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

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&

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

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

Galactic kinematics with modified Newtonian dynamics

We look for observational signatures that could discriminate between Newtonian and modified Newtonian (MOND) dynamics in the Milky Way, in view of the advent of large astrometric and spectroscopic surveys. Indeed, a typical signature of MOND is an apparent disk of "phantom" dark matter, which is uniquely correlated with the visible disk-density distribution. Due to this phantom dark disk, Newtonian models with a spherical halo have different signatures from MOND models close to the Galactic plane. The models can thus be differentiated by measuring dynamically (within Newtonian dynamics) the disk surface density at the solar radius, the radial mass gradient within the disk, or the velocity ellipsoid tilt angle above the Galactic plane. Using the most realistic possible baryonic mass model for the Milky Way, we predict that, if MOND applies, the local surface density measured by a Newtonist will be approximately 78 Msun/pc2 within 1.1 kpc of the Galactic plane, the dynamically measured disk scale-length will be enhanced by a factor of 1.25 with respect to the visible disk scale-length, and the local vertical tilt of the velocity ellipsoid at 1 kpc above the plane will be approximately 6 degrees. None of these tests can be conclusive for the present-day accuracy of Milky Way data, but they will be of prime interest with the advent of large surveys such as GAIA.Comment: 5 page

The inner structure of very massive elliptical galaxies: implications for the inside-out formation mechanism of z~2 galaxies

We analyze a sample of 23 supermassive elliptical galaxies (central velocity dispersion larger than 330 km s-1), drawn from the SDSS. For each object, we estimate the dynamical mass from the light profile and central velocity dispersion, and compare it with the stellar mass derived from stellar population models. We show that these galaxies are dominated by luminous matter within the radius for which the velocity dispersion is measured. We find that the sizes and stellar masses are tightly correlated, with Re ~ M*^{1.1}$, making the mean density within the de Vaucouleurs radius a steeply declining function of M*: rho_e ~ M*^{-2.2}. These scalings are easily derived from the virial theorem if one recalls that this sample has essentially fixed (but large) sigma_0. In contrast, the mean density within 1 kpc is almost independent of M*, at a value that is in good agreement with recent studies of z ~ 2 galaxies. The fact that the mass within 1 kpc has remained approximately unchanged suggests assembly histories that were dominated by minor mergers -- but we discuss why this is not the unique way to achieve this. Moreover, the total stellar mass of the objects in our sample is typically a factor of ~ 5 larger than that in the high redshift (z ~ 2) sample, an amount which seems difficult to achieve. If our galaxies are the evolved objects of the recent high redshift studies, then we suggest that major mergers were required at z > 1.5, and that minor mergers become the dominant growth mechanism for massive galaxies at z < 1.5.Comment: 11 pages, 8 figures, accepted in MNRA

The PN.S Elliptical Galaxy Survey: the dark matter in NGC 4494

We present new Planetary Nebula Spectrograph observations of the ordinary elliptical galaxy NGC 4494, resulting in positions and velocities of 255 PNe out to 7 effective radii (25 kpc). We also present new wide-field surface photometry from MMT/Megacam, and long-slit stellar kinematics from VLT/FORS2. The spatial and kinematical distributions of the PNe agree with the field stars in the region of overlap. The mean rotation is relatively low, with a possible kinematic axis twist outside 1 Re. The velocity dispersion profile declines with radius, though not very steeply, down to ~70 km/s at the last data point. We have constructed spherical dynamical models of the system, including Jeans analyses with multi-component LCDM-motivated galaxies as well as logarithmic potentials. These models include special attention to orbital anisotropy, which we constrain using fourth-order velocity moments. Given several different sets of modelling methods and assumptions, we find consistent results for the mass profile within the radial range constrained by the data. Some dark matter (DM) is required by the data; our best-fit solution has a radially anisotropic stellar halo, a plausible stellar mass-to-light ratio, and a DM halo with an unexpectedly low central density. We find that this result does not substantially change with a flattened axisymmetric model. Taken together with other results for galaxy halo masses, we find suggestions for a puzzling pattern wherein most intermediate-luminosity galaxies have very low concentration halos, while some high-mass ellipticals have very high concentrations. We discuss some possible implications of these results for DM and galaxy formation.Comment: 29 pages, 17 figures. MNRAS, accepte