Combined Solar System and rotation curve constraints on MOND

The Modified Newtonian Dynamics (MOND) paradigm generically predicts that the external gravitational field in which a system is embedded can produce effects on its internal dynamics. In this communication, we first show that this External Field Effect can significantly improve some galactic rotation curves fits by decreasing the predicted velocities of the external part of the rotation curves. In modified gravity versions of MOND, this External Field Effect also appears in the Solar System and leads to a very good way to constrain the transition function of the theory. A combined analysis of the galactic rotation curves and Solar System constraints (provided by the Cassini spacecraft) rules out several classes of popular MOND transition functions, but leaves others viable. Moreover, we show that LISA Pathfinder will not be able to improve the current constraints on these still viable transition functions.Comment: 13 pages, 7 figures, accepted for publication in MNRA

The wedding of modified dynamics and non-exotic dark matter in galaxy clusters

We summarize the status of Modified Newtonian Dynamics (MOND) in galaxy clusters. The observed acceleration is typically larger than the acceleration threshold of MOND in the central regions, implying that some dark matter is necessary to explain the mass discrepancy there. A plausible resolution of this issue is that the unseen mass in MOND is in the form of ordinary neutrinos with masses just below the experimentally detectable limit. In particular, we show that the lensing mass reconstructions of the clusters 1E0657-56 (the bullet cluster) and Cl0024+17 (the ring) do not pose a new challenge to this scenario. However, the mass discrepancy for cool X-ray emitting groups, in which neutrinos cannot cluster, pose a more serious problem, meaning that dark baryons could present a more satisfactory solution to the problem of unseen mass in MOND clusters.Comment: to appear in World Scientific, proceedings of DARK 200

The abundance of galaxy clusters in MOND: Cosmological simulations with massive neutrinos

We present a new Particle-Mesh cosmological N-body code for accurately solving the modified Poisson equation of the Quasi Linear formulation of MOND. We generate initial conditions for the Angus (2009) cosmological model, which is identical to LCDM except that the cold dark matter is switched for a single species of thermal sterile neutrinos. We set the initial conditions at z=250 for a (512 Mpc/h)^3 box with 256^3 particles and we evolve them down to z=0. We clearly demonstrate the necessity of MOND for developing the large scale structure in a hot dark matter cosmology and contradict the naive expectation that MOND cannot form galaxy clusters. We find that the correct order of magnitude of X-ray clusters (with T_X > 4.5 keV) can be formed, but that we overpredict the number of very rich clusters and seriously underpredict the number of lower mass clusters. The latter is a shortcoming of the resolution of our simulations, whereas we suggest that the over production of very rich clusters might be prevented by incorporating a MOND acceleration constant that varies with redshift and an expansion history that cannot be described by the usual Friedmann models. We present evidence that suggests the density profiles of our simulated clusters are compatible with those of observed X-ray clusters in MOND. It remains to be seen if the low mass end of the cluster mass function can be reproduced and if the high densities of dark matter in the central 20 kpc of groups and clusters of galaxies, measured in the MOND framework, can be achieved. As a last test, we computed the relative velocity between pairs of halos within 10 Mpc and find that pairs with velocities larger than 3000 km/s like the bullet cluster, can form without difficulty.Comment: 9 pages, 7 figur

Resolving the timing problem of the globular clusters orbiting the Fornax dwarf galaxy

We re-investigate the old problem of the survival of the five globular clusters orbiting the Fornax dwarf galaxy in both standard and modified Newtonian dynamics. For the first time in the history of the topic, we use accurate mass models for the Fornax dwarf, obtained through Jeans modelling of the recently published line of sight velocity dispersion data, and we are also not resigned to circular orbits for the globular clusters. Previously conceived problems stem from fixing the starting distances of the globulars to be less than half the tidal radius. We relax this constraint since there is absolutely no evidence for it and show that the dark matter paradigm, with either cusped or cored dark matter profiles, has no trouble sustaining the orbits of the two least massive globular clusters for a Hubble time almost regardless of their initial distance from Fornax. The three most massive globulars can remain in orbit as long as their starting distances are marginally outside the tidal radius. The outlook for modified Newtonian dynamics is also not nearly as bleak as previously reported. Although dynamical friction inside the tidal radius is far stronger in MOND, outside dynamical friction is negligible due to the absence of stars. This allows highly radial orbits to survive, but more importantly circular orbits at distances more than 85% of Fornax's tidal radius to survive indefinitely. The probability of the globular clusters being on circular orbits at this distance compared with their current projected distances is discussed and shown to be plausible. Finally, if we ignore the presence of the most massive globular (giving it a large line of sight distance) we demonstrate that the remaining four globulars can survive within the tidal radius for the Hubble time with perfectly sensible orbits.Comment: 8 pages, 10 figures, 1 table, MNRAS in pres

Isolated and non-isolated dwarfs in terms of modified Newtonian dynamics

Within the framework of modified Newtonian dynamics (MOND) we investigate the kinematics of two dwarf spiral galaxies belonging to very different environments, namely KK 246 in the Local Void and Holmberg II in the M81 group. A mass model of the rotation curve of KK 246 is presented for the first time, and we show that its observed kinematics are consistent with MOND. We re-derive the outer rotation curve of Holmberg II, by modelling its HI data cube, and find that its inclination should be closer to face-on than previously derived. This implies that Holmberg II has a higher rotation velocity in its outer parts, which, although not very precisely constrained, is consistent with the MOND prediction.Comment: Accepted in A&A as a Research Note. 6 pages, 3 figure

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

Analysis of galactic tides and stars on CDM microhalos

A special purpose N-body simulation has been built to understand the tidal heating of the smallest dark matter substructures (10^{-6}\msun and 0.01pc) from the grainy potential of the Milky Way due to individual stars in the disk and the bulge. To test the method we first run simulations of single encounters of microhalos with an isolated star, and compare with analytical predictions of the dark particle bound fraction as a function of impact parameter. We then follow the orbits of a set of microhalos in a realistic flattened Milky Way potential. We concentrate on (detectable) microhalos passing near the Sun with a range of pericenter and apocenter. Stellar perturbers near the orbital path of a microhalo would exert stochachstic impulses, which we apply in a Monte Carlo fashion according to the Besancon model for the distribution of stars of different masses and ages in our Galaxy. Also incorporated are the usual pericenter tidal heating and disk-shocking heating. We give a detailed diagnosis of typical microhalos and find microhalos with internal tangential anisotropy are slightly more robust than the ones with radial anisotropy. In addition, the dark particles generally go through of a random walk in velocity space and diffuse out of the microhalos. We show that the typical destruction time scales are strongly correlated with the stellar density averaged along a microhalo's orbit over the age of the stellar disk. We also present the morphology of a microhalo at several epochs which may hold the key to dark matter detections.Comment: 15 pages, 12 figure

Are sterile neutrinos consistent with clusters, the CMB and MOND?

If a single sterile neutrino exists such that $m_{\nu_s}\sim11eV$, it can serendipitously solve all outstanding issues of the Modified Newtonian Dynamics. With it one can explain the dark matter of galaxy clusters without influencing individual galaxies, match the angular power spectrum of the cosmic microwave background and potentially fit the matter power spectrum. This model is flat with $\Omega_{\nu_s}\sim0.23$ and the usual baryonic and dark energy components, thus the Universe has the same expansion history as the \lcdm model and only differs at the galactic scale where the Modified Dynamics outperforms \lcdm significantly.Comment: 5 pages, 3 figures, 1 tabl

Milky Way Mass Models and MOND

Using the Tuorla-Heidelberg model for the mass distribution of the Milky Way, I determine the rotation curve predicted by MOND. The result is in good agreement with the observed terminal velocities interior to the solar radius and with estimates of the Galaxy's rotation curve exterior thereto. There are no fit parameters: given the mass distribution, MOND provides a good match to the rotation curve. The Tuorla-Heidelberg model does allow for a variety of exponential scale lengths; MOND prefers short scale lengths in the range 2.0 to 2.5 kpc. The favored value of scale length depends somewhat on the choice of interpolation function. There is some preference for the `simple' interpolation function as found by Famaey & Binney. I introduce an interpolation function that shares the advantages of the simple function on galaxy scales while having a much smaller impact in the solar system. I also solve the inverse problem, inferring the surface mass density distribution of the Milky Way from the terminal velocities. The result is a Galaxy with `bumps and wiggles' in both its luminosity profile and rotation curve that are reminiscent of those frequently observed in external galaxies.Comment: Accepted for publication in the Astrophysical Journal. 31 pages including 8 figures and 3 table