Benoit Famaey

THE RADIAL VELOCITY EXPERIMENT (RAVE): FOURTH DATA RELEASE

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 Two Micron All Sky Survey 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, aluminum, silicon, titanium, iron, and nickel) are also given, based on a special-purpose pipeline that 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 Web site and the Vizier database.

Local-Group tests of dark-matter concordance cosmology - Towards a new paradigm for structure formation

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 Mwithin 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.

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, Science, 316, 1166) 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.

Universality of galactic surface densities within one dark halo scale-length

It was recently discovered that the mean dark matter surface density within one dark halo scale-length (the radius within which the volume density profile of dark matter remains approximately flat) is constant across a wide range of galaxies. This scaling relation holds for galaxies spanning a luminosity range of 14 magnitudes and the whole Hubble sequence. Here we report that the luminous matter surface density is also constant within one scale-length of the dark halo. This means that the gravitational acceleration generated by the luminous component in galaxies is always the same at this radius. Although the total luminous-to-dark matter ratio is not constant, within one halo scale-length it is constant. Our finding can be interpreted as a close correlation between the enclosed surface densities of luminous and dark matter in galaxies.

Overlapping abundance gradients and azimuthal gradients related to the spiral structure of the Galaxy

The connection between some features of the metallicity gradient in the Galactic disc, best revealed by Open Clusters and Cepheids, and the spiral structure, has been explored. The step-like abrupt decrease in metallicity at 8.5kpc (with R0= 7.5kpc, or at 9.5kpc if R0= 8.5kpc is adopted) is well explained by the corotation ring-shaped gap in the density of gas, which isolates the internal and external regions of the disc one from the other. This solves the long-standing problem of a lack of understanding of the different chemical characteristics of the inner and outer parts of the disc. The time required to build up the metallicity difference between the two sides of the step is a measure of the minimal lifetime of the present grand-design spiral pattern structure, of the order of 3 Gyr. The plateaux observed on both sides of the step are interpreted in terms of the large-scale radial motion of the stars and of the gas flow induced by the spiral structure. The star formation rate revealed by the density of open clusters is maximum in the Galactic radial range from 6 to 12kpc (with an exception of a narrow gap at corotation), coinciding with the region where the four-arms mode is allowed to exist. We argue that most of the old open clusters situated at large Galactocentric radii were born in this inner region where conditions more favourable for star formation are found. The ratio of α-elements to Fe of the sample of Cepheids does not vary appreciably with the Galactic radius, which reveals a homogeneous history of star formation. Different arguments are forwarded to show that the usual approximations of chemical evolution models, which assume fast mixing of metallicity in the azimuthal direction and ignore the existence of the spiral arms, are poor ones.

Co-orbiting satellite galaxy structures are still in conflict with the distribution of primordial dwarf galaxies

Both major galaxies in the Local Group host planar distributions of co-orbiting satellite galaxies, the Vast Polar Structure (VPOS) of the Milky Way and the Great Plane of Andromeda (GPoA). The

Insight into the baryon-gravity relation in galaxies

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Radial and vertical flows induced by galactic spiral arms: likely contributors to our ‘wobbly Galaxy’

CDM cosmological model did not predict these features. However, according to three recent studies the properties of the GPoA and the flattening of the VPOS are common features among sub-halo based

On the age heterogeneity of the Pleiades, Hyades, and Sirius moving groups

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Can MOND take a bullet? Analytical comparisons of three versions of MOND beyond spherical symmetry

CDM satellite systems, and the GPoA can be naturally explained by satellites being acquired along cold gas streams. We point out some methodological issues in these studies: either the selection of model satellites is different from that of the observed ones, or an incomplete set of observational constraints has been considered, or the observed satellite distribution is inconsistent with basic assumptions. Once these issues have been addressed, the conclusions are different: features like the VPOS and GPoA are very rare (each with probability