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Intermediate black holes: Scientists have found the cosmic object’s awkward phase

Between stellar and supermassive black holes, there is an evolutionary phase of intermediate black holes.

NASA

Black holes typically come in two sizes: stellar-mass black holes, which are five to ten times the mass of the Sun, or supermassive black holes, which are millions or billions of times the mass of the Sun.

Astronomers have long suspected that there may be another, third size — an awkward phase if you like — but confirming black holes exist within this in-between range has proven difficult. Several false or ambiguous detections for these so-called intermediate black holes have muddied the waters, too.

But a new paper published in Nature Astronomy may be the best evidence yet for the existence of these elusive creatures. The findings stem from observations of the aftermath of an explosion that took place when the universe was just 3 billion years old.

Intermediate black holes are those which have a mass somewhere between stellar and supermassive black holes — in fact, astronomers theorize they are an evolutionary in-between phase for these cosmic behemoths.

Intermediate black holes are particularly interesting because they may hold the key to understanding how these curious beings grow and evolve over time. But these cosmic adolescents are shrouded in mystery, as they appear to be scarce throughout the universe.

The new discovery could provide insight into how black holes form, especially in the early universe.

“If a significant intermediate-mass black hole population exists, it could provide the seeds for the growth of supermassive black holes in the early Universe,” the authors write in the new study.

What are black holes?

To understand an intermediate black hole, we need to zoom out and consider what we know about these behemoths as a whole. A black hole is a region of space with a gravitational pull so strong that nothing can escape its influence, not even light itself.

  • Black holes were first identified in Einstein’s Theory of General Relativity
  • You can’t directly see a black hole, only its effects on the surrounding environment
  • The event horizon is the boundary that outlines the limits of the black hole
Radio data from the National Science Foundation’s Very Large Array facility was used to construct this image of Cygnus A, a galaxy with a supermassive black hole at its center with jets of material firing off that span across half a million light years.NRAO/AUI

The largest-known black holes lurk at the center of galaxies, and scientists believe that most large galaxies have a supermassive black hole at their center.

Black holes grow by feeding off their surrounding material, swallowing up gas from neighboring stars or other celestial objects.

But the two main types of black holes have vastly different formation mechanisms:

  • Stellar black holes form in the aftermath of the collapse of a large star
  • The birth of supermassive black holes is not well understood
  • Supermassive black holes have existed for billions of years, and power the centers of large galaxies
  • Scientists are not sure how supermassive black holes formed and evolved over time to reach their massive sizes today

What are intermediate-mass black holes?

Intermediate-mass black holes weigh in at between 100 to 10,000 solar masses.

Astronomers have long predicted the existence of intermediate black holes, but finding them in the universe is another matter. Plenty of candidates have come to light over the years, but they have proven difficult to confirm. Then, in May 2019, scientists detected something incredible: A new, intermediate black hole during formation.

The National Science Foundation’s Laser Interferometer Gravitational-wave Observatory (LIGO) detected gravitational waves from the merger of two stellar-mass black holes — the collision appeared to birth an intermediate black hole.

A LIGO animation demonstrating the formation of an intermediate-mass black hole.

The two colliding black holes had masses of about 85 and 66 times the mass of the Sun respectively. As they merged, they created an even larger black hole, which had a mass of about 142 solar masses.

The newborn black hole marked the first time scientists have observed the birth of an intermediate-mass black hole — and it helped to confirm that these beasts do indeed exist beyond a theory.

What’s new — In the new Nature Astronomy study, scientists detected a gamma-ray burst dubbed GRB 950830, which is a high-energy explosion from a distant galaxy. Although black holes themselves don’t emit light, matter falling into black holes heats up and they emit ultraviolet light, X-rays, and gamma rays.

How do astronomers find black holes?

Gamma rays like GRB 950830 have been used to identify black holes before.

The astronomers behind the new study were able to peer back into the early universe using gravitational lensing. Gravitational lensing is a technique whereby the light being emitted by a distant source in the universe acts as a lens, with the light becoming distorted as a result of the distribution of matter between the source and the object it is traveling towards.

Looking back to when the universe was just 3 billion years old (for context, the universe is believed to be about 13.8 billion years old), the scientists behind the new study spotted distorted light from a gamma ray explosion in the early universe caused by an object of about 10,000 solar masses.

The study suggests that the object in question serves as a good candidate for an intermediate black hole.

The recent observation has implications for scientists’ understanding of how black holes formed in the early universe, and how long it took for them to grow to their massive sizes.

What’s next — The more we learn about black holes, the more we discover that no two are the same.

Black holes are generally a product of their environment, with the size of the black hole believed to be connected to the size of the galaxy in which it is located. However, a lot of the physical mechanisms that govern black holes are yet to be understood as these objects are hard to observe.

However, scientists have made great strides in the past couple of years with the first image of a black hole released in April 2019.

Follow-up observations are planned for black hole M87, the first to be captured by an array of telescopes, in order to better understand how these massive creatures behave, and how they formed in the early universe.

Abstract: If gamma-ray bursts are at cosmological distances, they must be gravitationally lensed occasionally. The detection of lensed images with millisecond-to-second time delays provides evidence for intermediate-mass black holes, a population that has been difficult to observe. Several studies have searched for these delays in gamma-ray burst light curves, which would indicate an intervening gravitational lens3–6. Among the ~104 gamma-ray bursts observed, there have been a handful of claimed lensing detections, but none have been statistically robust. Here we present a Bayesian analysis identifying gravitational lensing in the light curve of GRB 950830. The inferred lens mass Ml depends on the unknown lens redshift zl , and is given by ð1 þ zlÞMl ¼ 5:5þ1:7 �0:9 ´ 104 M (90% credibility), which we interpret as evidence for an I intermediate-mass black hole. The most probable configuration, with a lens redshift zl≈ 1 and a gamma-ray burst redshift zs≈ 2, yields a present-day number density of about 2:3þ4:9 �1:6 ´ 103 Mpc�3 I (90% credibility) with a dimensionless energy density ΩIMBH  4:6þ9:8 �3:3 ´ 10�4 I . The false alarm probability for this detection is ~0.6% with trial factors. While it is possible that GRB 950830 was lensed by a globular cluster, it is unlikely as we infer a cosmic density inconsistent with predictions for globular clusters ΩGC≈ 8 × 10−6 at 99.8% credibility. If a significant intermediate-mass black hole population exists, it could provide the seeds for the growth of supermassive black holes in the early Universe.
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