Editorial Feature

Massive Black Holes: Where Do They Come From?

As we learn more about the Universe, observing it in electromagnetic radiation and with gravitational waves, the mystery of how massive black holes reach tremendous sizes has also grown. 

black holes, massive black holes, supermassive black holes, stellar, space, gravitational, gravity

 Computer-simulated image showing a supermassive black hole at the core of a galaxy. Image Credits: NASA, ESA, and D. Coe, J. Anderson, and R. van der Marel (STScI)

The more we learn about the Universe, the better we understand the spacetime events called black holes. However, black holes, first predicted in Einstein’s theory of general relativity, are still hiding some mysteries. 

Until recently we had observed a wide mass divide between so-called stellar-mass black holes, with masses of between around 5 and 10 times that of our sun, and much larger supermassive black holes that dwell at the centre of most galaxies with masses millions and billions of times that of our sun. However, recent discoveries have revealed that black holes exist between these two ends of the black hole mass spectrum. 

Observations from the Chandra, XMM-Newton, and Hubble telescopes have found evidence of so-called ‘intermediate-mass black holes.’ Additionally, as the Laser Interferometer Gravitational-Wave Observatory (LIGO) began making its measurements of ripples in spacetime, we began to notice something strange. 

Some of the mergers observed by LIGO and its gravitational wave detecting partner, Virgo, involve ‘seed’ black holes that are way too massive to have been formed by the collapse of a star, the ‘traditional’ method of black hole formation, as stars in binary systems don not get large enough to leave behind such a massive stellar remnant. 

Further, there is yet another concerning element regarding massive black holes: researchers are increasingly finding huge black holes in the early universe. This presents a problem for the idea that black holes grow through a series of mergers, as this should not happen so quickly.

All this means that as the mass divide between stellar-mass black holes and supermassive black holes becomes less of a desert, researchers are left with more questions than answers; namely, how do massive black grow, and how do they do it so quickly?

Click Here to Hear What a Black Hole Sounds Like

A new paper published in the journal Nature Review Physics looks at recent developments in the field of astrophysics and highlights possible solutions to the black hole growth problem. The authors form two mass categories for black holes, low-mass stellar black holes, and massive black holes. This latter category runs from upper mass stellar black holes all the way to supermassive black holes, like those found at the centre of most galaxies.

The Seeds of Black Holes

It is very clear that there must be different formation methods at play for black holes of varying masses. It is almost certain that stellar-mass black holes at the low mass end of the spectrum form when stars with stellar cores 3 times larger than the sun run out of nuclear fuel and can no longer support themselves against gravitational collapse.

When the black holes that this process creates are in binary systems, material from a companion star can ‘feed’ the black hole, enabling it to grow in size. Likewise, if the black hole is in a region of gas and dust, this material can be fed to the black hole, enabling it to grow.

The problem is that this is much too slow a process to account for massive black holes. These could only form if smaller black holes merge. This couldn’t just happen once, however; these black holes then go on to merge with other black holes. 

This process of hierarchical mergers continues until the merger of galaxies with supermassive black holes at their centres brings these truly massive black holes together to form the most massive of these spacetime events. 

 The Nature paper suggests that the seeds for massive black holes could have a primordial origin forming in the early Universe from perturbations in high-density regions of space. 

They explain another, more recent method of formation for these massive black hole seeds: “Seeds could then have been generated in galaxies, from the collapse of massive stars or from mergers of stars or stellar black holes.”

The problem is that there is not yet a single coherent mechanism that describes how these seeds could come together and form massive black holes. Some proposed mechanisms can explain the abundance of observed massive black holes, others can explain the presence of such black holes in the early Universe.

NASA | Massive Black Hole Shreds Passing Star

Video Credit: NASA Goddard/Youtube.com

However, it’s rare for a mechanism to be able to explain both, and because black holes influence how galaxies form and change over time, there is a big hole in our understanding of how the Universe has evolved to take the shape that we currently observe.

Not all is lost with regards to explaining the growth of massive black holes, however. This year alone, a multitude of studies have revealed a multitude of systems that astronomers, astrophysicists, and cosmologists could study to reveal the secrets of these mergers and explain where massive black holes come from. 

A Solution to the Black Hole Growth Mystery Could Be 89 Million Light Years Away

One recent piece of research may have the answer to black hole growth problems. While black hole mergers are usually observed after the fact, scientists have discovered a pair of black holes close to Earth prior to their collision.

The supermassive black holes in the galaxy NGC 7727, just 89 million light-years from Earth, have the closest separation of any supermassive black hole pairing yet discovered. And, excitingly, it seems that they were brought into such close proximity by a merger between galaxies.

Though the black holes won’t collide and merge for at least another 250 million years, a short timespan on a cosmological scale, a paper published in the journal Astronomy & Astrophysics suggests that by studying this system astronomers could finally crack the secret of what puts the ‘massive’ into massive black holes. 

In another study published in The Astrophysical Journal, astronomers revealed that they have discovered a dwarf galaxy orbiting the Milky Way with a supermassive black hole at its centre that is almost as large as that sat at the heart of the Milky Way, a much larger galaxy.

This relates to the question of how massive black holes form, as researchers are currently struggling to explain how a dwarf galaxy, Leo I, came to have a supermassive black hole at least 3 million times as massive as the sun.

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The most likely explanation is that a series of mergers between galaxies has seen the black hole undergo multiple collisions absorbing smaller black holes and growing as a result.

The authors of the Nature paper assessing the merger mechanism of creating black holes say that problems still exist with this plausible way of forming massive black holes. 

This includes explaining how these massive black hole seeds can continue merging without being kicked from the galaxies they inhabit by gravitational interactions. 

Other mechanisms are suggested by the researchers that could account for the growth of massive black holes, including the collapse of regions in the early Universe before galaxy formation had even occurred.

While there is no observational evidence for this currently, one of the key benefits of the James Webb Space Telescope set to launch in December is its ability to allow astronomers to gaze further into the Universe, and thus further back in time, than ever before.

This could potentially allow them to catch a glimpse of the processes that led to the existence of the Universe’s most titanic, powerful, and mysterious objects.

References and Further Reading 

Volonteri. M., Habouzit. M., Colpi. M., [2021], ‘The origins of massive black holes,’ [https://www.nature.com/articles/s42254-021-00364-9]

Voggel. K. T., Seth. C., Baumgardt. H., et al, [2021], ‘First direct dynamical detection of a dual super-massive black hole system at sub-kpc separation,’ Astronomy & Astrophysics, [https://doi.org/10.1051/0004-6361/202140827]

Bustamante-Rosell, M.J., et al, [2021], ‘Dynamical Analysis of the Dark Matter and Central Black Hole Mass in the Dwarf Spheroidal Leo I,’ The Astrophysical Journal, [https://iopscience.iop.org/article/10.3847/1538-4357/ac0c79/pdf]

Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.

Robert Lea

Written by

Robert Lea

Robert is a Freelance Science Journalist with a STEM BSc. He specializes in Physics, Space, Astronomy, Astrophysics, Quantum Physics, and SciComm. Robert is an ABSW member, and aWCSJ 2019 and IOP Fellow.

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