A wealth of astronomical data indicate the presence of mass discrepancies in the Universe. The motions observed in a variety of classes of extragalactic systems exceed what can be explained by the mass visible in stars and gas. Either (i) there is a vast amount of unseen mass in some novel form — dark matter — or (ii) the data indicate a breakdown of our understanding of dynamics on the relevant scales, or (iii) both. Here, we first review a few outstanding challenges for the dark matter interpretation of mass discrepancies in galaxies, purely based on observations and independently of any alternative theoretical framework. We then show that many of these puzzling observations are predicted by one single relation — Milgrom’s law — involving an acceleration constant a0 (or a characteristic surface density Σ† = a0/G) on the order of the square-root of the cosmological constant in natural units. This relation can at present most easily be interpreted as the effect of a single universal force law resulting from a modification of Newtonian dynamics (MOND) on galactic scales. We exhaustively review the current observational successes and problems of this alternative paradigm at all astrophysical scales, and summarize the various theoretical attempts (TeVeS, GEA, BIMOND, and others) made to effectively embed this modification of Newtonian dynamics within a relativistic theory of gravity.

The availability of the Hipparcos Catalogue triggered many kinematic and dynamical studies of the solar neighbourhood. Nevertheless, those studies generally lacked the third component of the space velocities, i.e., the radial velocities. This work presents the kinematic analysis of 5952 K and 739 M giants in the solar neighbourhood which includes for the first time radial velocity data from a large survey performed with the CORAVEL spectrovelocimeter. It also uses proper motions from the Tycho-2 catalogue, which are expected to be more accurate than the Hipparcos ones. An important by-product of this study is the observed fraction of only 5.7% of spectroscopic binaries among M giants as compared to 13.7% for K giants. After excluding the binaries for which no center-of-mass velocity could be estimated, 5311 K and 719 M giants remain in the final sample. The UV-plane constructed from these data for the stars with precise parallaxes (σ π /π ≤ 20%) reveals a rich small-scale structure, with several clumps corresponding to the Hercules stream, the Sirius moving group, and the Hyades and Pleiades superclusters. A maximum-likelihood method, based on a bayesian approach, has been applied to the data, in order to make full use of all the available stars (not only those with precise parallaxes) and to derive the kinematic properties of these subgroups. Isochrones in the Hertzsprung-Russell diagram reveal a very wide range of ages for stars belonging to these groups. These groups are most probably related to the dynamical perturbation by transient spiral waves (as recently modelled by De Simone et al. 2004) rather than to cluster remnants. A possible explanation for the presence of young group/clusters in the same area of the UV-plane is that they have been put there by the spiral wave associated with their formation, while the kinematics of the older stars of our sample has also been disturbed by the same wave. The emerging picture is thus one of dynamical streams pervading the solar neighbourhood and travelling in the Galaxy with similar space velocities. The term dynamical stream is more appropriate than the traditional term supercluster since it involves stars of different ages, not born at the same place nor at the same time. The position of those streams in the UV-plane is responsible for the vertex deviation of 16.2 • ± 5.6 • for the whole sample. Our study suggests that the vertex deviation for younger populations could in fact have the same dynamical origin. The underlying velocity ellipsoid, extracted by the maximum-likelihood method after removal of the streams, is not centered on the value commonly accepted for the radial antisolar motion: it is centered on U = −2.78 ± 1.07 km s −1 . However, the full data set (including the various streams) does yield the usual value for the radial solar motion, when properly accounting for the biases inherent to this kind of analysis (namely, U = −10.25 ± 0.15 km s −1 ). This discrepancy clearly raises the essential question of how to derive the solar motion in the presence o...

While it has long been known that a large number of short-lived transient spirals can cause stellar migration, here we report that another mechanism is also effective at mixing disks of barred galaxies. The resonance overlap of the bar and spiral structure induces a nonlinear response leading to a strong redistribution of angular momentum in the disk. We find that, depending on the amplitudes of the perturbers, the changes in angular momentum, ∆L, could occur up to an order of magnitude faster than in the case of recurrent spirals. The signature of this mechanism is a bimodality in ∆L with maxima near the bar's corotation and its outer Lindblad resonance; this is independent of the properties of the spiral structure. For parameters consistent with the Milky Way the disk mixes in about 3 Gyr and the stellar velocity dispersion increases with time in a manner roughly consistent with observations. This new mechanism could account for both the observed age-velocity relation and the absence of age-metallicity relation in the solar neighborhood. Spiral-bar interaction could also explain observations showing that strongly barred galaxies have weaker metallicity gradients than weakly barred or non-barred galaxies.

Data Release 5 (DR5) of the Radial Velocity Experiment (RAVE) is the fifth data release from a magnitude-limited (9 < I < 12) survey of stars randomly selected in the southern hemisphere. The RAVE medium-resolution spectra (R ∼ 7500) covering the Ca-triplet region (8410-8795Å) span the complete time frame from the start of RAVE observations in 2003 to their completion in 2013. Radial velocities from 520 781 spectra of 457 588 unique stars are presented, of which 255 922 stellar observations have parallaxes and proper motions from the Tycho-Gaia astrometric solution (TGAS) in Gaia DR1. For our main DR5 catalog, stellar parameters (effective temperature, surface gravity, and overall metallicity) are computed using the RAVE DR4 stellar pipeline, but calibrated using recent K2 Campaign 1 seismic gravities and Gaia benchmark stars, as well as results obtained from highresolution studies. Also included are temperatures from the Infrared Flux Method, and we provide a catalogue of red giant stars in the dereddened color (J − Ks) 0 interval (0.50,0.85) for which the gravities were calibrated based only on seismology. Further data products for sub-samples of the RAVE stars include individual abundances for Mg, Al, Si, Ca, Ti, Fe, and Ni, and distances found using isochrones. Each RAVE spectrum is complemented by an error spectrum, which has been used to determine uncertainties on the parameters. The data can be accessed via the RAVE Web site or the Vizier database.

Both microlensing surveys and radio-frequency observations of gas flow imply that the inner Milky Way is completely dominated by baryons, contrary to the predictions of standard cold dark matter (CDM) cosmology. We investigate the predictions of the modified Newtonian dynamics (MOND) formula for the Galaxy given the measured baryon distribution. Satisfactory fits to the observationally determined terminal-velocity curve are obtained for different choices of MOND's interpolating function µ(x). However, with simple analytical forms of µ(x), the local circular speed v c (R 0 ) can be as large as 220 km s −1 only for values of the parameter a 0 that are excluded by observations of NGC 3198. Only a numerically specified interpolating function can produce v c (R 0 ) = 220 km s −1 , which is therefore an upper limit in MOND, while the asymptotic velocity is predicted to be v c (∞) = 170 ± 5 km s −1 . The data are probably not consistent with the functional form of µ(x) that has been explored as a toy model in the framework of Bekenstein's covariant theory of gravity.

We present the stellar atmospheric parameters (effective temperature, surface gravity, overall metallicity), radial velocities, individual abundances and distances determined for 425 561 stars, which constitute the fourth public data release of the RAdial Velocity Experiment (RAVE). The stellar atmospheric parameters are computed using a new pipeline, based on the algorithms of MATISSE and DEGAS. The spectral degeneracies and the 2MASS photometric information are now better taken into consideration, improving the parameter determination compared to the previous RAVE data releases. The individual abundances for six elements (magnesium, aluminium, silicon, titanium, iron and nickel) are also given, based on a special-purpose pipeline which is also improved compared to that available for the RAVE DR3 and Chemical DR1 data releases. Together with photometric information and proper motions, these data can be retrieved from the RAVE collaboration website and the Vizier database.

Recently, a new form of dark matter has been suggested to naturally reproduce the empirically successful aspects of Milgrom's law in galaxies. The dark matter particle candidates are axion-like, with masses of order eV and strong self-interactions. They Bose-Einstein condense into a superfluid phase in the central regions of galaxy halos. The superfluid phonon excitations in turn couple to baryons and mediate an additional long-range force. For a suitable choice of the superfluid equation of state, this force can mimic Milgrom's law. In this paper we develop in detail some of the main phenomenological consequences of such a formalism, by revisiting the expected dark matter halo profile in the presence of an extended baryon distribution. In particular, we show how rotation curves of both high and low surface brightness galaxies can be reproduced, with a slightly rising rotation curve at large radii in massive high surface brightness galaxies, thus subtly different from Milgrom's law. We finally point out other expected differences with Milgrom's law, in particular in dwarf spheroidal satellite galaxies, tidal dwarf galaxies, and globular clusters, whose Milgromian or Newtonian behavior depends on the position with respect to the superfluid core of the host galaxy. We also expect ultra-diffuse galaxies within galaxy clusters to have velocities slightly above the baryonic Tully-Fisher relation. Finally, we note that, in this framework, photons and gravitons follow the same geodesics, and that galaxy-galaxy lensing, probing larger distances within galaxy halos than rotation curves, should follow predictions closer to the standard cosmological model than those of Milgrom's law.

Predictions of the concordance cosmological model (CCM) of the structures in the environment of large spiral galaxies are compared with observed properties of Local Group galaxies. Five new, most probably irreconcilable problems are uncovered: 1) A wide variety of published CCM models consistently predict some form of relation between dark-matter-mass and luminosity for the Milky Way (MW) satellite galaxies, but none is observed.2) The mass function of luminous sub-haloes predicted by the CCM contains too few satellites with dark matter (DM) mass ≈10 7 M within their innermost 300 pc than in the case of the MW satellites.3) The Local Group galaxies and data from extragalactic surveys indicate there is a correlation between bulge-mass and the number of luminous satellites that is not predicted by the CCM. 4) The 13 new ultra-faint MW satellites define a disc-of-satellites (DoS) that is virtually identical to the DoS previously found for the 11 classical MW satellites, implying that most of the 24 MW satellites are correlated in phase-space. 5) The occurrence of two MW-type DM halo masses hosting MW-like galaxies is unlikely in the CCM. However, the properties of the Local Group galaxies provide information leading to a solution of the above problems. The DoS and bulge-satellite correlation suggest that dissipational events forming bulges are related to the processes forming phase-space correlated satellite populations. These events are well known to occur since in galaxy encounters energy and angular momentum are expelled in the form of tidal tails, which can fragment to form populations of tidal-dwarf galaxies (TDGs) and associated star clusters. If Local Group satellite galaxies are to be interpreted as TDGs then the substructure predictions of the CCM are internally in conflict. All findings thus suggest that the CCM does not account for the Local Group observations and that therefore existing as well as new viable alternatives have to be further explored. These are discussed and natural solutions for the above problems emerge.