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A Hidden Astronomical Wonder: How Does Direct Black Hole Collapse Unravel the Mystery of Supermassive Black Hole Formation?
Direct collapse black holes (DCBHs) are a type of high-mass black hole prototype that form from the direct collapse of large amounts of matter. These black holes are speculated to have formed in the redshift range z=15–30, when the universe was about 100 to 250 million years old. Unlike black hole progenitors formed by first-generation stars (also called Type III stars), direct-collapse black hole seeds are formed through direct general relativistic instabilities. When these black holes form, they typically have a mass of about 10^5 M☉.
This type of black hole prototype was originally proposed to address the challenge of observing supermassive black holes at redshift z~7, which has been confirmed by many observations.
Formation mechanism
Direct-collapse black holes (DCBHs) are thought to be the prototypes of giant black holes formed in the high-redshift universe, with masses of about 10^5 M☉ when they form, but can range from 10^4 M☉ to 10^6 M☉. The environmental conditions for the formation of DCBH are as follows:
- No metal gases (only hydrogen and helium).
- Atoms cool the gas.
- The Lyman-Wierth photon flux is large enough to disrupt the hydrogen molecules, which makes the gas cooling less efficient.
Satisfying the above conditions can prevent gas cooling and thus prevent the fragmentation of the original gas cloud. Gas clouds that fail to fragment and form stars undergo global gravitational collapse and reach extremely high matter densities in their cores, about 10^7 g/cm3. At such densities, the object would experience general relativistic instabilities, eventually forming a black hole with a mass of about 10^5 M☉, or even up to 1 million M☉. The occurrence of this instability, together with the absence of an intermediate stellar stage, leads to this type of black hole being called a direct-collapse black hole.
Quantity Statistics
Direct-collapse black holes are thought to be extremely rare in the high-redshift universe because it is quite challenging to meet the three basic conditions for their formation at once. Current cosmological simulations suggest that the number density of DCBHs at redshift 15 may be only about 1 per cubic gigapascal. In the most optimistic case, based on the minimum Lyman-Wierth photon flux required for formation, the number density could reach about 10^7 DCBHs per cubic gigapascal.
Detection Progress
In 2016, a team led by Harvard University astrophysicist Fabio Pacucci used data from the Hubble Space Telescope and the Chandra X-ray Observatory to identify the first two direct-collapse black hole candidates. Both candidates have redshifts greater than 6 and have spectral properties consistent with those predicted for such sources. In particular, these sources are predicted to have significant excess infrared emission at higher redshifts.
In the future, further observations, especially using the James Webb Space Telescope, will be crucial to investigate the properties of these sources and confirm their nature.
Differences from other black hole types
Primordial black holes are those formed by the direct collapse of energetic or ionized matter during periods of expansion or radiation-dominated periods, while direct collapse black holes are the result of the collapse of unusually dense and large gas regions. It is worth noting that black holes formed by the collapse of Type III stars are not considered "direct" collapses.
In the process of exploring the mysteries of the universe, the importance of direct collapse of black holes has gradually been recognized, and future research may reveal more clues about the mystery of the formation of supermassive black holes. In this mysterious universe, how many unknown black holes are waiting for humans to discover?
