19 research outputs found
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
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
Dissipationless collapse, weak homology and central cores of elliptical galaxies
By means of high-resolution N-body simulations we revisited the
dissipationless collapse scenario for galaxy formation. We considered both
single-component collapses and collapses of a cold stellar distribution in a
live dark matter halo. Single-component collapses lead to stellar systems whose
projected profiles are fitted very well by the Sersic R^(1/m) law with 3.6 < m
< 8. The stellar end-products of collapses in a dark matter halo are still well
described by the R^(1/m) law, but with 1.9 < m < 12, where the lowest m values
are obtained when the halo is dominant. In all the explored cases the profiles
at small radii deviate from their global best-fit R^(1/m) model, being
significantly flatter. The break-radius values are comparable with those
measured in `core' elliptical galaxies, and are directly related to the
coldness of the initial conditions. The dissipationless collapse of initially
cold stellar distributions in pre-existing dark matter haloes may thus have a
role in determining the observed weak homology of elliptical galaxies.Comment: Accepted for publication in MNRAS (11 pages, 10 figures
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
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
Axisymmetric and triaxial MOND density-potential pairs
We present a simple method, based on the deformation of spherically symmetric
potentials, to construct explicit axisymmetric and triaxial MOND
density-potential pairs. General guidelines to the choice of suitable
deformations, so that the resulting density distribution is nowhere negative,
are presented. This flexible method offers for the first time the possibility
to study the MOND gravitational field for sufficiently general and realistic
density distributions without resorting to sophisticated numerical codes. The
technique is illustrated by constructing the MOND density-potential pair for a
triaxial galaxy model that, in the absence of deformation, reduces to the
Hernquist sphere. Such analytical solutions are also relevant to test and
validate numerical codes. Here we present a new numerical potential solver
designed to solve the MOND field equation for arbitrary density distributions:
the code is tested with excellent results against the analytic MOND triaxial
Hernquist model and the MOND razor-thin Kuzmin disk, and a simple application
is finally presented.Comment: 21 pages, 5 figures. Accepted for publication in the Astrophysical
Journa
Dissipationless collapses in Modified Newtonian Dynamics
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.Comment: 15 pages, 6 figures, Accepted for publication in Ap