Capture of field stars by dark substructures

Abstract

We use analytical and N-body methods to study the capture of field stars by gravitating substructures moving across a galacticenvironment. The majority of stars captured by a substructure move on temporarily bound orbits that are lost to galactic tidesafter a few orbital revolutions. In numerical experiments where a substructure model is immersed into a sea of field particleson a circular orbit, we find a population of particles that remain bound to the substructure potential for indefinitely long times.This population is absent from substructure models, initially placed outside the galaxy on an eccentric orbit. We show thatgravitational capture is most efficient in dwarf spheroidal galaxies (dSphs) on account of their low velocity dispersions and highstellar phase-space densities. In these galaxies, ‘dark’ sub-subhaloes, which do not experience in situ star formation, may capturefield stars and become visible as stellar overdensities with unusual properties: (i) they would have a large size for their luminosity,(ii) contain stellar populations indistinguishable from the host galaxy, and (iii) exhibit dark matter (DM)-dominated mass-to-light ratios. We discuss the nature of several ‘anomalous’ stellar systems reported as star clusters in the Fornax and EridanusII dSphs that exhibit some of these characteristics. DM sub-subhaloes with a mass function dN/dM• ∼ M −α• are expected togenerate stellar systems with a luminosity function, dN/dM ∼ M −β , where β = (2α + 1)/3 = 1.6 for α = 1.9. Detecting andcharacterizing these objects in dSphs would provide unprecedented constraints on the particle mass and cross-section of a largerange of DM particle candidates.