Do black holes grow
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Do Black Holes Grow? Analyzing the Evidence
Black Hole Growth Mechanisms
Chaotic Accretion and Spin Dynamics
Black holes can grow significantly through a process known as chaotic accretion. This involves the black hole gaining mass from a series of randomly oriented accretion episodes. The Bardeen-Petterson effect, which causes counteralignment of the black hole and disc spins under certain conditions, allows for rapid mass growth without the black hole's spin increasing excessively. This is crucial because a high spin would otherwise limit the accretion efficiency and slow down the growth process .
Intermediate Mass Black Holes as Seeds
Intermediate Mass Black Holes (IMBHs), formed at high redshift, are considered the seeds from which supermassive black holes (SMBHs) grow. These IMBHs can accrete primordial gas within their parent dark matter halos, leading to substantial growth. This process is essential for explaining the presence of SMBHs less than 1 billion years after the Big Bang .
Super-Eddington Accretion
Super-Eddington accretion is another mechanism that allows black holes to grow rapidly. This involves accretion rates that exceed the Eddington limit, which is the balance point where radiation pressure from the accreting material counteracts the gravitational pull of the black hole. In dense, gas-rich environments, black holes can sustain super-Eddington accretion, leading to significant mass increases 810.
Growth Efficiency and Constraints
Feeding-Dominated vs. Feedback-Limited Regimes
The growth efficiency of high-redshift black holes is influenced by the balance between feeding-dominated and feedback-limited regimes. In the feeding-dominated regime, black holes can accrete a large fraction of the available gas, leading to rapid growth. Conversely, in the feedback-limited regime, intermittent accretion due to feedback processes limits the growth rate .
Physical Limits to Black Hole Mass
There is a theoretical upper limit to the mass a black hole can achieve through luminous accretion, approximately 5 × 10^10 solar masses under typical conditions. Beyond this limit, black holes cannot grow further through luminous accretion and must rely on non-luminous means such as mergers with other black holes .
Observational Evidence
Accretion in Active Galactic Nuclei (AGN)
Observations of narrow line Seyfert 1 galaxies (NLS1s) show that black holes can grow substantially during high-accretion phases. These galaxies often lie below the MBH-σ relation of normal galaxies, indicating significant mass growth during periods of high accretion rates .
Star-Forming Galaxies and Quasars
Ultra-deep X-ray observations of distant star-forming galaxies reveal that black holes in these galaxies grow almost continuously during periods of intense star formation. This suggests a close association between black hole growth and star formation in the early universe, supporting the idea of a pre-quasar phase where black holes grow rapidly before becoming optically luminous quasars .
Conclusion
In summary, black holes do indeed grow, and they do so through various mechanisms including chaotic accretion, super-Eddington accretion, and accretion in star-forming galaxies. The growth efficiency is influenced by the balance between feeding-dominated and feedback-limited regimes, and there are theoretical limits to how massive black holes can become through luminous accretion. Observational evidence from AGNs and star-forming galaxies supports these theoretical models, providing a comprehensive understanding of black hole growth across different cosmic epochs.
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