Xufen Wu

Milky Way potentials in cold dark matter and MOdified Newtonian Dynamics. Is the Large Magellanic Cloud on a bound orbit

We compute the Milky Way potential in different cold dark matter (CDM) based models, and compare these with the modified Newtonian dynamics (MOND) framework. We calculate the axis ratio of the potential in various models, and find that isopotentials are less spherical in MOND than in CDM potentials. As an application of these models, we predict the escape velocity as a function of the position in the Galaxy. This could be useful in comparing with future data from planned or already-underway kinematic surveys (RAVE, SDSS, SEGUE, SIM, GAIA or the hypervelocity stars survey). In addition, the predicted escape velocity is compared with the recently measured high proper motion velocity of the Large Magellanic Cloud (LMC). To bind the LMC to the Galaxy in a MOND model, while still being compatible with the RAVE-measured local escape speed at the Suns position, we show that an external field modulus of less than

The mass and angular momentum distribution of simulated massive early-type galaxies to large radii

0.03 a_0

Galactic rotation curves, the baryon-to-dark-halo-mass relation and space–time scale invariance

is needed.

Galactic kinematics with modified Newtonian dynamics

We study the dark and luminous mass distributions, circular velocity curves (CVC), line-of-sight kinematics, and angular momenta for a sample of 42 cosmo- logical zoom simulations of galaxies with stellar masses from 2.0 × 10 10 M⊙ h −1 to 3.4× 10 11 M⊙ h −1 . Using a temporal smoothing technique, we are able to reach large radii. We find that: (i) The dark matter halo density profiles outside a few kpc follow simple power-law models, with flat dark matter CVCs for lower-mass systems, and rising CVCs for high-mass haloes. The projected stellar density distributions at large radii can be fitted by Sersic functions with n � 10, larger than for typical early-type galaxies (ETGs). (ii) The massive systems have nearly flat total (luminous plus dark matter) CVCs at large radii, while the less massive systems have mildly decreasing CVCs. The slope of the circular velocity at large radii correlates with circular velocity itself. (iii) The dark matter fractions within the projected stellar half mass radius Re are in the range 15-30% and increase to 40-65% at 5Re. Larger and more massive galaxies have higher dark matter fractions. The fractions and trends with mass and size are in agreement with observational estimates, even though the stellar-to-total mass ratio is �2-3 times higher than estimated for ETGs. (iv) The short axes of simulated galaxies and their host dark matter haloes are well aligned and their short-to-long axis ratios are correlated. (v) The stellar root mean square velocity vrms(R) profiles are slowly declining, in agreement with planetary nebulae observations in the outer haloes of most ETGs. (vi) The line-of-sight velocity fieldsshow that rotation properties at small and large radii are correlated. Most radial profiles for the cumulative specific angular momentum parameter λ(R) are nearly flat or slightly rising, with values in (0.06, 0.75) from 2Re to 5Re. A few cases show local maxima in |¯|/σ(R). These prop- erties agree with observations of ETGs at large radii. (vii) Stellar mass, ellipticity at large radii ǫ(5Re), and λ(5Re) are correlated: the more massive systems have less an- gular momentum and are rounder, as for observed ETGs. (viii) More massive galaxies with a large fraction of accreted stars have radially anisotropic velocity distributions outside Re. Tangential anisotropy is seen only for galaxies with high fraction of in-situ stars.

THE GALACTIC POTENTIAL AND THE ASYMMETRIC DISTRIBUTION OF HYPERVELOCITY STARS

Low-acceleration space-time scale invariant dynamics (SID, Milgrom 2009a) predicts two fundamental correlations known from observational galactic dynamics: the baryonic Tully-Fisher relation (BTFR) and a correlation between the observed mass discrepancy and acceleration (MDA) in the low acceleration regime for disc galaxies. SID corresponds to the deep MOdified Newtonian Dynamics (MOND) limit. The MDA data emerging in cold/warm dark matter (C/WDM) cosmological simulations disagree significantly with the tight MDA correlation of the observed galaxies. Therefore, the most modern simulated disc galaxies, which are delicately selected to have a quiet merging history in a standard dark-matter-cosmological model, still do not represent the correct rotation curves. Also, the observed tight correlation contradicts the postulated stochastic formation of galaxies in low-mass DM halos. Moreover, we find that SID predicts a baryonic to apparent virial halo (dark matter) mass relation which agrees well with the correlation deduced observationally assuming Newtonian dynamics to be valid, while the baryonic to halo mass relation predicted from CDM models does not. The distribution of the observed ratios of dark-matter halo masses to baryonic masses may be empirical evidence for the external field effect, which is predicted in SID as a consequence of the forces acting between two galaxies depending on the position and mass of a third galaxy. Applying the external field effect, we predict the masses of galaxies in the proximity of the dwarf galaxies in the Miller et al. sample. Classical non-relativistic gravitational dynamics is thus best described as being Milgromian, rather than Newtonian.

SELF-CONSISTENT MODELS OF TRIAXIAL GALAXIES IN MOND GRAVITY

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 di erent signatures from MOND models close to the Galactic plane. The models can thus be di erentiated 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 M =pc 2 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

Lopsidedness of cluster galaxies in modified gravity

In recent years several hypervelocity stars (HVSs) have been observed in the halo of our Galaxy. Such HVSs have possibly been ejected from the Galactic center and then propagated in the Galactic potential up to their current position. The recent survey for candidate HVSs show an asymmetry in the kinematics of candidate HVSs (position and velocity vectors), where more outgoing stars than ingoing stars (i.e., positive Galactocentric velocities versus negative ones) are observed. We show that such kinematic asymmetry, which is likely due to the finite lifetime of the stars and Galactic potential structure, could be used in a novel method to probe and constrain the Galactic potential, identify the stellar type of the stars in the survey and estimate the number of HVSs. Kinematics-independent identification of the stellar types of the stars in such surveys (e.g., spectroscopic identification) could further improve these results. We find that the observed asymmetry between ingoing and outgoing stars favors specific Galactic potential models. It also implies a lower limit of ~54 ? 8 main-sequence HVSs in the survey sample (648 ? 96 in the Galaxy), assuming that all of the MS stars in the survey originate from the GC. The other stars in the survey are likely to be hot blue horizontal branch stars born in the halo rather than stars ejected from the GC.

N-body simulations for testing the stability of triaxial galaxies in MOND

The Bekenstein-Milgrom gravity theory with a modified Poisson equation is tested here for the existence of triaxial equilibrium solutions. Using the nonnegative least-square method, we show that self-consistent triaxial galaxies exist for baryonic models with a mild density cusp, -->ρ ~ Σ/r. Self-consistency is achieved for a wide range of central concentrations, -->Σ ~ 10-1000 M☉ pc−2, representing low to high surface brightness galaxies. Our results demonstrate for the first time that the orbit superposition technique is fruitful for constructing galaxy models beyond Newtonian gravity, and triaxial cuspy galaxies might exist without the help of cold dark matter.

Stirring N‐body systems: universality of end states

We point out an interesting theoretical prediction for elliptical galaxies residing inside galaxy clusters in the framework of modified Newtonian dynamics (MOND), that could be used to test this paradigm. Apart from the central brightest cluster galaxy, other galaxies close enough to the centre experience a strong gravitational influence from the other galaxies of the cluster. This influence manifests itself only as tides in standard Newtonian gravity, meaning that the systematic acceleration of the centre of mass of the galaxy has no consequence. However, in the context of MOND, a consequence of the breaking of the strong equivalence principle is that the systematic acceleration changes the own self-gravity of the galaxy. We show here that, in this framework, initially axisymmetric elliptical galaxies become lopsided along the external fields direction, and that the centroid of the galaxy, defined by the outer density contours, is shifted by a few hundreds parsecs with respect to the densest point.

Calibrating the α parameter of convective efficiency using observed stellar properties

We perform a stability test of triaxial models in Modified Newtonian Dynamics (MOND) using N-body simulations. The triaxial models considered here have densities that vary with r −1 in the centre and r −4 at large radii. The total mass of the model varies from 10 8 to 10 10 M� , representing the mass scale of dwarfs to medium-mass elliptical galaxies, respectively, from deep MOND to quasi-Newtonian gravity. We build triaxial galaxy models using the Schwarzschild technique, and evolve the systems for 200 Keplerian dynamical times (at the typical length-scale of 1.0 kpc). We find that the systems are virial overheating, and in quasi-equilibrium with the relaxation taking approximately 5 Keplerian dynamical times (1.0 kpc). For all systems, the change of the inertial (kinetic) energy is less than 10 per cent (20 per cent) after relaxation. However, the central profile of the model is flattened during the relaxation and the (overall) axis ratios change by roughly 10 per cent within 200 Keplerian dynamical times (at 1.0 kpc) in our simulations. We further find that the systems are stable once they reach the equilibrium state.