Black Holes Grow Through Eating, Not Merging

Giant black holes primarily grow by gobbling up gas, not by crashing into other black holes.

New research into Massive Black Holes (MBHs) reveals that these cosmic behemoths gain most of their weight by "eating" surrounding gas, a process called accretion. Mergers, while dramatic, play a much smaller role in their overall growth.

How Black Holes Grow

Astronomers sought to understand how the universe's biggest black holes become so immense. These cosmic monsters lurk at the hearts of most galaxies, influencing how entire galaxies change over time.

There are two primary mechanisms by which black holes accrue mass:

Accretion: Pulling in surrounding gas and dust.
Mergers: Colliding and combining with other black holes.

The Research Approach

To investigate, the research team employed:

Cosmic Recipes: Theoretical models based on cold dark matter cosmogony. This framework describes the universe's large-scale structure and formation.
Simulations: The models simulated how MBHs evolve within general populations of black holes and their host galaxies, rather than focusing on specific individual cases.

The simulations accounted for several factors:

How black hole pairs interact and move.
The "kick" they receive from gravitational waves during mergers.
The mass added through the process of gas accretion.

Key Findings

The study conclusively found that accretion is the heavyweight champion of black hole growth. Merging contributed "relatively little to their final mass."

Impact of Mergers

Gravitational Kicks: Gravitational waves from mergers can be so powerful they can even fling black holes out of their home galaxies.
Early Universe Losses: At very early times (redshift Z > 10), between 50% to 90% of merging black hole pairs were "lost" due to these kicks.

Accretion and Spin

Black holes that steadily feed on gas for prolonged periods can achieve very high spin rates.
According to the authors, "The differences in accretion histories for black holes hosted in elliptical or disc galaxies may reflect on different spin distributions."

Implications and Future Research

This discovery profoundly assists scientists in comprehending the grand partnership between black holes and their host galaxies.

It suggests that bright, active black holes known as quasars, observed across vast cosmic distances, likely possess very high spins due to their extensive feeding history.

The study acknowledges certain unknowns, particularly concerning the exact formation mechanisms of the very first black holes.

Future research, especially with upcoming tools like the Laser Interferometer Space Antenna (LISA), aims to detect gravitational waves from black hole mergers. This will provide new and crucial clues about how these colossal objects formed and evolved.

Ultimately, these cosmic giants grow primarily by devouring hot gas, thus shaping their galaxies through powerful cosmic meals.

Citation: arXiv:0709.1722v2 [astro-ph] 16 Sep 2007