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

Gaseous Flows in Galaxies

The gas component plays a major role in the dynamics of spiral galaxies, because of its dissipative character, and its ability to exchange angular momentum with stars in the disk. Due to its small velocity dispersion, it triggers gravitational instabilities, and the corresponding non-axisymmetric patterns produce gravity torques, which mediate these angular momentum exchanges. When a srong bar pattern develops with the same pattern speed all over the disk, only gas inside corotation can flow towards the center. But strong bars are not long lived in presence of gas, and multiple-speed spiral patterns can develop between bar phases, and help the galaxy to accrete external gas flowing from cosmic filaments. The gas is then intermittently driven to the galaxy center, to form nuclear starbursts and fuel an active nucleus. The various time-scales of these gaseous flows are described.Comment: 10 pages, 6 figures, review paper in the Proceedings of the IAU Symposium 245, "Formation and Evolution of Galaxy Bulges", held at Oxford, U.K., July 2007, Eds. M. Bureau, E. Athanassoula, B. Barbu

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&

Quasi-linear formulation of MOND

A new formulation of MOND as a modified-potential theory of gravity is propounded. In effect, the theory dictates that the MOND potential phi produced by a mass distribution rho is a solution of the Poisson equation for the modified source density rho*=-(1/4 pi G)divergence(g), where g=nu(|gN|/a0)gN, and gN is the Newtonian acceleration field of rho. This makes phi simply the scalar potential of the algebraic acceleration field g. The theory thus involves solving only linear differential equations, with one nonlinear, algebraic step. It is derivable from an action, satisfies all the usual conservation laws, and gives the correct center-of-mass acceleration to composite bodies. The theory is akin in some respects to the nonlinear Poisson formulation of Bekenstein and Milgrom, but it is different from it, and is obviously easier to apply. The two theories are shown to emerge as natural modifications of a Palatini-type formulation of Newtonian gravity, and are members in a larger class of bi-potential theories.Comment: 23 pages. Published in MNRAS. Minor changes to match the published versio

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

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