Hunting for wandering massive black holes

TL;DR

This study uses hydrodynamical simulations and spectral modeling to explore the detectability of wandering massive black holes in galaxy outskirts, proposing millimeter observations as a promising method.

Contribution

It introduces a combined simulation and spectral model approach to identify wandering black holes via millimeter-wave observations, extending detection limits.

Findings

Accretion rates are limited to 10-20% of the Bondi-Hoyle-Littleton rate.

Predicted spectra peak at millimeter wavelengths accessible by ALMA.

Detection limits could reach black hole masses of ~2×10^7 M_sun in ellipticals and ~10^5 M_sun in the Milky Way.

Abstract

We investigate low-density accretion flows onto massive black holes (BHs) with masses of $\gtrsim 10^5~M_\odot$ orbiting around in the outskirts of their host galaxies, performing three-dimensional hydrodynamical simulations. Those wandering BHs are populated via ejection from the galactic nuclei through multi-body BH interactions and gravitational wave recoils associated with galaxy and BH coalescences. We find that when a wandering BH is fed with hot and diffuse plasma with density fluctuations, the mass accretion rate is limited at $\sim 10-20\%$ of the canonical Bondi-Hoyle-Littleton rate owing to a wide distribution of inflowing angular momentum. We further calculate radiation spectra from radiatively inefficient accretion flows onto the wandering BH using a semi-analytical two-temperature disk model and find that the predicted spectra have a peak at the millimeter band, where the Atacama Large Millimeter/submillimeter Array (ALMA) has the highest sensitivity and spatial resolution. Millimeter observations with ALMA and future facilities such as the next generation Very Large Array (ngVLA) will enable us to hunt for a population of wandering BHs and push the detectable mass limit down to $M_\bullet \simeq 2\times10^7~M_\odot$ for massive nearby ellipticals, e.g., M87, and $M_\bullet \simeq 10^5~M_\odot$ for the Milky Way. This radiation spectral model, combined with numerical simulations, will be applied to give physical interpretations of off-nuclear BHs detected in dwarf galaxies, which may constrain BH seed formation scenarios.