12,221 research outputs found
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 cores all the way down
We use high resolution simulations of isolated dwarf galaxies to study the
physics of dark matter cusp-core transformations at the edge of galaxy
formation: M200 = 10^7 - 10^9 Msun. We work at a resolution (~4 pc minimum cell
size; ~250 Msun per particle) at which the impact from individual supernovae
explosions can be resolved, becoming insensitive to even large changes in our
numerical 'sub-grid' parameters. We find that our dwarf galaxies give a
remarkable match to the stellar light profile; star formation history;
metallicity distribution function; and star/gas kinematics of isolated dwarf
irregular galaxies. Our key result is that dark matter cores of size comparable
to the stellar half mass radius (r_1/2) always form if star formation proceeds
for long enough. Cores fully form in less than 4 Gyrs for the M200 = 10^8 Msun
and 14 Gyrs for the 10^9 Msun dwarf. We provide a convenient two parameter
'coreNFW' fitting function that captures this dark matter core growth as a
function of star formation time and the projected stellar half mass radius.
Our results have several implications: (i) we make a strong prediction that
if LCDM is correct, then 'pristine' dark matter cusps will be found either in
systems that have truncated star formation and/or at radii r > r_1/2; (ii)
complete core formation lowers the projected velocity dispersion at r_1/2 by a
factor ~2, which is sufficient to fully explain the 'too big to fail problem';
and (iii) cored dwarfs will be much more susceptible to tides, leading to a
dramatic scouring of the subhalo mass function inside galaxies and groups.Comment: 20 pages; 9 figures; final version to appear in MNRAS including typos
corrected in proo
Recommended from our members
Geometrical Comparison of Conventional and Gerotor-Type Positive Displacement Screw Machines
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
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
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
New multi-channel electron energy analyzer with cylindrically symmetrical electrostatic field
This paper discusses an electron energy analyzer with a cylindrically
symmetrical electrostatic field, designed for rapid Auger analysis. The device
was designed and built. The best parameters of the analyzer were estimated and
then experimentally verified.Comment: 5 pages, 4 figure
Extending the Globular Cluster System-Halo Mass Relation to the Lowest Galaxy Masses
High mass galaxies, with halo masses , reveal
a remarkable near-linear relation between their globular cluster (GC) system
mass and their host galaxy halo mass. Extending this relation to the mass range
of dwarf galaxies has been problematic due to the difficulty in measuring
independent halo masses. Here we derive new halo masses based on stellar and HI
gas kinematics for a sample of nearby dwarf galaxies with GC systems. We find
that the GC system mass--halo mass relation for galaxies populated by GCs holds
from halo masses of down to below
, although there is a substantial increase in scatter
towards low masses. In particular, three well-studied ultra diffuse galaxies,
with dwarf-like stellar masses, reveal a wide range in their GC-to-halo mass
ratios. We compare our GC system--halo mass relation to the recent model of El
Badry et al., finding that their fiducial model does not reproduce our data in
the low mass regime. This may suggest that GC formation needs to be more
efficient than assumed in their model, or it may be due to the onset of
stochastic GC occupation in low mass halos. Finally, we briefly discuss the
stellar mass-halo mass relation for our low mass galaxies with GCs, and we
suggest some nearby dwarf galaxies for which searches for GCs may be fruitful.Comment: 16 pages, 5 figures, accepted for publication in MNRA
A simplified model of the Martian atmosphere - Part 1: a diagnostic analysis
In this paper we derive a reduced-order approximation to the vertical and horizontal structure of a simplified model of the baroclinically unstable Martian atmosphere. The original model uses the full hydrostatic primitive equations on a sphere, but has only highly simplified schemes to represent the detailed physics of the Martian atmosphere, e.g. forcing towards a plausible zonal mean temperature state using Newtonian cooling. Three different norms are used to monitor energy conversion processes in the model and are then compared. When four vertical modes (the barotropic and first three baroclinic modes) are retained in the reduced-order approximation, the correlation norm captures approximately 90% of the variance, while the kinetic energy and total energy norms capture approximately 83% and 78% of the kinetic and total energy respectively. We show that the leading order Proper Orthogonal Decomposition (POD) modes represent the dominant travelling waves in the baroclinically-unstable, winter hemisphere. In part 2 of our study we will develop a hierarchy of truncated POD-Galerkin expansions of the model equations using up to four vertical modes
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