The origins of massive black holes

TL;DR

This paper reviews the origins of massive black holes, discussing seed formation channels, their growth into MBHs, and diagnostic methods, highlighting current challenges and recent developments in the field.

Contribution

It provides a comprehensive overview linking seed formation, growth processes, and observational diagnostics, emphasizing recent advances and unresolved issues.

Findings

Identification of multiple seed formation channels

Discussion of growth pathways from seeds to MBHs

Highlighting recent observational and theoretical developments

Abstract

Massive black holes (MBHs) inhabit galaxy centers, power luminous quasars and Active Galactic Nuclei (AGN) and shape their cosmic environment with the energy they produce. The origins of MBHs remain a mystery and the recent detection by LIGO/Virgo of an almost 150 solar mass black hole has revitalized the question of whether there is a continuum between "stellar" and "massive" black holes and what the seeds of MBHs are. Seeds could have formed in the first galaxies, or could be also related to the collapse of horizon-sized regions in the early Universe. Understanding the origins of MBHs straddles fundamental physics, cosmology and astrophysics and it bridges the fields of gravitational wave physics and traditional astronomy. With several facilities in the next 10-15 years we foresee the possibility of discovering MBHs' avenues of formation. In this article we link three main topics: the channels of black hole seed formation, the journey from seeds to massive black holes, the diagnostics on the origins of MBHs. We highlight and critically discuss current unsolved problems and touch on recent developments that stirred the community.