Gravitational gorge: How galactic mergers give birth to hidden supermassive black holes

Dusty region around a black hole

An artist’s impression of a dusty region around a black hole. More dust-covered black holes can completely block X-rays and visible light from escaping, but the same dust can be heated by a growing black hole and will glow brightly at infrared wavelengths. Credits: ESA/NASA, the AVO project and Paolo Padovani

New research conducted by Newcastle University and published in Monthly Notices of the Royal Astronomical Society revealed that dust-obscured supermassive black holes are more likely to grow and release enormous amounts of energy when inside galaxies expected to collide with a nearby galaxy.

Supermassive black holes, with masses millions or even billions of times that of our Sun, are found at the heart of galaxies, including our own Milky Way. These black holes grow in size by consuming the gas spiraling inside them. However, the factors that push gas close enough to black holes for consumption remain an ongoing investigation.

One possibility is that when galaxies are close enough together, they are likely to be gravitationally pulled towards each other and merge into a larger galaxy.

In the final stages of his journey in a black hole, the gas ignites and produces a huge amount of energy. This energy is typically detected using visible light or X-rays. However, the astronomers who conducted this study were only able to detect growing black holes using infrared light. The team used data from many different telescopes, including the Hubble Space Telescope and the infrared Spitzer Space Telescope.

Researchers have developed a new technique for determining the probability that two galaxies are very close to each other and are expected to collide in the future. They applied this new method to hundreds of thousands of galaxies in the distant universe (observing galaxies formed 2 to 6 billion years after the big Bang) in an effort to better understand the so-called cosmic noon, a time when most of the galaxy and black hole growth of the Universe is predicted to have taken place.

Understanding how black holes grew during this period is crucial in modern galactic research, especially as it could give us insight into the supermassive black hole located within the Milky Way and how our galaxy has evolved over time.

Because they are so far away, only a small number of cosmic midday galaxies meet the criteria required to obtain precise measurements of their distances. This makes it very difficult to know with high precision whether two galaxies are very close to each other.

This study presents a new statistical method to overcome the previous limitations of measuring the accurate distances of galaxies and supermassive black holes at cosmic noon. It applies a statistical approach to determining distances to galaxies using images at different wavelengths and eliminates the need for spectroscopic distance measurements for individual galaxies.

Data coming from James Webb Space Telescope in the next few years it should revolutionize infrared studies and reveal even more secrets about how these dusty black holes grow.

Sean Dougherty, a postgraduate student at Newcastle University and lead author of the paper, says: ‘Our new approach looks at hundreds of thousands of distant galaxies with a statistical approach and asks how likely it is that two galaxies are close to each other and therefore likely on a collision course.’

Dr. Study co-author Chris Harrison says these supermassive black holes are very hard to find because the X-ray light, which astronomers have typically used to find these growing black holes, is blocked and undetected by our telescopes. But these black holes themselves can be found using infrared light, produced by the hot dust that surrounds them.

He adds: ‘The difficulty in finding these black holes and establishing precise distance measurements explains why this result has previously been difficult to pin down precisely on these distant midday cosmic galaxies.’ With JWST we expect to find many more of these growing hidden black holes. JWST will be much better at finding them, so we’ll have many more to study, including those that are harder to find. From there, we can do more to understand the dust around them and discover how many are hidden in distant galaxies.

Reference: Enhancement of darkened AGN in galaxy pairs at cosmic noon: Evidence from a probabilistic treatment of photometric redshifts by Sean L Dougherty, CM Harrison, Dale D Kocevski, and DJ Rosario, May 8, 2023, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/stad1300

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