Where are the Intermediate Mass Black Holes?

TL;DR

This paper discusses the challenges in observing intermediate mass black holes (IMBHs), their potential formation as seeds for supermassive black holes, and how future gravitational wave observations by LISA could detect and characterize them to understand their role in galaxy evolution.

Contribution

It highlights the importance of gravitational wave detection with LISA for identifying IMBH mergers and constraining their formation and evolution.

Findings

LISA will detect IMBH mergers at high redshifts.

IMBHs are likely present in dwarf galaxies and galaxy halos.

Observations will inform models of black hole and galaxy formation.

Abstract

Observational evidence has been mounting for the existence of intermediate mass black holes (IMBHs, 10^2-10^5 Msun), but observing them at all, much less constraining their masses, is very challenging. In one theorized formation channel, IMBHs are the seeds for supermassive black holes in the early universe. As a result, IMBHs are predicted to exist in the local universe in dwarf galaxies, as well as wandering in more massive galaxy halos. However, these environments are not conducive to the accretion events or dynamical signatures that allow us to detect IMBHs. The Laser Interferometer Space Antenna (LISA) will demystify IMBHs by detecting the mergers of these objects out to extremely high redshifts, while measuring their masses with extremely high precision. These observations of merging IMBHs will allow us to constrain the formation mechanism and subsequent evolution of massive black holes, from the 'dark ages' to the present day, and reveal the role that IMBHs play in hierarchical galaxy evolution.