# Searching for correlations in GAIA DR2 unbound star trajectories

## TL;DR

This paper develops a Bayesian method to detect correlations in stellar trajectories from Gaia DR2, aiming to identify past scattering events with compact objects like primordial black holes, and applies it to various galactic data.

## Contribution

The paper introduces a novel Bayesian framework for analyzing stellar trajectories to find evidence of past scattering events in the Milky Way and nearby galaxies.

## Key findings

- Evidence for trajectory intersections with Sagittarius dSph 20-40 Myr ago under certain assumptions.
- Results depend on distance estimates, with some scenarios showing no confirmation of scattering events.
- Discussion of data quality issues and future prospects for detection with Gaia.

## Abstract

Scattering events with compact objects are expected in the primordial black hole (PBH) cold dark matter (CDM) scenario due to close encounters between stars and PBH in the dense environments of dwarf spheroidals. We develop a Bayesian framework to search for correlations among Milky Way stellar trajectories and those of globular clusters and dwarf galaxies in the halo, and other nearby galaxies. We apply the method to a selection of hypervelocity stars (HVS) and globular clusters from Gaia DR2 catalog, and known nearby (mostly dwarf) galaxies with full phase-space and size measurements. We report positive evidence for trajectory intersection $\sim$20-40 Myr ago of up to 2 stars, depending on priors, with the Sagittarius dwarf Spheroidal (dSph) galaxy when assuming Marchetti et al. (2019) distance estimates. We verify that the result is compatible with their evolutionary status, setting a lower bound for the stellar age of $\sim$100 Myr. However, such scattering events are not confirmed when assuming Anders et al. (2019) distance estimates. We discuss shortcomings related to present data quality and future prospects for detection of HVS with the full Gaia catalog and Sagittarius dSph.