Astrometric Microlensing by Local Dark Matter Subhalos

TL;DR

This paper explores the potential of astrometric microlensing to detect dark matter subhalos in the Milky Way, developing a formalism to estimate event rates and assessing current and future observational capabilities.

Contribution

It introduces a general formalism for calculating astrometric lensing cross sections across various subhalo profiles and predicts detection prospects with existing and upcoming telescopes.

Findings

Detection rates are low based on current models but could increase with denser subhalo cores.

Ground-based adaptive optics could detect intermediate-mass subhalos within hundreds of parsecs.

SIM could detect smaller, more distant subhalos, enhancing dark matter substructure studies.

Abstract

High-resolution N-body simulations of dark matter halos indicate that the Milky Way contains numerous subhalos. When a dark matter subhalo passes in front of a star, the light from that star will be deflected by gravitational lensing, leading to a small change in the star's apparent position. This astrometric microlensing signal depends on the inner density profile of the subhalo and can be greater than a few microarcseconds for an intermediate-mass subhalo (Mvir > 10000 solar masses) passing within arcseconds of a star. Current and near-future instruments could detect this signal, and we evaluate SIM's, Gaia's, and ground-based telescopes' potential as subhalo detectors. We develop a general formalism to calculate a subhalo's astrometric lensing cross section over a wide range of masses and density profiles, and we calculate the lensing event rate by extrapolating the subhalo mass function predicted by simulations down to the subhalo masses potentially detectable with this technique. We find that, although the detectable event rates are predicted to be low on the basis of current simulations, lensing events may be observed if the central regions of dark matter subhalos are more dense than current models predict (>1 solar mass within 0.1 pc of the subhalo center). Furthermore, targeted astrometric observations can be used to confirm the presence of a nearby subhalo detected by gamma-ray emission. We show that, for sufficiently steep density profiles, ground-based adaptive optics astrometric techniques could be capable of detecting intermediate-mass subhalos at distances of hundreds of parsecs, while SIM could detect smaller and more distant subhalos.