2,055 research outputs found
Combined Solar System and rotation curve constraints on MOND
The Modified Newtonian Dynamics (MOND) paradigm generically predicts that the
external gravitational field in which a system is embedded can produce effects
on its internal dynamics. In this communication, we first show that this
External Field Effect can significantly improve some galactic rotation curves
fits by decreasing the predicted velocities of the external part of the
rotation curves. In modified gravity versions of MOND, this External Field
Effect also appears in the Solar System and leads to a very good way to
constrain the transition function of the theory. A combined analysis of the
galactic rotation curves and Solar System constraints (provided by the Cassini
spacecraft) rules out several classes of popular MOND transition functions, but
leaves others viable. Moreover, we show that LISA Pathfinder will not be able
to improve the current constraints on these still viable transition functions.Comment: 13 pages, 7 figures, accepted for publication in MNRA
The wedding of modified dynamics and non-exotic dark matter in galaxy clusters
We summarize the status of Modified Newtonian Dynamics (MOND) in galaxy
clusters. The observed acceleration is typically larger than the acceleration
threshold of MOND in the central regions, implying that some dark matter is
necessary to explain the mass discrepancy there. A plausible resolution of this
issue is that the unseen mass in MOND is in the form of ordinary neutrinos with
masses just below the experimentally detectable limit. In particular, we show
that the lensing mass reconstructions of the clusters 1E0657-56 (the bullet
cluster) and Cl0024+17 (the ring) do not pose a new challenge to this scenario.
However, the mass discrepancy for cool X-ray emitting groups, in which
neutrinos cannot cluster, pose a more serious problem, meaning that dark
baryons could present a more satisfactory solution to the problem of unseen
mass in MOND clusters.Comment: to appear in World Scientific, proceedings of DARK 200
The abundance of galaxy clusters in MOND: Cosmological simulations with massive neutrinos
We present a new Particle-Mesh cosmological N-body code for accurately
solving the modified Poisson equation of the Quasi Linear formulation of MOND.
We generate initial conditions for the Angus (2009) cosmological model, which
is identical to LCDM except that the cold dark matter is switched for a single
species of thermal sterile neutrinos. We set the initial conditions at z=250
for a (512 Mpc/h)^3 box with 256^3 particles and we evolve them down to z=0. We
clearly demonstrate the necessity of MOND for developing the large scale
structure in a hot dark matter cosmology and contradict the naive expectation
that MOND cannot form galaxy clusters. We find that the correct order of
magnitude of X-ray clusters (with T_X > 4.5 keV) can be formed, but that we
overpredict the number of very rich clusters and seriously underpredict the
number of lower mass clusters. The latter is a shortcoming of the resolution of
our simulations, whereas we suggest that the over production of very rich
clusters might be prevented by incorporating a MOND acceleration constant that
varies with redshift and an expansion history that cannot be described by the
usual Friedmann models. We present evidence that suggests the density profiles
of our simulated clusters are compatible with those of observed X-ray clusters
in MOND. It remains to be seen if the low mass end of the cluster mass function
can be reproduced and if the high densities of dark matter in the central 20
kpc of groups and clusters of galaxies, measured in the MOND framework, can be
achieved. As a last test, we computed the relative velocity between pairs of
halos within 10 Mpc and find that pairs with velocities larger than 3000 km/s
like the bullet cluster, can form without difficulty.Comment: 9 pages, 7 figur
Resolving the timing problem of the globular clusters orbiting the Fornax dwarf galaxy
We re-investigate the old problem of the survival of the five globular
clusters orbiting the Fornax dwarf galaxy in both standard and modified
Newtonian dynamics. For the first time in the history of the topic, we use
accurate mass models for the Fornax dwarf, obtained through Jeans modelling of
the recently published line of sight velocity dispersion data, and we are also
not resigned to circular orbits for the globular clusters. Previously conceived
problems stem from fixing the starting distances of the globulars to be less
than half the tidal radius. We relax this constraint since there is absolutely
no evidence for it and show that the dark matter paradigm, with either cusped
or cored dark matter profiles, has no trouble sustaining the orbits of the two
least massive globular clusters for a Hubble time almost regardless of their
initial distance from Fornax. The three most massive globulars can remain in
orbit as long as their starting distances are marginally outside the tidal
radius. The outlook for modified Newtonian dynamics is also not nearly as bleak
as previously reported. Although dynamical friction inside the tidal radius is
far stronger in MOND, outside dynamical friction is negligible due to the
absence of stars. This allows highly radial orbits to survive, but more
importantly circular orbits at distances more than 85% of Fornax's tidal radius
to survive indefinitely. The probability of the globular clusters being on
circular orbits at this distance compared with their current projected
distances is discussed and shown to be plausible. Finally, if we ignore the
presence of the most massive globular (giving it a large line of sight
distance) we demonstrate that the remaining four globulars can survive within
the tidal radius for the Hubble time with perfectly sensible orbits.Comment: 8 pages, 10 figures, 1 table, MNRAS in pres
Isolated and non-isolated dwarfs in terms of modified Newtonian dynamics
Within the framework of modified Newtonian dynamics (MOND) we investigate the
kinematics of two dwarf spiral galaxies belonging to very different
environments, namely KK 246 in the Local Void and Holmberg II in the M81 group.
A mass model of the rotation curve of KK 246 is presented for the first time,
and we show that its observed kinematics are consistent with MOND. We re-derive
the outer rotation curve of Holmberg II, by modelling its HI data cube, and
find that its inclination should be closer to face-on than previously derived.
This implies that Holmberg II has a higher rotation velocity in its outer
parts, which, although not very precisely constrained, is consistent with the
MOND prediction.Comment: Accepted in A&A as a Research Note. 6 pages, 3 figure
Analysis of galactic tides and stars on CDM microhalos
A special purpose N-body simulation has been built to understand the tidal
heating of the smallest dark matter substructures (10^{-6}\msun and 0.01pc)
from the grainy potential of the Milky Way due to individual stars in the disk
and the bulge. To test the method we first run simulations of single encounters
of microhalos with an isolated star, and compare with analytical predictions of
the dark particle bound fraction as a function of impact parameter. We then
follow the orbits of a set of microhalos in a realistic flattened Milky Way
potential. We concentrate on (detectable) microhalos passing near the Sun with
a range of pericenter and apocenter. Stellar perturbers near the orbital path
of a microhalo would exert stochachstic impulses, which we apply in a Monte
Carlo fashion according to the Besancon model for the distribution of stars of
different masses and ages in our Galaxy. Also incorporated are the usual
pericenter tidal heating and disk-shocking heating. We give a detailed
diagnosis of typical microhalos and find microhalos with internal tangential
anisotropy are slightly more robust than the ones with radial anisotropy. In
addition, the dark particles generally go through of a random walk in velocity
space and diffuse out of the microhalos.
We show that the typical destruction time scales are strongly correlated with
the stellar density averaged along a microhalo's orbit over the age of the
stellar disk. We also present the morphology of a microhalo at several epochs
which may hold the key to dark matter detections.Comment: 15 pages, 12 figure
Neutrinos as galactic dark matter in the Ursa Major galaxy group?
We present the analysis of 23 published rotation curves of disk galaxies
belonging to the Ursa Major group of galaxies, with kinematics free of
irregularities. The rotation curves are analysed in the context of MOND
(Modified Newtonian Dynamics). We add an extra component to the rotation curve
fits, in addition to the stellar and gaseous disks: a speculative halo of
constant density made of, e.g., neutrinos, which would solve the bulk of the
problem currently faced by MOND on rich galaxy clusters scales. We find that
this additional unseen mass density is poorly constrained (as expected a
priori, given that a neutrino halo never dominates the kinematics), but we also
find that the best-fit value is non-zero: rho = 3.8 x 10^{-27} g/cm^3, and that
a zero-density is marginally excluded with 87% confidence; also, the 95%
confidence upper limit for the density is rho = 9.6 x 10^{-27} g/cm^3. These
limits are slightly above the expectations from the Tremaine-Gunn phase space
constraints on ordinary 2 eV neutrinos, but in accordance with the maximum
density expected for one or two species of 5 eV sterile neutrinos.Comment: Accepted for publication in MNRAS Letters. 5 pages, 2 figure
Implications for dwarf spheroidal mass content from interloper removal
Using the caustic method, we identify the member stars of five dwarf
spheroidal (dSph) galaxies of the Milky Way, the smallest dark matter (DM)
dominated systems in the Universe. After our interloper rejection, we compute
line-of-sight velocity dispersion profiles that are substantially smoother than
previous results. Moreover, two dSphs have line-of-sight velocity dispersions
20% smaller than previous calculations suggested. Our Jeans modelling confirms
that the DM content interior to 300pc is roughly constant with satellite
luminosity. Finally, if we assume that MOND provides the true law of gravity,
our identification of interlopers implies that four dSphs have mass-to-light
ratios in agreement with stellar population synthesis models, whereas Carina
still has a mass-to-light ratio a factor of two too large and remains a problem
for MOND.Comment: A&A accepted, 13 pages, 14 figures, 4 table
Can MONDian vector theories explain the cosmic speed up ?
Generalized Einstein - Aether vector field models have been shown to provide,
in the weak field regime, modifications to gravity which can be reconciled with
the successfull MOND proposal. Very little is known, however, on the function
F(K) defining the vector field Lagrangian so that an analysis of the viability
of such theories at the cosmological scales has never been performed. As a
first step along this route, we rely on the relation between F(K) and the MOND
interpolating function to assign the vector field Lagrangian thus
obtaining what we refer to as "MONDian vector models". Since they are able by
construction to recover the MOND successes on galaxy scales, we investigate
whether they can also drive the observed accelerated expansion by fitting the
models to the Type Ia Supernovae data. Should be this the case, we have a
unified framework where both dark energy and dark matter can be seen as
different manifestations of a single vector field. It turns out that both
MONDian vector models are able to well fit the low redshift data on Type Ia
Supernovae, while some tension could be present in the high z regime.Comment: 15 pages, 5 tables, 4 figures, accepted for publication on Physical
Review
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