24 research outputs found
MOND and the Galaxies
We review galaxy formation and dynamics under the MOND hypothesis of modified
gravity, and compare to similar galaxies in Newtonian dynamics with dark
matter. The aim is to find peculiar predictions both to discriminate between
various hypotheses, and to make the theory progress through different
constraints, touching the interpolation function, or the fundamental
acceleration scale. Galaxy instabilities, forming bars and bulges at longer
term, evolve differently in the various theories, and help to bring
constraints, together with the observations of bar frequency. Dynamical
friction and the predicted merger rate could be a sensitive test of theories.
The different scenarios of galaxy formation are compared within the various
theories and observations.Comment: 8 pages, 7 figures, Invited paper to "The Invisible Universe
International Conference", ed. J-M. Alimi, A. Fuzfa, P-S. Corasaniti, AIP pu
Polar ring galaxies as tests of gravity
Polar ring galaxies are ideal objects with which to study the
three-dimensional shapes of galactic gravitational potentials since two
rotation curves can be measured in two perpendicular planes. Observational
studies have uncovered systematically larger rotation velocities in the
extended polar rings than in the associated host galaxies. In the dark matter
context, this can only be explained through dark halos that are systematically
flattened along the polar rings. Here, we point out that these objects can also
be used as very effective tests of gravity theories, such as those based on
Milgromian dynamics (MOND). We run a set of polar ring models using both
Milgromian and Newtonian dynamics to predict the expected shapes of the
rotation curves in both planes, varying the total mass of the system, the mass
of the ring with respect to the host, as well as the size of the hole at the
center of the ring. We find that Milgromian dynamics not only naturally leads
to rotation velocities being typically higher in the extended polar rings than
in the hosts, as would be the case in Newtonian dynamics without dark matter,
but that it also gets the shape and amplitude of velocities correct. Milgromian
dynamics thus adequately explains this particular property of polar ring
galaxies.Comment: 9 pages, 8 Figures, 1 Table, Accepted for publication by MNRA
Evolution of spiral galaxies in modified gravity: II- Gas dynamics
The stability of spiral galaxies is compared in modified Newtonian Dynamics
(MOND) and Newtonian dynamics with dark matter (DM). We extend our previous
simulations that involved pure stellar discs without gas, to deal with the
effects of gas dissipation and star formation. We also vary the interpolating
function between the MOND and Newtonian regime. Bar formation is compared in
both dynamics, from initial conditions identical in visible component. One
first result is that the MOND galaxy evolution is not affected by the choice of
the mu-function, it develops bars with the same frequency and strength. The
choice of the mu-function significantly changes the equivalent DM models, in
changing the dark matter to visible mass ratio and, therefore, changing the
stability. The introduction of gas shortens the timescale for bar formation in
the DM model, but is not significantly shortened in the MOND model. As a
consequence, with gas, the MOND and DM bar frequency histograms are now more
similar than without gas. The thickening of the plane occurs through vertical
resonance with the bar and peanut formation, and even more quickly with gas.
Since the mass gets more concentrated with gas, the radius of the peanut is
smaller, and the appearance of the pseudo-bulge is more boxy. The bar strength
difference is moderated by saturation, and feedback effects, like the bar
weakening or destruction by gas inflow due to gravity torques. Averaged over a
series of models representing the Hubble sequence, the MOND models have still
more bars, and stronger bars, than the equivalent DM models, better fitting the
observations. Gas inflows driven by bars produce accumulations at Lindblad
resonances, and MOND models can reproduce observed morphologies quite well, as
was found before in the Newtonian dynamics.Comment: 9 pages, 11 figures, accepted in A&
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
Tidal dwarf galaxies as a test of fundamental physics
Within the cold dark matter (CDM) framework tidal dwarf galaxies (TDGs)
cannot contain dark matter, so the recent results by Bournaud et al. (2007)
that 3 rotating TDGs do show significant evidence for being dark matter
dominated is inconsistent with the current concordance cosmological theory
unless yet another dark matter component is postulated. We confirm that the TDG
rotation curves are consistent with Newtonian dynamics only if either an
additional dark matter component is postulated, or if all 3 TDGs happen to be
viewed nearly edge-on, which is unlikely given the geometry of the tidal
debris. We also find that the observed rotation curves are very naturally
explained without any free parameters within the modified Newtonian dynamics
(MOND) framework if inclinations are adopted as derived by Bournaud et al. We
explore different inclination angles and two different assumptions about the
external field effect. The results do not change significantly, and we conclude
therefore that Newtonian dynamics has severe problems while MOND does
exceedingly well in explaining the observed rotation curves of the 3 TDGs
studied by Bournaud et al.Comment: Accepted for publication in A&A Letters, 5 pages, 3 figure
MOND and the dark baryons
We consider for the first time the implications on the modified gravity MOND
model of galaxies, of the presence of dark baryons, under the form of cold
molecular gas in galaxy discs. We show that MOND models of rotation curves are
still valid and universal, but the critical acceleration a0 separating the
Newtonian and MONDian regimes has a lower value. We quantify this modification,
as a function of the scale factor c between the total gas of the galaxy and the
measured atomic gas. The main analysis concerns 43 resolved rotation curves and
allows us to find the best pair (a0 = 0.96 10e-10 m.s-2, c = 3), which is also
compatible to the one obtained from a second method by minimizing the scatter
in the baryonic Tully-Fisher relation.Comment: 11 pages, 8 figures, accepted in A&
Tides in colliding galaxies
Long tails and streams of stars are the most noticeable upshots of galaxy
collisions. Their origin as gravitational, tidal, disturbances has however been
recognized only less than fifty years ago and more than ten years after their
first observations. This Review describes how the idea of galactic tides
emerged, in particular thanks to the advances in numerical simulations, from
the first ones that included tens of particles to the most sophisticated ones
with tens of millions of them and state-of-the-art hydrodynamical
prescriptions. Theoretical aspects pertaining to the formation of tidal tails
are then presented. The third part of the review turns to observations and
underlines the need for collecting deep multi-wavelength data to tackle the
variety of physical processes exhibited by collisional debris. Tidal tails are
not just stellar structures, but turn out to contain all the components usually
found in galactic disks, in particular atomic / molecular gas and dust. They
host star-forming complexes and are able to form star-clusters or even
second-generation dwarf galaxies. The final part of the review discusses what
tidal tails can tell us (or not) about the structure and content of present-day
galaxies, including their dark components, and explains how tidal tails may be
used to probe the past evolution of galaxies and their mass assembly history.
On-going deep wide-field surveys disclose many new low-surface brightness
structures in the nearby Universe, offering great opportunities for attempting
galactic archeology with tidal tails.Comment: 46 pages, 13 figures, Review to be published in "Tidal effects in
Astronomy and Astrophysics", Lecture Notes in Physics. Comments are most
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