Complexity on dwarf galaxies scale: A bimodal distribution function in Sculptor

In previous work we have presented Schwarzschild models of the Sculptor dSph, demonstrating that this system could be embedded in dark matter halos that are either cusped or cored. Here we show that the non-parametric distribution function recovered through Schwarschild's method is bimodal in energy and angular momentum space for all best fitting mass models explored. We demonstrate that this bimodality is directly related to the two components known to be present in Sculptor through stellar populations analysis, although our method is purely dynamical in nature and does not use this prior information. It therefore constitutes independent confirmation of the existence of two physically distinct dynamical components in Sculptor and suggests a rather complex assembly history for this dwarf galaxy.Comment: 4 pages, 4 figures, 1 table, accepted to ApJ Letter

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

The Galaxy and its stellar halo: insights on their formation from a hybrid cosmological approach

We use a series of high-resolution simulations of a `Milky-Way' halo coupled to semi-analytic methods to study the formation of our own Galaxy and of its stellar halo. The physical properties of our model Milky Way, as well as the age and metallicity distribution of stars in the different components, are in relatively good agreement with observational measurements. Assuming that the stellar halo builds up from the cores of the satellite galaxies that merged with the Milky Way over its life-time, we are able to study the physical and structural properties of this component. In agreement with previous work, we find that the largest contribution to the stellar halo should come from a few relatively massive (10^8 - 10^10 Msun) satellites, accreted at early times. Our "stellar halo" does not exhibit any clear metallicity gradient, but higher metallicity stars are more centrally concentrated than stars of lower abundance. This indicates that the probability of observing low-metallicity halo stars increases with distance from the Galactic centre. We find that the proposed "dual" nature of the Galactic stellar halo can be explained in our model as a result of a mass-metallicity relation imprinted in the building blocks of this component.Comment: 20 pages, 14 figures. Minor revisions to match version accepted to MNRAS. Version with high-resolution figures available at: http://www.mpa-garching.mpg.de/~lucia/astro-ph/mw_delucia_rev.pd

The time evolution of gaps in tidal streams

We model the time evolution of gaps in tidal streams caused by the impact of a dark matter subhalo, while both orbit a spherical gravitational potential. To this end, we make use of the simple behaviour of orbits in action-angle space. A gap effectively results from the divergence of two nearby orbits whose initial phase-space separation is, for very cold thin streams, largely given by the impulse induced by the subhalo. We find that in a spherical potential the size of a gap increases linearly with time for sufficiently long timescales. We have derived an analytic expression that shows how the growth rate depends on the mass of the perturbing subhalo, its scale and its relative velocity with respect to the stream. We have verified these scalings using N-body simulations and find excellent agreement. For example, a subhalo of mass 10^8 Msun directly impacting a very cold thin stream on an inclined orbit can induce a gap that may reach a size of several tens of kpc after a few Gyr. The gap size fluctuates importantly with phase on the orbit, and it is largest close to pericentre. This indicates that it may not be fully straightforward to invert the spectrum of gaps present in a stream to recover the mass spectrum of the subhalos.Comment: 4 pages, 4 figures, ApJ Letters in pres

Dark influences: imprints of dark satellites on dwarf galaxies

In the context of the current $\Lambda$CDM cosmological model small dark matter haloes are abundant and satellites of dwarf galaxies are expected to be predominantly dark. Since low mass galaxies have smaller baryon fractions interactions with these satellites may leave particularly dramatic imprints. We uncover the influence of the most massive of these dark satellites on disky dwarf galaxies and the possible dynamical and morphological transformations that result from these interactions. We use a suite of carefully set-up, controlled simulations of isolated dwarf galaxies. The primary dwarf galaxies have solely a stellar disk in the dark matter halo and the secundaries are completely devoid of baryons. We vary the disk mass, halo concentration, initial disk thickness and inclination of the satellite orbit. The disky dwarf galaxies are heated and disrupted due to the minor merger event, more extremely for higher satellite over disk mass ratios, and the morphology and kinematics are significantly altered. Moreover, for less concentrated haloes the minor merger can completely destroy the disk leaving a low-luminosity spheroidal-like galaxy instead. We conclude that dwarf galaxies are very much susceptible to being disturbed by dark galaxies and that even a minor merger event can significantly disrupt and alter the structure and kinematics of a dwarf galaxy. This process may be seen as a new channel for the formation of dwarf spheroidal galaxies.Comment: 16 pages, 13 figures, A&A accepted. For movies or a higher resolution version see http://www.astro.rug.nl/~starkenb/dwarfsanddarks.htm

A new fitting-function to describe the time evolution of a galaxy's gravitational potential

We present a new simple functional form to model the evolution of a spherical mass distribution in a cosmological context. Two parameters control the growth of the system and this is modelled using a redshift dependent exponential for the scale mass and scale radius. In this new model, systems form inside out and the mass of a given shell can be made to never decrease, as generally expected. This feature makes it more suitable for studying the smooth growth of galactic potentials or cosmological halos than other parametrizations often used in the literature. This is further confirmed through a comparison to the growth of dark matter halos in the Aquarius simulations.Comment: 5 pages, 4 figures; example figures replaced by their correct versions. Accepted for publication in A&