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

TL;DR

This paper predicts observable signatures of a wandering massive black hole and its star cluster in the halo of M31 resulting from galaxy merging, providing guidance for future detections and insights into galaxy-BH coevolution.

Contribution

It offers the first detailed observational predictions for a wandering black hole and associated star cluster in M31's halo, based on merger simulations and accretion models.

Findings

Radio emission from the wandering BH is detectable with EVLA, ALMA, and SKA.

The star cluster bound to the BH could match observed clusters in PAndAS survey.

A relic BH in M31's halo can inform galaxy and black hole coevolution studies.

Abstract

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