‘Mysterious’ life histories of supermassive black holes decoded by study

Black holes radiating different light signatures are actually in different life cycle stages, according to a new study.

The research, published last month in The Astrophysical Journal, challenges a model of supermassive black holes which characterises them as all having the same properties.

Such black holes were previously thought to be the same kind of cosmic objects being viewed from different angles.

Scientists, including those from Dartmouth College in the US, said the findings could allow researchers to create more precise models about the evolution of the universe and shed more light on how black holes develop.

Previous studies suggest supermassive black holes are at the centre of nearly all large galaxies, including the Milky Way, devouring galactic gas, dust as well as stars – and sometimes becoming heavier than small galaxies.

As dusty material falls into these black holes at incredible speeds, they light up and emit radiation in infrared or X-ray wavelengths that outshine the entire host galaxy.

Astronomers refer to active supermassive black holes at the centre of their host galaxy which grow by the accretion of matter as Active Galactic Nuclei (AGNs).

“These objects have mystified researchers for over a half-century,” said study lead author Tonima Tasnim Ananna, a postdoctoral research associate at Dartmouth College.

“Over time, we’ve made many assumptions about the physics of these objects. Now we know that the properties of obscured black holes are significantly different from the properties of AGNs that are not as heavily hidden,” Dr Ananna said.

Scientists have been studying AGNs for decades by assessing their light signatures.

Since the 1980s, analysing their X-ray wavelength light signatures, they assumed AGNs usually had a doughnut-shaped ring – or “torus” – of gas and dust around them.

Researchers believed the different brightness and colors associated with the objects were due to the angle from which the AGNs were being observed and how much of the torus was obscuring the view.

Based on this belief, the unified theory of AGNs became the prevalent understanding, scientists said.

In the new study, researchers assessed how quickly black holes are feeding on space matter or their accretion rates.

They found 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.

“This provides support for the idea that the torus structures around black holes are not all the same. There is a relationship between the structure and how it is growing,” explained Ryan Hickox, professor of physics and astronomy and a co-author of the study.

The new research suggests the amount of dust surrounding an AGN is directly related to how much it is feeding, indicating there are differences beyond orientation between different populations of AGNs.

An actively accreting black hole blows away dust and gas and is more likely to appear brighter while a less active AGN is surrounded by a denser torus and appears fainter, scientists said.

“In the past, it was uncertain how the obscured AGN population varied from their more easily observable, unobscured counterparts. This new research definitively shows a fundamental difference between the two populations that goes beyond viewing angle,” Dr Ananna said.

By knowing a black hole’s mass and how actively it is feeding, scientists said they can determine when a majority of supermassive black holes underwent most of their growth.

Researchers believe the latest findings solve a critical piece of the puzzle of “where supermassive black holes come from” and provide valuable information about the evolution of such cosmic entities and the universe.