On the nature and origin of the dwarf galaxies in the Local Group: Are they tidal dwarf galaxies?

Marcel Pawlowski

CWRU Astronomy

There are thought to be two different types of dwarf galaxies in the Universe, primordial dwarf galaxies (PDGs) and second-generation tidal dwarf galaxies (TDGs) formed from the debris of galaxy collisions. While PDGs and ancient TDGs can have similar internal properties, populations of PDGs and TDGs have very different phase-space distributions because the latter form as a correlated population. By making use of this difference I investigate the most-likely origin of the satellite galaxy population of the Milky Way (MW). I find that the MW is surrounded by a vast polar structure (VPOS), a thin planar distribution of satellite galaxies and young halo globular clusters. Stellar and gaseous streams – which trace the orbital planes of tidally disrupted systems – preferentially align with the VPOS and the orbits of most MW satellite galaxies are also confined to the VPOS, which is therefore rotationally stabilized. This highly flattened phase-space distribution of co-orbiting satellites is indicative of them being TDGs. By comparison with cosmological high-resolution simulations of MW-like haloes I demonstrate that the observed distribution of satellites is inconsistent with the expected distribution of dark-matter dominated PDGs, a finding which poses a major problem for cold-dark-matter based standard cosmology.

If the MW satellite galaxies are TDGs, this implies that a major galaxy encounter has happened in the Local Group about 10 Gyr ago. This event must result in signatures on a Local Group scale. Searching for such features, I discovered that the non-satellite galaxies in the Local Group are confined to two thin and symmetric planes. These and the mutual alignments of additional features – such as the Magellanic Stream, the orbit of the MW-M31 system, high-velocity clouds and hypervelocity stars – indicate an intriguing connection between the different dwarf and satellite galaxy structures in the Local Group, which might allow us to disentangle the dynamical history of our cosmic neighborhood.