Relics of Galaxy Merging: Observational Predictions for a Wandering Massive Black Hole and Accompanying Star Cluster in the Halo of M31

摘要

Galaxies and massive black holes (BHs) presumably grow via galactic mergingevents and subsequent BH coalescence. As a case study, we investigate themerging event between the Andromeda galaxy (M31) and a satellite galaxy. Wecompute the expected observational appearance of the massive BH that was at thecenter of the satellite galaxy prior to the merger, and is currently wanderingin the M31 halo. We demonstrate that a radiatively inefficient accretion flowwith a bolometric luminosity of a few tens of solar luminosities develops whenHoyle-Lyttleton accretion onto the BH is assumed. We compute the associatedbroadband spectrum and show that the radio band (observable with EVLA, ALMA andSKA) is the best frequency range to detect the emission. We also evaluate themass and the luminosity of the stars bound by the wandering BH and find thatsuch a star cluster is sufficiently luminous that it could correspond to one ofthe star clusters found by the PAndAS survey. The discovery of a relic massiveBH wandering in a galactic halo will provide a direct means to investigate indetail the coevolution of galaxies and BHs. It also means a new population ofBHs (off-center massive BHs), and offers targets for clean BH imaging thatavoids strong interstellar scattering in the center of galaxies.