Recent observations have shown that the majority of the Andromeda galaxys satellites are aligned in a thin plane. On the theoretical side it has been proposed that galaxies acquire their gas via cold streams. In addition, numerical simulations show t
hat the same streams also deliver gas clumps which could potentially develop into satellite galaxies. Assuming that cold streams are a major source of satellite systems around galaxies we calculate the probabilities in two different models to find a certain fraction of satellites within a thin plane around the central galaxy of the host halo with and without having the same sense of rotation. Using simple geometrical considerations and adopting a random orientation of the streams we demonstrate that the vast thin disk of satellites detected around Andromeda can naturally be explained within this framework. In fact, without any satellite scattering, two streams would lead to too many satellites in the thin plane, compared with the observations. Three streams reproduce the observations very well. Natural implications from our model are that all massive galaxies should have a thin plane of satellites and that the satellites should naturally distribute themselves not only into a single plane but into several inclined ones. We estimate the effect of additional satellites accreted from random directions and find it to be of minor relevance for a mild inflow of satellites from random directions.
We perform a detailed investigation into the disruption of central cusps via the transfer of energy from sinking massive objects. Constant density inner regions form at the radius where the enclosed mass approximately matches the mass of the infallin
g body. We explore parameter space using numerical simulations and give an empirical relation for the size of the resulting core within structures that have different initial cusp slopes. We find that infalling bodies always stall at the edge of these newly formed cores, experiencing no dynamical friction over many dynamical times. As applications, we consider the resulting decrease in the dark matter annihilation flux due to centrally destroyed cusps, and we present a new theory for the formation of close binary nuclei -- the `stalled binary model. We focus on one particularly interesting binary nucleus system, the dwarf spheroidal galaxy VCC 128 which is dark matter dominated at all radii. We show that its nuclei would rapidly coalesce within a few million years if it has a central dark matter cusp slope steeper than r^{-1}. However, if its initial dark matter cusp is slightly shallower than a log slope of -0.75 at ~0.1% of the virial radius, then the sinking nuclei naturally create a core equal to their observed separation and stall. This is close to the log slope measured a recent billion particle CDM halo simulation.
