Lisbon, Symposium 2 "Environment and the formation of galaxies: 30 years
later", comments welcom
Does the Fornax dwarf spheroidal have a central cusp or core?
The dark matter dominated Fornax dwarf spheroidal has five globular clusters
orbiting at ~1 kpc from its centre. In a cuspy CDM halo the globulars would
sink to the centre from their current positions within a few Gyrs, presenting a
puzzle as to why they survive undigested at the present epoch. We show that a
solution to this timing problem is to adopt a cored dark matter halo. We use
numerical simulations and analytic calculations to show that, under these
conditions, the sinking time becomes many Hubble times; the globulars
effectively stall at the dark matter core radius. We conclude that the Fornax
dwarf spheroidal has a shallow inner density profile with a core radius
constrained by the observed positions of its globular clusters. If the phase
space density of the core is primordial then it implies a warm dark matter
particle and gives an upper limit to its mass of ~0.5 keV, consistent with that
required to significantly alleviate the substructure problem.Comment: 6 pages, 5 figures, accepted for publication in MNRAS, high
resolution simulations include
On the physical origin of dark matter density profiles
The radial mass distribution of dark matter haloes is investigated within the
framework of the spherical infall model. We present a new formulation of
spherical collapse including non-radial motions, and compare the analytical
profiles with a set of high-resolution N-body simulations ranging from galactic
to cluster scales. We argue that the dark matter density profile is entirely
determined by the initial conditions, which are described by only two
parameters: the height of the primordial peak and the smoothing scale. These
are physically meaningful quantities in our model, related to the mass and
formation time of the halo. Angular momentum is dominated by velocity
dispersion, and it is responsible for the shape of the density profile near the
centre. The phase-space density of our simulated haloes is well described by a
power-law profile, rho/sigma^3 = 10^{1.46\pm0.04} (rho_c/Vvir^3)
(r/Rvir)^{-1.90\pm0.05}. Setting the eccentricity of particle orbits according
to the numerical results, our model is able to reproduce the mass distribution
of individual haloes.Comment: 12 pages, 13 figures, submitted to MNRA
A Method for Measuring (Slopes of) the Mass Profiles of Dwarf Spheroidal Galaxies
We introduce a method for measuring the slopes of mass profiles within dwarf
spheroidal (dSph) galaxies directly from stellar spectroscopic data and without
adopting a dark matter halo model. Our method combines two recent results: 1)
spherically symmetric, equilibrium Jeans models imply that the product of
halflight radius and (squared) stellar velocity dispersion provides an estimate
of the mass enclosed within the halflight radius of a dSph stellar component,
and 2) some dSphs have chemo-dynamically distinct stellar
\textit{sub}components that independently trace the same gravitational
potential. We devise a statistical method that uses measurements of stellar
positions, velocities and spectral indices to distinguish two dSph stellar
subcomponents and to estimate their individual halflight radii and velocity
dispersions. For a dSph with two detected stellar subcomponents, we obtain
estimates of masses enclosed at two discrete points in the same mass profile,
immediately defining a slope. Applied to published spectroscopic data, our
method distinguishes stellar subcomponents in the Fornax and Sculptor dSphs,
for which we measure slopes and , respectively. These
values are consistent with 'cores' of constant density within the central
few-hundred parsecs of each galaxy and rule out `cuspy' Navarro-Frenk-White
(NFW) profiles ( at all radii) with significance \ga
96% and \ga 99%, respectively. Tests with synthetic data indicate that our
method tends systematically to overestimate the mass of the inner stellar
subcomponent to a greater degree than that of the outer stellar subcomponent,
and therefore to underestimate the slope (implying that the stated NFW
exclusion levels are conservative).Comment: Accepted for publication in The Astrophysical Journal (added
references
Unbiased constraints on ultralight axion mass from dwarf spheroidal galaxies
It has been suggested that the internal dynamics of dwarf spheroidal galaxies
(dSphs) can be used to test whether or not ultralight axions with are a preferred dark matter candidate. However, comparisons
to theoretical predictions tend to be inconclusive for the simple reason that
while most cosmological models consider only dark matter, one observes only
baryons. Here we use realistic kinematic mock data catalogs of Milky Way dSph's
to show that the "mass-anisotropy degeneracy" in the Jeans equations leads to
biased bounds on the axion mass in galaxies with unknown dark matter halo
profiles. In galaxies with multiple chemodynamical components this bias can be
partly removed by modelling the mass enclosed within each subpopulation.
However, analysis of the mock data reveals that the least-biased constraints on
the axion mass result from fitting the luminosity-averaged velocity dispersion
of the individual chemodynamical components directly. Applying our analysis to
two dSph's with reported stellar subcomponents, Fornax and Sculptor, and
assuming that the halo profile has not been acted on by baryons, yields core
radii kpc and kpc respectively, and at 97.5\% confidence. These bounds are in tension with the
number of observed satellites derived from simple (but conservative) estimates
of the subhalo mass function in Milky Way-like galaxies. We discuss how
baryonic feedback might affect our results, and the impact of such a small
axion mass on the growth of structures in the Universe.Comment: 17 pages, 12 figures. Version to match MNRAS. Analysis extended to
anisotropic mocks. Main conclusions unchange
Dark Matter in the Milky Way's Dwarf Spheroidal Satellites
The Milky Way's dwarf spheroidal satellites include the nearest, smallest and
least luminous galaxies known. They also exhibit the largest discrepancies
between dynamical and luminous masses. This article reviews the development of
empirical constraints on the structure and kinematics of dSph stellar
populations and discusses how this phenomenology translates into constraints on
the amount and distribution of dark matter within dSphs. Some implications for
cosmology and the particle nature of dark matter are discussed, and some
topics/questions for future study are identified.Comment: A version with full-resolution figures is available at
http://www.cfa.harvard.edu/~mwalker/mwdsph_review.pdf; 70 pages, 22 figures;
invited review article to be published in Vol. 5 of the book "Planets, Stars,
and Stellar Systems", published by Springe