Direct collapse black hole

Direct collapse black holes (DCBHs) are massive black hole seeds theorized to have formed in the high-redshift Universe and with typical masses at formation of ~10⁵ M_☉, but spanning between 10⁴ M_☉ and 10⁶ M_☉. The environmental physical conditions to form a DCBH (as opposed to a cluster of stars) are the following:[3][4]

1. Metal-free gas (gas containing only hydrogen and helium).
2. Atomic-cooling gas.
3. Sufficiently large flux of Lyman-Werner photons, in order to destroy hydrogen molecules, which are very efficient gas coolants.[12][13]

The previous conditions are necessary to avoid gas cooling and, hence, fragmentation of the primordial gas cloud. Unable to fragment and form stars, the gas cloud undergoes a gravitational collapse of the entire structure, reaching extremely large values of the matter density at the core, of the order of ${\displaystyle \sim 10^{7}g/cm^{3}}$.[14] At this density, the object undergoes a general relativistic instability,[14] which leads to the formation of a black hole of a typical mass ~10⁵ M_☉, and up to 1 million M_☉. The occurrence of the general relativistic instability, as well as the absence of the intermediate stellar phase, led to the denomination of direct collapse black hole. In other words, these objects collapse directly from the primordial gas cloud, not from a stellar progenitor as prescribed in standard black hole models.[15]

A computer simulation reported in July 2022 showed that a halo at the rare convergence of strong, cold accretion flows can create massive black holes seeds without the need for ultraviolet backgrounds, supersonic streaming motions or even atomic cooling. Cold flows produced turbulence in the halo, which suppressed star formation. In the simulation, no stars formed in the halo until it had grown to 40 million solar masses at a redshift of 25.7 when the halo’s gravity was finally able to overcome the turbulence; the halo then collapsed and formed two supermassive stars that died as DCBHs of 31,000 and 40,000 M_☉.[16][17]

Direct collapse black holes are generally thought to be extremely rare objects in the high-redshift Universe, because the three fundamental conditions for their formation (see above in section Formation) are challenging to be met all together in the same gas cloud.[18][19] Current cosmological simulations suggest that DCBHs could be as rare as only about 1 per cubic gigaparsec at redshift 15.[19] The prediction on their number density is highly dependent on the minimum flux of Lyman-Werner photons required for their formation[20] and can be as large as ${\displaystyle \sim 10^{7}}$ DCBHs per cubic gigaparsec in the most optimistic scenarios.[19]

In 2016, a team led by Harvard University astrophysicist Fabio Pacucci identified the first two candidate direct collapse black holes,[21][22] using data from the Hubble Space Telescope and the Chandra X-ray Observatory.[23][24][25][26] The two candidates, both at redshift ${\displaystyle z>6}$, were found in the CANDELS GOODS-S field and matched the spectral properties predicted for this type of astrophysical sources.[27] In particular, these sources are predicted to have a significant excess of infrared radiation, when compared to other categories of sources at high redshift.[21] Additional observations, in particular with the James Webb Space Telescope, will be crucial to investigate the properties of these sources and confirm their nature.[28]

A primordial black hole is the result of the direct collapse of energy, ionized matter, or both, during the inflationary or radiation-dominated eras, while a direct collapse black hole is the result of the collapse of unusually dense and large regions of gas.[29] Note that a black hole formed by the collapse of a Population III star is not considered "direct" collapse.

• Pandey, Kanhaiya L.; Mangalam, A. (2018). "Role of primordial black holes in the direct collapse scenario of supermassive black hole formation at high redshifts". Journal of Astrophysics and Astronomy. 39 (1): 9. arXiv:1801.06649. Bibcode:2018JApA...39....9P. doi:10.1007/s12036-018-9513-x. S2CID 255489158.
• Mayer, Lucio; Bonoli, Silvia (2019). "The route to massive black hole formation via merger-driven direct collapse: A review". Reports on Progress in Physics. 82 (1): 016901. arXiv:1803.06391. Bibcode:2019RPPh...82a6901M. doi:10.1088/1361-6633/aad6a5. PMID 30057369. S2CID 51865966.
• Haemmerlé, Lionel; Heger, Alexander; Woods, Tyrone E. (2020). "On monolithic supermassive stars". Monthly Notices of the Royal Astronomical Society. 494 (2): 2236–2243. arXiv:2003.10467. doi:10.1093/mnras/staa763.