Hai-Bo Yu

How a supermassive black hole originates

Anonymous — Thu, 01 Jul 2021 18:24:49 +0000

How a supermassive black hole originates Anonymous (not verified) Thu, 07/01/2021 - 11:24 More News Iqbal Pittalwala | UCR News June 16, 2021

Supermassive black holes, or SMBHs, are black holes with masses that are several million to billion times the mass of our sun. The Milky Way hosts an SMBH with mass a few million times the solar mass. Surprisingly, astrophysical observations show that SMBHs already existed when the universe was very young. For example, a billion solar mass black holes are found when the universe was just 6% of its current age, 13.7 billion years. How do these SMBHs in the early universe originate?

A team led by a theoretical physicist at the University of California, Riverside, has come up with an explanation: a massive seed black hole that the collapse of a dark matter halo could produce.

Dark matter halo is the halo of invisible matter surrounding a galaxy or a cluster of galaxies. Although dark matter has never been detected in laboratories, physicists remain confident this mysterious matter that makes up 85% of the universe’s matter exists. Were the visible matter of a galaxy not embedded in a dark matter halo, this matter would fly apart.

Hai-Bo Yu is a theoretical physicist with expertise in the particle properties of dark matter.
(Samantha Tieu)

“Physicists are puzzled why SMBHs in the early universe, which are located in the central regions of dark matter halos, grow so massively in a short time,” said Hai-Bo Yu, an associate professor of physics and astronomy at UC Riverside, who led the study that appears in Astrophysical Journal Letters. “It’s like a 5-year-old child that weighs, say, 200 pounds. Such a child would astonish us all because we know the typical weight of a newborn baby and how fast this baby can grow. Where it comes to black holes, physicists have general expectations about the mass of a seed black hole and its growth rate. The presence of SMBHs suggests these general expectations have been violated, requiring new knowledge. And that’s exciting.”

A seed black hole is a black hole at its initial stage — akin to the baby stage in the life of a human.

“We can think of two reasons,” Yu added. “The seed — or ‘baby’ — black hole is either much more massive or it grows much faster than we thought, or both. The question that then arises is what are the physical mechanisms for producing a massive enough seed black hole or achieving a fast enough growth rate?”

“It takes time for black holes to grow massive by accreting surrounding matter,” said co-author Yi-Ming Zhong, a postdoctoral researcher at the Kavli Institute for Cosmological Physics at the University of Chicago. “Our paper shows that if dark matter has self-interactions then the gravothermal collapse of a halo can lead to a massive enough seed black hole. Its growth rate would be more consistent with general expectations.”

In astrophysics, a popular mechanism used to explain SMBHs is the collapse of pristine gas in protogalaxies in the early universe.

“This mechanism, however, cannot produce a massive enough seed black hole to accommodate newly observed SMBHs — unless the seed black hole experienced an extremely fast growth rate,” Yu said. “Our work provides an alternative explanation: a self-interacting dark matter halo experiences gravothermal instability and its central region collapses into a seed black hole.”

The explanation Yu and his colleagues propose works in the following way:

Dark matter particles first cluster together under the influence of gravity and form a dark matter halo. During the evolution of the halo, two competing forces — gravity and pressure — operate. While gravity pulls dark matter particles inward, pressure pushes them outward. If dark matter particles have no self-interactions, then, as gravity pulls them toward the central halo, they become hotter, that is, they move faster, the pressure increases effectively, and they bounce back. However, in the case of self-interacting dark matter, dark matter self-interactions can transport the heat from those “hotter” particles to nearby colder ones. This makes it difficult for the dark matter particles to bounce back.

Yu explained that the central halo, which would collapse into a black hole, has angular momentum, meaning, it rotates. The self-interactions can induce viscosity, or “friction,” that dissipates the angular momentum. During the collapse process, the central halo, which has a fixed mass, shrinks in radius and slows down in rotation due to viscosity. As the evolution continues, the central halo eventually collapses into a singular state: a seed black hole. This seed can grow more massive by accreting surrounding baryonic — or visible — matter such as gas and stars.

“The advantage of our scenario is that the mass of the seed black hole can be high since it is produced by the collapse of a dark matter halo,” Yu said. “Thus, it can grow into a supermassive black hole in a relatively short timescale.”

The new work is novel in that the researchers identify the importance of baryons—ordinary atomic and molecular particles — for this idea to work.

“First, we show the presence of baryons, such as gas and stars, can significantly speed up the onset of the gravothermal collapse of a halo and a seed black hole could be created early enough,” said Wei-Xiang Feng, Yu’s graduate student and a co-author on the paper. “Second, we show the self-interactions can induce viscosity that dissipates the angular momentum remnant of the central halo. Third, we develop a method to examine the condition for triggering general relativistic instability of the collapsed halo, which ensures a seed black hole could form if the condition is satisfied.”

Over the past decade, Yu has explored novel predictions of dark matter self-interactions and their observational consequences. His work has shown that self-interacting dark matter can provide a good explanation for the observed motion of stars and gas in galaxies.

“In many galaxies, stars and gas dominate their central regions,” he said. “Thus, it’s natural to ask how the presence of this baryonic matter affects the collapse process. We show it will speed up the onset of the collapse. This feature is exactly what we need to explain the origin of supermassive black holes in the early universe. The self-interactions also lead to viscosity that can dissipate angular momentum of the central halo and further help the collapse process.”

The study was funded by the U.S. Department of Energy; NASA; the Kavli Institute for Cosmological Physics; and the John Templeton Foundation.

The research paper is titled “Seeding Supermassive Black Holes with Self-Interacting Dark Matter: A Unified Scenario with Baryons.”

Header image credit: Event Horizon Telescope Collaboration.

Physicists explain mysterious dark matter deficiency in galaxy pair

Anonymous — Thu, 10 Sep 2020 03:18:51 +0000

Physicists explain mysterious dark matter deficiency in galaxy pair Anonymous (not verified) Wed, 09/09/2020 - 20:18 More News Iqbal Pittalwala | UCR News September 09, 2020

Anew theory about the nature of dark matter helps explain why a pair of galaxies about 65 million light-years from Earth contains very little of the mysterious matter, according to a study led by a physicist at the University of California, Riverside.

Dark matter is nonluminous and cannot be seen directly. Thought to make up 85% of matter in the universe, its nature is not well understood. Unlike normal matter, it does not absorb, reflect, or emit light, making it difficult to detect.

The prevailing dark matter theory, known as cold dark matter, or CDM, assumes dark matter particles are collisionless, aside from gravity. A newer second theory, called self-interacting dark matter, or SIDM, proposes dark matter particles self-interact through a new dark force. Both theories explain how the overall structure of the universe emerges, but they predict different dark matter distributions in the inner regions of a galaxy. SIDM suggests dark matter particles strongly collide with one another in a galaxy’s inner halo, close to its center.

Hai-Bo Yu is a theoretical physicist with expertise in the particle properties of dark matter. (Samantha Tieu)

Typically, a visible galaxy is hosted by an invisible dark matter halo — a concentrated clump of material, shaped like a ball, that surrounds the galaxy and is held together by gravitational forces. Recent observations of two ultra-diffuse galaxies, NGC 1052-DF2 and NGC 1052-DF4, show, however, that this pair of galaxies contains very little, if any, dark matter, challenging physicists’ understanding of galaxy formation. Astrophysical observations suggest NGC 1052-DF2 and NGC 1052-DF4 are likely satellite galaxies of NGC1052.

“It is commonly thought that dark matter dominates the overall mass in a galaxy,” said Hai-Bo Yu, an associate professor of physics and astronomy at UCR, who led the study. “Observations of NGC 1052-DF2 and -DF4 show, however, that the ratio of their dark matter to their stellar masses is about 1, which is 300 times lower than expected. To resolve the discrepancy, we considered that the DF2 and DF4 halos may be losing the majority of their mass through tidal interactions with the massive NGC 1052 galaxy.”

Using sophisticated simulations, the UCR-led team reproduced the properties of NGC 1052-DF2 and NGC 1052-DF4 through tidal stripping — the stripping away of material by galactic tidal forces — by NGC1052. Because the satellite galaxies cannot hold the stripped mass with their own gravitational forces, it effectively gets added to NGC 1052’s mass.

The researchers considered both CDM and SIDM scenarios. Their results, published in Physical Review Letters, indicate SIDM forms dark-matter-deficient galaxies like NGC 1052-DF2 and -DF4 far more favorably than CDM, as the tidal mass loss of the inner halo is more significant and the stellar distribution is more diffuse in SIDM.

The research paper has been selected as an “editors’ suggestion” by the journal, an honor that only a select few papers receive each week to promote reading across fields.

Yu explained tidal mass loss could occur in both CDM and SIDM halos. In CDM, the inner halo structure is “stiff” and resilient to tidal stripping, which makes it difficult for a typical CDM halo to lose sufficient inner mass in the tidal field to accommodate observations of NGC 1052-DF2 and -DF4. In contrast, in SIDM, dark matter self-interactions could push dark matter particles from the inner to the outer regions, making the inner halo “fluffier” and enhancing the tidal mass loss accordingly. Further, the stellar distribution becomes more diffuse.

“A typical CDM halo remains too massive in the inner regions even after tidal evolution,” Yu said.

Next, the team will perform a more comprehensive study of the NGC 1052 system and explore newly discovered galaxies with novel properties in an effort to better understand the nature of dark matter.

Yu was joined in the study by Daneng Yang and Haipeng An of Tsinghua University in Beijing, China. Yu was supported by grants from the U.S. Department of Energy and the U.S. National Science Foundation.

The title of the research paper is “Self-Interacting Dark Matter and the Origin of Ultradiffuse Galaxies NGC1052-DF2 and -DF4.”

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Header image credit: NASA, ESA, and P. van Dokkum (Yale University).

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