Title: Direct-collapse supermassive black holes from relic particle decay
Authors: Yifan Lu, Zachary S. C. Picker, Alexander Kusenko
First Author’s Institution: Department of Physics and Astronomy, University of California Los Angeles, California
Status: Published in Physical Review Letters [closed access]
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The origin story of supermassive black holes is one of astronomy’s greatest mysteries. Supermassive black holes are found at the centers of most galaxies, and some actively gobble up surrounding gas through a process called accretion, making them some of the most luminous objects in the universe. Although they have existed for billions of years, how they formed and became so massive remains unclear. One leading idea is that the first stars collapsed into “seed” black holes that grew into the supermassive black holes we see today. However, recent discoveries by JWST are shaking up this theory. Some of the black holes found by the James Webb Space Telescope (JWST) are too massive to have grown from seed black holes within the available time.
Another possible way to create seed black holes is through the direct collapse of a dense gas cloud. Normally, the gas clouds from the early universe break into smaller clumps (via a process called fragmentation) that cool and form stars. But if the cooling is somehow prevented, the entire gas cloud can collapse directly to form a black hole. This process would produce much heavier seed black holes, giving them enough time to grow into the massive ones observed by JWST. The conditions that allow the direct collapse of a gas cloud in the early universe are rare. In today’s paper, the authors investigate the role of dark matter in aiding the direct collapse process.
Can we have less H₂, please?
In the early universe, molecular hydrogen (H₂) played a key role in rapidly cooling gas clouds, reducing pressure and leading to fragmentation that eventually formed stars. To avoid this fragmentation, the amount of molecular hydrogen would have to be suppressed. This can happen in two ways: i) Radiation composed of Lyman-Werner photons (they have energy between 11.2-13.6 eV) can break the hydrogen molecule into hydrogen atoms via a process called photodissociation, and ii) photons can also absorb an electron from a hydrogen anion H⁻, which is an intermediate product in H₂ formation, through a process called photodetachment.
Both of these reactions require radiation within a specific energy range, but where does this radiation come from? The authors propose that decaying dark matter particles could be the source of the photons. While the nature of dark matter is yet another mystery, it has been speculated that they are made of unstable particles that decay into photons. If enough radiation from decaying dark matter is present, it can suppress cooling, preventing fragmentation and allowing larger gas clouds to collapse directly into black holes.
Shields disengaged
In previous studies of the direct collapse theory, the Lyman-Werner radiation was assumed to come from external sources, primarily from stars. However, this background radiation would struggle to penetrate dense regions of H₂ in the gas cloud. The molecular hydrogen acts as a shield, significantly reducing the rate of dissociation, thus making it difficult to form a direct collapse black hole.
= 0.5), both the dark matter particle decay can cross the critical reaction rates required. Source: Figure 4 in the paperThe self-shielding effect of H₂ is greatly reduced in the dark matter scenario. Since dark matter is present throughout the gas cloud, the radiation from particle decay will be homogeneous and follow the density distribution of dark matter. This means that the radiation no longer needs to penetrate through the gas cloud; it can reach even the densest regions to effectively dissociate H₂. In Figure 1, the authors show that with reduced hydrogen shielding, the dark matter particle decays effectively and crosses the critical reaction rates required for suppressing molecular hydrogen thereby enabling direct collapse.
If this theory proves true, it would solve two puzzles at once: the nature of dark matter and the origin of supermassive black holes. The authors note that the light from dark matter particle decay could be observed in the optical and UV ranges of the electromagnetic spectrum. This could be observed in the cosmic optical background (COB), which represents the summed emission of light from objects beyond the Milky Way. Future observations of COB by the Hubble Space Telescope will help us look for light from the dark matter particle decay and test the mechanism proposed in this paper.
Astrobite edited by Junellie Gonzalez Quiles
Featured image credit: Pranav Satheesh
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