107 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
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
The thickness of HI in galactic discs under MOND: theory and application to the Galaxy
The outskirts of galaxies are a very good laboratory for testing the nature
of the gravitational field at low accelerations. By assuming that the neutral
hydrogen gas is in hydrostatic equilibrium in the gravitational potential of
the host galaxy, the observed flaring of the gas layer can be used to test
modified gravities. For the first time we construct a simple framework to
derive the scaleheight of the neutral hydrogen gas disc in the MOND scenario
and apply this to the Milky Way. It is shown that using a constant gas velocity
dispersion of ~9 km/s, MOND is able to give a very good fit to the observed HI
flaring beyond a galactocentric distance of 17 kpc up to the last measured
point (~40 kpc). Between 10 and 16 kpc, however, the observed scaleheight is
about 40% more than what MOND predicts for the standard interpolating function
and 70% for the form suggested by Famaey & Binney. Given the uncertainties in
the non-thermal pressure support by cosmic rays and magnetic fields, MOND seems
to be a plausible alternative to dark matter in explaining the Milky Way
flaring. Studying the flaring of extended HI discs in external edge-on galaxies
may be a promising approach to assess the viability of MOND.Comment: 13 pages, 4 figure
An extensive study of dynamical friction in dwarf galaxies: the role of stars, dark matter, halo profiles and MOND
We investigate the in-spiraling timescales of globular clusters in dwarf
spheroidal (dSph) and dwarf elliptical (dE) galaxies, due to dynamical
friction. We address the problem of these timescales having been variously
estimated in the literature as much shorter than a Hubble time. Using
self-consistent two-component (dark matter and stars) models, we explore
mechanisms which may yield extended dynamical friction timescales in such
systems in order to explain why dwarf galaxies often show globular cluster
systems. As a general rule, dark matter and stars both give a comparable
contribution to the dynamical drag. By exploring various possibilities for
their gravitational make-up, it is shown that these studies help constrain the
parameters of the dark matter haloes in these galaxies, as well as to test
alternatives to dark matter. Under the assumption of a dark haloes having a
constant density core, dynamical friction timescales are naturally extended
upwards of a Hubble time. Cuspy dark haloes yield timescales 4.5
Gyr, for any dark halo parameters in accordance with observations of stellar
line-of-sight velocity dispersion in dwarf spheroidal galaxies. We find that
under the hypothesis of MOND dynamics, due to the enhanced dynamical drag of
the stars, the dynamical friction timescales would be extremely short. Taking
the well-measured structural parameters of the Fornax dSph and its globular
cluster system as a case study, we conclude that requiring dynamical friction
timescales comparable to the Hubble time strongly favours dark haloes with a
central core.Comment: 18 pages, four figures, final version, accepted in MNRA
An upper limit on the mass of the black hole in Ursa Minor dwarf galaxy
The well-established correlations between the mass of massive black holes
(BHs) in the nuclei of most studied galaxies and various global properties of
their hosting galaxy lend support to the idea that dwarf galaxies and globular
clusters could also host a BH in their centers. Direct kinematic detection of
BHs in dwarf spheroidal (dSph) galaxies are seriously hindered by the small
number of stars inside the gravitational influence region of the BH. The aim of
this Letter is to establish an upper dynamical limit on the mass of the
putative BH in the Ursa Minor (UMi) dSph galaxy. We present direct N-body
simulations of the tidal disruption of the dynamical fossil observed in UMi,
with and without a massive BH. We find that the observed substructure is
incompatible with the presence of a massive BH of (2-3)x10^4 Msun within the
core of UMi. These limits are consistent with the extrapolation of the
M_{BH}-sigma relation to the M_{BH}<10^6 Msun regime. We also show that the BH
may be off-center with respect to the center of symmetry of the whole galaxy.Comment: 6 pages, 3 figures, ApJL (in press
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