Constraints on the shape of the Milky Way dark matter halo from the Sagittarius stream

We propose a new model for the dark matter halo of the Milky Way that fits the properties of the stellar stream associated with the Sagittarius dwarf galaxy. Our dark halo is oblate with q_z = 0.9 for r < 10 kpc, and can be made to follow the Law & Majewski model at larger radii. However, we find that the dynamical perturbations induced by the Large Magellanic Cloud on the orbit of Sgr cannot be neglected when modeling its streams. When taken into account, this leads us to constrain the Galaxy's outer halo shape to have minor-to-major axis ratio (c/a)_\Phi = 0.8 and intermediate-to-major axis ratio (b/a)_\Phi = 0.9, in good agreement with cosmological expectations.Comment: 5 pages, 5 figures. Accepted for publication in ApJ Letters. Minor changes to match published versio

Matching the dark matter profiles of dSph galaxies with those of simulated satellites: a two parameter comparison

We compare the dark matter halos' structural parameters derived for four Milky Way dwarf spheroidal galaxies to those of subhalos found in cosmological $N$-body simulations. We confirm that estimates of the mass at a single fixed radius are fully consistent with the observations. However, when a second structural parameter such as the logarithmic slope of the dark halo density profile measured close to the half-light radius is included in the comparison, we find little to no overlap between the satellites and the subhalos. Typically the right mass subhalos have steeper profiles at these radii than measurements of the dSph suggest. Using energy arguments we explore if it is possible to solve this discrepancy by invoking baryonic effects. Assuming that feedback from supernovae can lead to a reshaping of the halos, we compute the required efficiency and find entirely plausible values for a significant fraction of the subhalos and even as low as 0.1%. This implies that care must be taken not to exaggerate the effect of supernovae feedback as this could make the halos too shallow. These results could be used to calibrate and possibly constrain feedback recipes in hydrodynamical simulations.Comment: 6 pages, 3 figures, submitted to ApJ

The Local Group in LCDM - Shapes and masses of dark halos

In dit proefschrift bestuderen we de eigenschappen van donkere materie halo's in het LCDM paradigma. Het eerste deel richt zich op de vorm van de massadistributie van dergelijke objecten. We hebben gevonden dat de vorm van ge"isoleerde Melkweg-achtige donkere materie halo's significant afwijkt van bolsymmetrie. De lokale omgeving heeft invloed op de halo's en deze worden daarbij sterk be"invloed door de manier waarop massa aangroeit. We hebben ook de structuur en de baanstructuur van de satellieten van dergelijke halo's in detail onderzocht. In het algemeen zijn deze objecten sferischer dan de halo's zelf. Ze vertonen ook duidelijke afdrukken van getijdenwerking in zowel hun geometrische vorm als in de baanstructuur. Daarnagebruiken we het aantal massieve objecten rond de Melkweg om limieten te zetten op de totale massa van de donkere materie halo van de Melkweg. De eigenschappen van de massaverdeling van de Melkweg worden verder onderzocht in het laatste hoofdstuk. Daar maken we gebruik van de Sagittarius sterstroom om de vorm van de galactische potentiaal beter te bepalen. We komen met een nieuw model dat rekening houdt met de galactische schijf en de invloed van satellietstelsels en die bovendien consistent is met het LCDM paradigma.In this Thesis we study the properties of dark matter halos assembledin the LCDM paradigm and its substructures. The first part focuses on the geometry of the mass distribution of such objects. We find that the shapes of isolated Milky Way-like dark halos significantly deviate from the spherical symmetry and respond to the local environment, directly correlating with the way in which mass accretion takesplace. The shape and orbital structure for the satellite structures inside these halos is also analyzed in detail. In general, this objects are rounder than their more massive hosts, and exhibit clear imprints of tidal disruption in both their geometrical shapes and internal orbital structure. After this, we use the number of massive structures around the Milky Way to put constraints on the total mass of the Galactic dark matter halo. The properties of the mass distribution of the Milky Way dark matter halo are investigated further in the last chapter, where we use the stellar stream assosiated to the Sagittarius dwarf galaxy to constraint the shape of the Galactic potential. We put forward a new model that ntegratesour knowledge about the formation of the Galactic disk, the impact of satellite galaxies on measurements of the Galactic potential, and that is finally consistent with the LCDM paradigm.<br/

The effect of radial migration on galactic disks

We study the radial migration of stars driven by recurring multi-arm spiral features in an exponential disk embedded in a dark matter halo. The spiral perturbations redistribute angular momentum within the disk and lead to substantial radial displacements of individual stars, in a manner that largely preserves the circularity of their orbits and that results, after 5 Gyr (~40 full rotations at the disk scalelength), in little radial heating and no appreciable changes to the vertical or radial structure of the disk. Our results clarify a number of issues related to the spatial distribution and kinematics of migrators. In particular, we find that migrators are a heavily biased subset of stars with preferentially low vertical velocity dispersions. This "provenance bias" for migrators is not surprising in hindsight, for stars with small vertical excursions spend more time near the disk plane and thus respond more readily to non-axisymmetric perturbations. We also find that the vertical velocity dispersion of outward migrators always decreases, whereas the opposite holds for inward migrators. To first order, newly arrived migrators simply replace stars that have migrated off to other radii, thus inheriting the vertical bias of the latter. Extreme migrators might therefore be recognized, if present, by the unexpectedly small amplitude of their vertical excursions. Our results show that migration, understood as changes in angular momentum that preserve circularity, can affect strongly the thin disk, but cast doubts on models that envision the Galactic thick disk as a relic of radial migration.Comment: 10 pages, 12 figures. ApJ in pres

THE IMPRINT of RADIAL MIGRATION on the VERTICAL STRUCTURE of GALAXY DISKS

We use numerical simulations to examine the effects of radial migration on the vertical structure of galaxy disks. The simulations follow three exponential disks of different mass but similar circular velocity, radial scalelength, and (constant) scale height. The disks develop different non-axisymmetric patterns, ranging from feeble, long-lived multiple arms to strong, rapidly evolving few-armed spirals. These fluctuations induce radial migration through secular changes in the angular momentum of disk particles, mixing the disk radially and blurring pre-existing gradients. Migration primarily affects stars with small vertical excursions, regardless of spiral pattern. This "provenance bias" largely determines the vertical structure of migrating stars: inward migrators thin down as they move in, whereas outward migrators do not thicken up but rather preserve the disk scale height at their destination. Migrators of equal birth radius thus develop a strong scale-height gradient, not by flaring out as commonly assumed, but by thinning down as they spread inward. Similar gradients have been observed for low-[α/Fe] mono-abundance populations (MAPs) in the Galaxy, but our results argue against interpreting them as a consequence of radial migration. This is because outward migration does not lead to thickening, implying that the maximum scale height of any population should reflect its value at birth. In contrast, Galactic MAPs have scale heights that increase monotonically outward, reaching values that greatly exceed those at their presumed birth radii. Given the strong vertical bias affecting migration, a proper assessment of the importance of radial migration in the Galaxy should take carefully into account the strong radial dependence of the scale heights of the various stellar populations. © 2016. The American Astronomical Society. All rights reserved

The shape of dark matter subhalos in the Aquarius simulations

We analyze the Aquarius simulations to characterize the shape of dark matter halos with peak circular velocity in the range 8<Vmax<200 km/s, and perform a convergence study using the various Aquarius resolution levels. For the converged objects, we determine the principal axis (a<b<c) of the normalized inertia tensor as a function of radius. We find that the triaxiality of field halos is an increasing function of halo mass, so that the smallest halos in our sample are ~40-50% rounder than Milky Way-like objects at the radius where the circular velocity peaks, rmax. We find that the distribution of subhalo axis ratios is consistent with that of field halos of comparable Vmax. Inner and outer contours within each object are well aligned, with the major axis preferentially pointing in the radial direction for subhalos closest to the center of their host halo. We also analyze the dynamical structure of subhalos likely to host luminous satellites comparable to the classical dwarf spheroidals in the Local Group. These halos have axis ratios that increase with radius, and which are mildly triaxial with ~0.75 and ~0.60 at r~1 kpc. Their velocity ellipsoid become strongly tangentially biased in the outskirts as a consequence of tidal stripping.Comment: 12 pages, 11 figures. MNRAS in pres