Formation of Supermassive Black Holes (1003.4404v1)

Abstract: Evidence shows that massive black holes reside in most local galaxies. Studies have also established a number of relations between the MBH mass and properties of the host galaxy such as bulge mass and velocity dispersion. These results suggest that central MBHs, while much less massive than the host (~ 0.1%), are linked to the evolution of galactic structure. In hierarchical cosmologies, a single big galaxy today can be traced back to the stage when it was split up in hundreds of smaller components. Did MBH seeds form with the same efficiency in small proto-galaxies, or did their formation had to await the buildup of substantial galaxies with deeper potential wells? I briefly review here some of the physical processes that are conducive to the evolution of the massive black hole population. I will discuss black hole formation processes for `seed' black holes that are likely to place at early cosmic epochs, and possible observational tests of these scenarios.

• The paper explores various supermassive black hole formation mechanisms, including remnants of Population III stars, gas-dynamical collapse, and stellar-dynamical processes leading to massive seed black holes.
• Observed correlations between black hole and host galaxy masses suggest a co-evolutionary relationship, while the presence of supermassive black holes in the early universe points to efficient growth mechanisms or massive initial seeds.
• Future observational efforts, such as gravitational wave astronomy with LISA and high-redshift electromagnetic observations with JWST, are highlighted as crucial for testing theoretical models of early black hole formation.

Formation of Supermassive Black Holes

The scientific investigation into the formation of supermassive black holes (SMBHs) in the cosmos has long intrigued astrophysicists due to the multifaceted processes that characterize their emergence. In this paper, Marta Volonteri provides an analytical treatise exploring the origins and early growth mechanisms of SMBHs, focusing on high-redshift galaxies where these massive entities serve as quasars.

Black Hole-Galaxy Co-evolution

The paper begins by discussing the significant correlations observed between the mass of SMBHs and various properties of their host galaxies, such as stellar velocity dispersion and bulge mass. These correlations suggest a co-evolutionary symbiosis between black holes and their galaxies. Empirical evidence indicates that the mass of a typical SMBH is about 0.1% of its host galaxy's spheroidal component—a relationship observed broadly across different galactic systems.

Formation Mechanisms

Volonteri explores various formation scenarios for the initial "seed" black holes, potentially precursors to SMBHs observed today. Three primary mechanisms are proposed:

1. Population III Star Remnants: Volonteri suggests that remnants of the first generation of stars, formed from primordial, zero-metallicity gas in minihalos, might collapse directly into black holes. These so-called Population III stars, given their high mass, have the potential to directly form black holes rather than explode as supernovae, depending on their initial mass distribution.
2. Gas-Dynamical Processes: Another possibility is the collapse of massive gaseous structures within proto-galaxies, forming intermediate-mass seed black holes. This pathway requires critical conditions to avoid fragmentation and enhance direct collapse. The efficiency of angular momentum transfer plays a crucial role in facilitating such a collapse.
3. Stellar-Dynamical Processes: Volonteri also explores the scenario where dense stellar systems experience runaway collisions, ultimately forming very massive stars (VMSs) that collapse into black holes.

Early Growth and Observational Implications

The paper addresses the surprising presence of SMBHs in the early universe when the cosmos was less than a billion years old—a scenario difficult to reconcile with standard accretion models given the short timescales available. Rapid accretion mechanisms or seed black hole formation via massive initial seeds are considered.

Gas Accretion and Feedback: Volonteri explores the notion that despite copious gas supplies in early protogalaxies, the gas accretion onto black holes is likely moderated by feedback processes, which pose limitations on growth rates.

Gravitational Wave Signatures: The paper highlights gravitational waves as a promising observational tool for understanding early SMBH formation. Instruments like LISA have the potential to detect these early events, offering insights into the seed black hole mass distribution and formation redshift distribution.

Future Directions and Theoretical Speculation

Looking forward, the paper suggests that further scientific advancements in gravitational wave astronomy and high-redshift electromagnetic observations (e.g., through the James Webb Space Telescope) will refine our understanding of black hole seeding mechanisms. Particularly, improving our comprehension of the interplay between black holes and their environments at cosmic dawn could unveil pathways for SMBH formation.

Volonteri concludes with a reflection on the ongoing challenges and prospects of deciphering the formation narrative of SMBHs. The future prospect of corroborating theoretical models with empirical data holds the promise of elucidating these cosmic phenomena, which lie at the intersection of galaxy evolution and complex astrophysical processes.