Recoiled star clusters in the Milky Way halo: N-body simulations and a candidate search through SDSS

TL;DR

This study uses N-body simulations and SDSS data to investigate and search for star clusters bound to ejected black holes in the Milky Way halo, predicting their properties and identifying potential candidates.

Contribution

It provides the first detailed modeling of the long-term evolution of recoiled star clusters and conducts a systematic search for candidates in SDSS data.

Findings

Stellar density profile follows a power-law with slope ~ -2.15.

20-90% of stars are removed from clusters over 10 billion years.

Approximately 100 candidate clusters identified in SDSS data.

Abstract

During the formation of the Milky Way, > 100 central black holes (BHs) may have been ejected from their small host galaxies as a result of asymmetric gravitational wave emission. We previously showed that many of these BHs are surrounded by a compact cluster of stars that remained bound to the BH during the ejection process. In this paper, we perform long term N-body simulations of these star clusters to determine the distribution of stars in these clusters today. These numerical simulations, reconciled with our Fokker-Planck simulations, show that stellar density profile follows a power-law with slope ~ -2.15, and show that large angle scattering and tidal disruptions remove 20 - 90% of the stars by ~10^10 yr. We then analyze the photometric and spectroscopic properties of recoiled clusters accounting for the small number of stars in the clusters. We use our results to perform a systematic search for candidates in the Sloan Digital Sky Survey. We find no spectroscopic candidates, in agreement with our expectations from the completeness of the survey. Using generic photometric models of present day clusters we identify ~100 recoiling cluster candidates. Follow-up spectroscopy would be able to determine the nature of these candidates.