401 research outputs found
Refining MOND interpolating function and TeVeS Lagrangian
The phenomena customly called Dark Matter or Modified Newtonian Dynamics
(MOND) have been argued by Bekenstein (2004) to be the consequences of a
covariant scalar field, controlled by a free function (related to the MOND
interpolating function) in its Lagrangian density. In the context of this
relativistic MOND theory (TeVeS), we examine critically the interpolating
function in the transition zone between weak and strong gravity. Bekenstein's
toy model produces too gradually varying functions and fits rotation curves
less well than the standard MOND interpolating function. However, the latter
varies too sharply and implies an implausible external field effect (EFE).
These constraints on opposite sides have not yet excluded TeVeS, but made the
zone of acceptable interpolating functions narrower. An acceptable "toy"
Lagrangian density function with simple analytical properties is singled out
for future studies of TeVeS in galaxies. We also suggest how to extend the
model to solar system dynamics and cosmology, and compare with strong lensing
data (see also astro-ph/0509590).Comment: accepted for publication in ApJ Letter
Loss of mass and stability of galaxies in MOND
The self-binding energy and stability of a galaxy in MOND-based gravity are
curiously decreasing functions of its center of mass acceleration towards
neighbouring mass concentrations. A tentative indication of this breaking of
the Strong Equivalence Principle in field galaxies is the RAVE-observed escape
speed in the Milky Way. Another consequence is that satellites of field
galaxies will move on nearly Keplerian orbits at large radii (100 - 500 kpc),
with a declining speed below the asymptotically constant naive MOND prediction.
But consequences of an environment-sensitive gravity are even more severe in
clusters, where member galaxies accelerate fast: no more Dark-Halo-like
potential is present to support galaxies, meaning that extended axisymmetric
disks of gas and stars are likely unstable. These predicted reappearance of
asymptotic Keplerian velocity curves and disappearance of "stereotypic
galaxies" in clusters are falsifiable with targeted surveys.Comment: 4 pages, 2 figures, ApJ Letter
Quasi integral of motion for axisymmetric potentials
We present an estimate of the third integral of motion for axisymmetric
three-dimensional potentials. This estimate is based on a Staeckel
approximation and is explicitly written as a function of the potential. We
tested this scheme for the Besancon Galactic model and two other disc-halo
models and find that orbits of disc stars have an accurately conserved third
quasi integral.
The accuracy ranges from of 0.1% to 1% for heights varying from z = 0~kpc to
z= 6 kpc and Galactocentric radii R from 5 to 15kpc.
We also tested the usefulness of this quasi integral in analytic distribution
functions of disc stellar populations: we show that the distribution function
remains approximately stationary and that it allows to recover the potential
and forces by applying Jeans equations to its moments.Comment: 9 pages, 9 figures, accepted for publication in Astron. and Astrophy
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
Potentials and distribution functions to be used for dynamical modeling with GAIA-like data
We present new tools to establish axisymmetric equilibrium models of the
Milky Way. The models we wish to establish are pairs (V,F) where V is the
gravitational potential generated by the whole mass distribution including the
dark matter, and F is the distribution function in phase space for late-type
tracer stars. We present a set of Stackel potentials that fit some fundamental
parameters of the Milky Way (mass density in the solar neighbourhood and Oort
constants). Then we define new component distribution functions that can be
combined with these potentials in order to reproduce kinematical data like
those that will be provided by GAIA.Comment: 2 pages, 2 figures, Proceeding of Les Houches summer school to appear
in J.PhysIV Franc
Is violation of Newton's second law possible?
Astrophysical observations (usually explained by dark matter) suggest that
classical mechanics could break down when the acceleration becomes extremely
small (the approach known as modified Newtonian dynamics, or MOND). I present
the first analysis of MOND manifestations in terrestrial (rather than
astrophysical) settings. A new effect is reported: around each equinox date, 2
spots emerge on the Earth where static bodies experience spontaneous
acceleration due to the possible violation of Newton's second law. Preliminary
estimates indicate that an experimental search for this effect can be feasible.Comment: 10 pages; minor changes to match the published versio
Testing MOND Over a Wide Acceleration Range in X-Ray Ellipticals
The gravitational fields of two isolated ellipticals, NGC 720 and NGC 1521,
have been recently measured to very large galactic radii (~100 and ~200 kpc),
assuming hydrostatic balance of the hot gas enshrouding them. They afford, for
the first time to my knowledge, testing MOND in ellipticals with force and
quality that, arguably, approach those of rotation-curve tests in disk
galaxies. In the context of MOND, it is noteworthy that the measured
accelerations span a wide range, from more than 10a0 to about a0/10,
unprecedented in individual ellipticals. I find that MOND predicts correctly
the measured dynamical mass runs (apart from a possible minor tension in the
inner few kpc of NGC 720, which might be due to departure from hydrostatic
equilibrium): The predicted mass discrepancy increases outward from none, near
the center, to ~10 at the outermost radii. The implications for the
MOND-vs-dark-matter controversy go far beyond the simple fact of two more
galaxies conforming to MOND.Comment: 5 pages, 2 figures. Minor changes to match published versio
Three-component Stackel potentials satisfying recent estimates of Milky Way parameters
We present a set of three-component Stackel potentials defined by five
parameters and designed to model the Milky Way. We review the fundamental
constraints that any model of the Milky Way must satisfy, including the most
recent ones derived from Hipparcos data, and we study how the parameters of the
presented potentials can vary in order to match these constraints. Five
different valid potentials are presented and analyzed in detail: they are
designed to be confronted with kinematical surveys in the future, by the
construction of three-integral analytic distribution functions.Comment: 13 pages, 5 figures, accepted for publication in MNRA
Modified Newtonian Dynamics in the Milky Way
Both microlensing surveys and radio-frequency observations of gas flow imply
that the inner Milky Way is completely dominated by baryons, contrary to the
predictions of standard cold dark matter (CDM) cosmology. We investigate the
predictions of the Modified Newtonian Dynamics (MOND) formula for the Galaxy
given the measured baryon distribution. Satisfactory fits to the
observationally determined terminal-velocity curve are obtained for different
choices of the MOND's interpolating function mu(x). However, with simple
analytical forms of mu(x), the local circular speed v_c(R_0) can be as large as
220 km/s only for values of the parameter a_0 that are excluded by observations
of NGC 3198. Only a numerically specified interpolating function can produce
v_c(R_0)=220 km/s, which is therefore an upper limit in MOND, while the
asymptotic velocity is predicted to be v_c(infty)=170+-5 km/s. The data are
probably not consistent with the functional form of mu(x) that has been
explored as a toy model in the framework of Bekenstein's covariant theory of
gravity.Comment: 7 pages, 4 figures, accepted for publication in MNRA
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