What Are The Stages In The Life Cycle Of A Supermassive Black Hole? Study Gives Answers To Mystery

A new study led by researchers at Dartmouth College has found that black holes with varying light signatures that were thought to be the same objects being viewed from different angles actually depict different stages of the life cycle. 

The study based on black holes known as “active galactic nuclei” was published in The Astrophysical Journal Supplement Series

What Is An Active Galactic Nucleus?

An active galactic nucleus (AGN) is a small region at the centre of some galaxies that is extremely luminous, and emits so much radiation that it can outshine the rest of the galaxy altogether. Active galactic nuclei are now understood to be active supermassive black holes that emit bright jets and winds, and shape their galaxies. 

The study authors noted there is a need to revise the widely used “unified model of AGN” that characterises supermassive black holes as all having the same properties.

The study gives answers to the mystery about the life cycle of a black hole, and will help scientists better understand the evolution of the universe. 

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In a statement released by Dartmouth College, Tonima Tasnim Ananna, the lead author on the paper, said supermassive black holes have mystified researchers for over a half-century, and that over time, scientists have made many assumptions about the physics of these objects. She added that now researchers know that the properties of obscured black holes are significantly different from the properties of active galactic nuclei that are not as heavily hidden.

Supermassive black holes are objects that devour galactic gas, dust and stars, and can become heavier than smaller galaxies, and are believed to reside at the centre of nearly all large galaxies. The supermassive black hole at the centre of the Milky Way galaxy is known as Sagittarius A*. 

What Do Light Signatures Of Active Galactic Nuclei Determine?

Scientists have always been interested to know more about the light signatures of active galactic nuclei. An active galactic nucleus is a type of supermassive black hole that is “accreting”, or in a rapid growth stage. The accumulation of matter under the influence of a gravitational field is known as accretion. 

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Scientists began studying the light signatures of active galactic nuclei in the late 1980s, and realised that light signatures coming from space ranging from radio wavelengths to X-rays could be attributed to active galactic nuclei. The objects were thought to have a doughnut-shaped ring or “torus” of gas and dust around them.

What Does The ‘Unified Theory Of AGNs’ Say?

Researchers believed that the angle from which active galactic nuclei were being observed and how much of the torus was obscuring the view of the bright objects determined the brightness and colours associated with the cosmic entities.

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Subsequently, the unified theory of AGNs was proposed. According to the theory, a black hole should appear faint if it is being viewed through its torus. The black hole should appear bright if it is being viewed from below or above the ring, the theory states. 

However, the new study has claimed that past research relied too heavily on data from the less obscured objects and skewed research results. 

Stages In The Life Cycle Of A Supermassive Black Hole

The study authors have focused on how quickly black holes are feeding on space matter, or their accretion rates. According to the study, the accretion rate does not depend upon the mass of a black hole, but varies significantly depending on how obscured it is by the gas and dust ring. 

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Ryan Hickox, a co-author on the paper, said this provides support for the idea that the torus structures around black holes are not all the same. He added that there is a relationship between the structure of the black hole and how it is growing. 

The amount of gas surrounding an active galactic nucleus is directly related to how much it is feeding, the study found. This confirms that there are differences beyond orientation between different populations of active galactic nuclei.

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According to the study, a black hole accreting at a high rate generates large amounts of energy which blow away dust and gas, causing the black hole to be unobscured and appear brighter. Meanwhile, an active galactic nucleus which is less active is surrounded by a denser torus, and appears fainter. 

How Was The Study Conducted?

Ananna said that in the past, it was uncertain how the obscured active galactic nuclei population varied from their more easily observable, unobscured counterparts. She stated that the new research definitively shows a fundamental difference between the two populations that goes beyond the viewing angle. 

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Swift-BAT, a high-energy NASA X-ray telescope, has detected several nearby active galactic nuclei since it was commissioned. For a decade, the study authors analysed the active galactic nuclei detected by the telescope.

Ananna said this is a big win for high-energy X-ray telescopes because researchers have never had such a large sample of X-ray detected obscured local active galactic nuclei before. The study authors had analysed active galactic nuclei in the past as well. They used the knowledge about active galactic nuclei gathering as part of previous research to complete the new study. 

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Ananna assessed the effect of obscuring matter on observed properties of black holes with the help of a computational technique. 

What Mystery Does A Black Hole’s Mass And Its Accretion Speed Unravel?

Researchers can determine when most supermassive black holes underwent most of their growth, by knowing a black hole’s mass and how fast it is feeding, the study said. In this way, the paper provides valuable information about the evolution of black holes and the universe. 

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Hickox said one of the biggest questions in the field of astronomy is where do supermassive black holes come from, and that the new study provides a critical piece that can help scientists answer that question.

Next, the researchers aim to study wavelengths that will allow them to search beyond the local universe. Their goal is to understand what triggers active galactic nuclei to go into high accretion mode, and how long it takes rapidly accelerating active galactic nuclei to transition from heavily obscured to unobscured.