18 février 2020A vast polar structure of satellites around the Milky Way

Marcel PAWLOWSKI
AIfA Bonn, Allemagne

The Milky Way is surrounded by numerous satellite objects: dwarf galaxies, globular clusters and streams of disrupted systems. I will show that these are part of, and mostly co-orbit within, a vast polar structure (VPOS), a thin plane spreading to Galactocentric distances as large as 250 kpc. In addition, it was recently discovered that about half of the satellite galaxies of Andromeda/M31 define and mostly co-orbit in a thin plane. The existence of similar, co-rotating phase-space structures in the two only satellite galaxy systems for which 3D positions are known emphasizes the need to develop an understanding of their origin. I will discuss why suggested formation scenarios which interpret the satellite galaxies as tracers of dark matter sub-halos, often being based on their accretion along cosmic filaments, fail to deliver a satisfactory explanation for the VPOS. As an alternative origin I suggest the formation of tidal dwarf galaxies (TDGs) in the phase-space correlated debris of interacting galaxies. Numerical models of galaxy interactions demonstrate that such tidal debris reproduce the observed phase-space properties of the VPOS. I will discuss the consistencies between this dwarf galaxy origin and the observed Local Group and present several scenarios for a past galaxy encounter.