209 research outputs found
N-body simulations in modified Newtonian dynamics
We describe some results obtained with N-MODY, a code for N-body simulations
of collisionless stellar systems in modified Newtonian dynamics (MOND). We
found that a few fundamental dynamical processes are profoundly different in
MOND and in Newtonian gravity with dark matter. In particular, violent
relaxation, phase mixing and galaxy merging take significantly longer in MOND
than in Newtonian gravity, while dynamical friction is more effective in a MOND
system than in an equivalent Newtonian system with dark matter.Comment: 4 pages, no figures. To appear in EAS Publication Series (Proceedings
of Symposium 7 of the JENAM 2008, Vienna
The role of thermal evaporation in galaxy formation
In colour-magnitude diagrams most galaxies fall in either the ``blue cloud''
or the ``red sequence'', with the red sequence extending to significantly
brighter magnitudes than the blue cloud. The bright-end of the red sequence
comprises elliptical galaxies (Es) with boxy isophotes and luminosity profiles
with shallow central cores, while fainter Es have disky isophotes and power-law
inner surface-brightness (SB) profiles. An analysis of published data reveals
that the centres of galaxies with power-law central SB profiles have younger
stellar populations than the centres of cored galaxies. We argue that thermal
evaporation of cold gas by virial-temperature gas plays an important role in
determining these phenomena. In less massive galaxies, thermal evaporation is
not very efficient, so significant amounts of cold gas can reach the galaxy
centre and fill a central core with newly formed stars, consistent with the
young stellar ages of the cusps of Es with power-law SB profiles. In more
massive galaxies, cold gas is evaporated within a dynamical time, so star
formation is inhibited, and a core in the stellar density profile produced by
dissipationless dynamics cannot be refilled. The different observed properties
of AGN in higher-mass and lower-mass ellipticals are also explained because in
the former the central black holes invariably accrete hot gas, while in the
latter they typically accrete cold gas. An important consequence of our results
is that at the present time there cannot be blue, star-forming galaxies in the
most massive galactic halos, consistent with the observed truncation of the
blue cloud at L*. [abridged]Comment: MNRAS, accepted. Added discussion and references, conclusions
unchanged. 14 pages, 6 figures (2 color
Phase mixing in MOND
Dissipationless collapses in Modified Newtonian Dynamics (MOND) have been
studied by using our MOND particle-mesh N-body code, finding that the projected
density profiles of the final virialized systems are well described by Sersic
profiles with index m<4 (down to m~2 for a deep-MOND collapse). The simulations
provided also strong evidence that phase mixing is much less effective in MOND
than in Newtonian gravity. Here we describe "ad hoc" numerical simulations with
the force angular components frozen to zero, thus producing radial collapses.
Our previous findings are confirmed, indicating that possible differences in
radial orbit instability under Newtonian and MOND gravity are not relevant in
the present context.Comment: 10 pages, 3 figures. To appear in the Proceedings of the
International Workshop "Collective Phenomena in Macroscopic Systems", G.
Bertin, R. Pozzoli, M. Rome, and K.R. Sreenivasan, eds., World Scientific,
Singapor
Magnetorotational instability in cool cores of galaxy clusters
Clusters of galaxies are embedded in halos of optically thin, gravitationally
stratified, weakly magnetized plasma at the system's virial temperature. Due to
radiative cooling and anisotropic heat conduction, such intracluster medium
(ICM) is subject to local instabilities, which are combinations of the thermal,
magnetothermal and heat-flux-driven buoyancy instabilities. If the ICM rotates
significantly, its stability properties are substantially modified and, in
particular, also the magnetorotational instability (MRI) can play an important
role. We study simple models of rotating cool-core clusters and we demonstrate
that the MRI can be the dominant instability over significant portions of the
clusters, with possible implications for the dynamics and evolution of the cool
cores. Our results give further motivation for measuring the rotation of the
ICM with future X-ray missions such as ASTRO-H and ATHENA.Comment: 17 pages, 10 figures, accepted for publication in Journal of Plasma
Physics, Special Issue "Complex Plasma Phenomena in the Laboratory and in the
Universe
The effect of tides on the Sculptor dwarf spheroidal galaxy
Dwarf spheroidal galaxies (dSphs) appear to be some of the most dark matter
dominated objects in the Universe. Their dynamical masses are commonly derived
using the kinematics of stars under the assumption of equilibrium. However,
these objects are satellites of massive galaxies (e.g.\ the Milky Way) and thus
can be influenced by their tidal fields. We investigate the implication of the
assumption of equilibrium focusing on the Sculptor dSph by means of ad-hoc
-body simulations tuned to reproduce the observed properties of Sculptor
following the evolution along some observationally motivated orbits in the
Milky Way gravitational field. For this purpose, we used state-of-the-art
spectroscopic and photometric samples of Sculptor's stars. We found that the
stellar component of the simulated object is not directly influenced by the
tidal field, while the mass of the more diffuse DM halo is
stripped. We conclude that, considering the most recent estimate of the
Sculptor proper motion, the system is not affected by the tides and the stellar
kinematics represents a robust tracer of the internal dynamics. In the
simulations that match the observed properties of Sculptor, the present-day
dark-to-luminous mass ratio is within the stellar half-light radius
( kpc) and within the maximum radius of the analysed dataset
( kpc).Comment: 19 pages, 10 figures, accepted for publication in MNRAS. V3: updated
after editor comments See our playlist for simulation videos:
https://av.tib.eu/series/633/supplemental+videos+of+the+paper+the+effect+of+tides+on+the+sculptor+dwarf+spheroidal+galax
Globular clusters in modified Newtonian dynamics: velocity-dispersion profiles from self-consistent models
We test the modified Newtonian dynamics (MOND) theory with the
velocity-dispersion profiles of Galactic globular clusters populating the
outermost region of the Milky Way halo, where the Galactic acceleration is
lower than the characteristic MOND acceleration a_0. For this purpose, we
constructed self-consistent, spherical models of stellar systems in MOND, which
are the analogues of the Newtonian King models. The models are spatially
limited, reproduce well the surface-brightness profiles of globular clusters,
and have velocity-dispersion profiles that differ remarkably in shape from the
corresponding Newtonian models. We present dynamical models of six globular
clusters, which can be used to efficiently test MOND with the available
observing facilities. A comparison with recent spectroscopic data obtained for
NGC2419 suggests that the kinematics of this cluster might be hard to explain
in MOND.Comment: 13 pages, 9 figures, accepted for publication by MNRA
Galactic fountains and gas accretion
Star-forming disc galaxies such as the Milky Way need to accrete \gsim 1
of gas each year to sustain their star formation. This gas
accretion is likely to come from the cooling of the hot corona, however it is
still not clear how this process can take place. We present simulations
supporting the idea that this cooling and the subsequent accretion are caused
by the passage of cold galactic-fountain clouds through the hot corona. The
Kelvin-Helmholtz instability strips gas from these clouds and the stripped gas
causes coronal gas to condense in the cloud's wake. For likely parameters of
the Galactic corona and of typical fountain clouds we obtain a global accretion
rate of the order of that required to feed the star formation.Comment: 2 pages, 1 figure, to appear in "Hunting for the Dark: The Hidden
Side of Galaxy Formation", Malta, 19-23 Oct. 2009, eds. V.P. Debattista &
C.C. Popescu, AIP Conf. Se
Fountain-driven gas accretion by the Milky Way
Accretion of fresh gas at a rate of ~ 1 M_{sun} yr^{-1} is necessary in
star-forming disc galaxies, such as the Milky Way, in order to sustain their
star-formation rates. In this work we present the results of a new hydrodynamic
simulation supporting the scenario in which the gas required for star formation
is drawn from the hot corona that surrounds the star-forming disc. In
particular, the cooling of this hot gas and its accretion on to the disc are
caused by the passage of cold galactic fountain clouds through the corona.Comment: 2 pages, 1 figure. To appear in the proceedings of the conference
"Assembling the Puzzle of the Milky Way", Le Grand-Bornand 17-22 April 2011,
European Physical Journal, editors C. Reyl\'e, A. Robin and M. Schulthei
Dissipationless collapses in MOND
Dissipationless collapses in Modified Newtonian Dynamics (MOND) are studied by using a new particle-mesh N-body code based on our numerical MOND potential solver. We found that low surface-density end-products have shallower inner density profile, flatter radial velocity-dispersion profile, and more radially anisotropic orbital distribution than high surface-density end-products. The projected density profiles of the final virialized systems are well described by Sersic profiles with index m~4, down to m~2 for a deep-MOND collapse. Consistently with observations of elliptical galaxies, the MOND end-products, if interpreted in the context of Newtonian gravity, would appear to have little or no dark matter within the effective radius. However, we found impossible (under the assumption of constant mass-to-light ratio) to simultaneously place the resulting systems on the observed Kormendy, Faber-Jackson and Fundamental Plane relations of elliptical galaxies. Finally, the simulations provide strong evidence that phase mixing is less effective in MOND than in Newtonian gravity
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