141 research outputs found
Gaseous drag on a gravitational perturber in Modified Newtonian Dynamics and the structure of the wake
We calculate the structure of a wake generated by, and the dynamical friction
force on, a gravitational perturber travelling through a gaseous medium of
uniform density and constant background acceleration g_ext, in the context of
Modified Newtonian Dynamics (MOND). The wake is described as a linear
superposition of two terms. The dominant part displays the same structure as
the wake generated in Newtonian gravity scaled up by a factor
mu^{-1}(g_ext/a_0), where a_{0} is the constant MOND acceleration and mu the
interpolating function. The structure of the second term depends greatly on the
angle between g_{ext} and and the velocity of the perturber. We evaluate the
dynamical drag force numerically and compare our MOND results with the
Newtonian case. We mention the relevance of our calculations to orbit evolution
of globular clusters and satellites in a gaseous proto-galaxy. Potential
differences in the X-ray emission of gravitational galactic wakes in MOND and
in Newtonian gravity with a dark halo are highlighted.Comment: 13 pages, 7 figures, accepted for publication in MNRA
Exploring cloudy gas accretion as a source of interstellar turbulence in the outskirts of disks
High--resolution 2D--MHD numerical simulations have been carried out to
investigate the effects of continuing infall of clumpy gas in extended HI
galactic disks. Given a certain accretion rate, the response of the disk
depends on its surface gas density and temperature. For Galactic conditions at
a galactocentric distance of ~20 kpc, and for mass accretion rates consistent
with current empirical and theoretical determinations in the Milky Way, the
rain of compact high velocity clouds onto the disk can maintain transonic
turbulent motions in the warm phase (~2500 K) of HI. Hence, the HI line width
is expected to be ~6.5 km/s for a gas layer at 2500 K, if infall were the only
mechanism of driving turbulence. Some statistical properties of the resulting
forcing flow are shown in this Letter. The radial dependence of the gas
velocity dispersion is also discussed.Comment: 13 pages, 3 figures, accepted for publication in ApJ Letter
Bounds on the mass and abundance of dark compact objects and black holes in dwarf spheroidal galaxy halos
We establish new dynamical constraints on the mass and abundance of compact
objects in the halo of dwarf spheroidal galaxies. In order to preserve
kinematically cold the second peak of the Ursa Minor dwarf spheroidal (UMi
dSph) against gravitational scattering, we place upper limits on the density of
compact objects as a function of their assumed mass. The mass of the dark
matter constituents cannot be larger than 1000 solar masses at a halo density
in UMi's core of 0.35 solar masses/pc^3. This constraint rules out a scenario
in which dark halo cores are formed by two-body relaxation processes. Our
bounds on the fraction of dark matter in compact objects with masses >3000
solar masses improve those based on dynamical arguments in the Galactic halo.
In particular, objects with masses solar masses can comprise no
more than a halo mass fraction . Better determinations of the
velocity dispersion of old overdense regions in dSphs may result in more
stringent constraints on the mass of halo objects. For illustration, if the
preliminary value of 0.5 km/s for the secondary peak of UMi is confirmed,
compact objects with masses above solar masses could be excluded
from comprising all its dark matter halo.Comment: 6 pages, 2 figures, accepted for publication in ApJ Letter
Dynamical Friction of Double Perturbers in a Gaseous Medium
In many astrophysical situations, as in the coalescence of supermassive black
hole pairs at gas rich galactic nuclei, the dynamical friction experienced by
an object is a combination of its own wake as well as the wakes of its
companions. Using a semi-analytic approach, we investigate the composite wake
due to, and the resulting drag forces on, double perturbers that are placed at
the opposite sides of the orbital center and move on a circular orbit in a
uniform gaseous medium. The circular orbit makes the wake of each perturber
asymmetric, creating an overdense tail at the trailing side. The tail not only
drags the perturber backward but it also exerts a positive torque on the
companion. For equal-mass perturbers, the positive torque created by the
companion wake is, on average, a fraction ~40-50% of the negative torque
created by its own wake, but this fraction may be even larger for perturbers
moving subsonically. This suggests that the orbital decay of a perturber in a
double system, especially in the subsonic regime, can take considerably longer
than in isolation. We provide the fitting formulae for the forces due to the
companion wake and discuss our results in light of recent numerical simulations
for mergers of binary black holes.Comment: 4 pages, 3 figures, accepted for publication in ApJ
The survival of dynamical fossils in dwarf spheroidal galaxies in conventional and modified dynamics
The survival of unbound density substructure against orbital mixing imposes
strong constraints on the slope of the underlying gravitational potential and
provides a new test on modified gravities. Here we investigate whether the
interpretation that the stellar clump in Ursa Minor (UMi) dwarf spheroidal
galaxy is a `dynamical fossil' is consistent with Modified Newtonian dynamics
(MOND). For UMi mass models inferred by fitting the velocity dispersion
profile, the stellar clump around the second peak of UMi is erased very
rapidly, within 1.25 Gyr (6.5 orbits), even with the inclusion of self-gravity.
We find that the clump can hardly survive for more than 2 Gyr even under more
generous conditions. Alternative scenarios which could lead to a kinematically
cold clump are discussed but, so far, none of them were found to be fully
satisfactory. Our conclusion is that the cold clump in UMi poses a challenge
for both LambdaCDM and MOND.Comment: 14 pages, 13 figures, accepted for publication in MNRA
Dark Matter Subhalos in the Ursa Minor Dwarf Galaxy
Through numerical simulations, we study the dissolution timescale of the Ursa
Minor cold stellar clump, due to the combination of phase-mixing and
gravitational encounters with compact dark substructures in the halo of Ursa
Minor. We compare two scenarios; one where the dark halo is made up by a smooth
mass distribution of light particles and one where the halo contains 10% of its
mass in the form of substructures (subhalos). In a smooth halo, the stellar
clump survives for a Hubble time provided that the dark matter halo has a big
core. In contrast, when the point-mass dark substructures are added, the clump
survives barely for \sim 1.5 Gyr. These results suggest a strong test to the
\Lambda-cold dark matter scenario at dwarf galaxy scale.Comment: accepted for publication in Ap
Gravitational drag on a point mass in hypersonic motion through a gaseous medium
We explore a ballistic orbit model to infer the gravitational drag force on
an accreting point mass M, such as a black hole, moving at a hypersonic
velocity v_{0} through a gaseous environment of density \rho_{0}. The
streamlines blend in the flow past the body and transfer momentum to it. The
total drag force acting on the body, including the nonlinear contribution of
those streamlines with small impact parameter that bend significantly and pass
through a shock, can be calculated by imposing conservation of momentum. In
this fully analytic approach, the ambiguity in the definition of the lower
cut-off distance in calculations of the effect of dynamical
friction is removed. It turns out that .
Using spherical surfaces of control of different sizes, we carry out a
successful comparison between the predicted drag force and the one obtained
from a high resolution, axisymmetric, isothermal flow simulation. We
demonstrate that ballistic models are reasonably successful in accounting for
both the accretion rate and the gravitational drag.Comment: 8 pages, 6 figures, accepted to MNRA
Dynamical friction in a gaseous medium with a large-scale magnetic field
The dynamical friction force experienced by a massive gravitating body moving
through a gaseous medium is modified by sufficiently strong large-scale
magnetic fields. Using linear perturbation theory, we calculate the structure
of the wake generated by, and the gravitational drag force on, a body traveling
in a straight-line trajectory in a uniformly magnetized medium. The functional
form of the drag force as a function of the Mach number (V_0/c_s, where V_0 is
the velocity of the body and c_s the sound speed) depends on the strength of
the magnetic field and on the angle between the velocity of the perturber and
the direction of the magnetic field. In particular, the peak value of the drag
force is not near Mach number 1 for a perturber moving in a sufficiently
magnetized medium. As a rule of thumb, we may state that for supersonic motion,
magnetic fields act to suppress dynamical friction; for subsonic motion,
magnetic fields tend to enhance dynamical friction. For perturbers moving along
the magnetic field lines, the drag force at some subsonic Mach numbers may be
stronger than it is at supersonic velocities. We also mention the relevance of
our findings to black hole coalescence in galactic nuclei.Comment: 21 pages, 14 figures, accepted for publication in Ap
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