Core creation in galaxies and haloes via sinking massive objects

TL;DR

This paper investigates how sinking massive objects disrupt galactic cusps, forming cores, and explores implications for dark matter annihilation and binary nucleus formation, with simulations showing objects stall at core edges.

Contribution

It provides an empirical relation for core sizes formed by sinking objects and introduces a new 'stalled binary' model for binary nucleus formation in galaxies.

Findings

Sinking objects always stall at the edge of formed cores.

Core size correlates with initial cusp slope and infalling mass.

Binary nuclei can naturally form and stall depending on cusp steepness.

Abstract

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 infalling 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.