Capture of field stars by dark substructures

TL;DR

This paper investigates how dark matter substructures can capture field stars, potentially forming observable stellar overdensities in dwarf spheroidal galaxies, which could reveal properties of dark matter particles.

Contribution

It combines analytical and N-body methods to show that dark subhaloes can capture stars, creating unique stellar systems that inform dark matter properties.

Findings

Captured stars often on temporary orbits are lost after a few revolutions.

Dwarf spheroidal galaxies are most efficient at capturing stars due to low velocity dispersions.

Captured stellar systems could explain some anomalous star clusters in dSphs.

Abstract

We use analytical and $N$-body methods to study the capture of field stars by gravitating substructures moving across a galactic environment. The majority of stars captured by a substructure move on temporarily-bound orbits that are lost to galactic tides after a few orbital revolutions. In numerical experiments where a substructure model is immersed into a sea of field particles on 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 that gravitational capture is most efficient in dwarf spheroidal galaxies (dSphs) on account of their low velocity dispersions and high stellar phase-space densities. In these galaxies `dark' sub-subhaloes which do not experience in-situ star formation may capture field 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 Eridanus II dSphs which exhibit some of these characteristics. DM sub-subhaloes with a mass function $d N/d M_\bullet\sim M_\bullet^{-\alpha}$ are expected to generate stellar systems with a luminosity function, $d N/d M_\star\sim M_\star^{-\beta}$, where $\beta=(2\alpha+1)/3=1.6$ for $\alpha=1.9$. Detecting and characterizing these objects in dSphs would provide unprecedented constraints on the particle mass and cross section of a large range of DM particle candidates.