Supermassive black holes hidden in galaxy collisions

Dust-obscured supermassive black holes are more likely to grow and release enormous amounts of energy when they are inside galaxies that are expected to collide with neighboring galaxies, astronomers have found. The new work, led by researchers at Newcastle University, is published in Monthly Notices of the Royal Astronomical Society.

Galaxies, including our own Milky Way, contain supermassive black holes at their centers. They are millions or even billions of times the mass of the sun. These black holes grow by “eating” the gas that falls on them. However, what drives the gas close enough to the black hole for this to happen remains a mystery.

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

In the final stages of entering the black hole, the gas glows and generates a lot of energy. This energy is usually detected using visible light or X-rays. However, the astronomers who conducted the study could only detect the growing black hole using infrared light. The team drew on data from a number of different telescopes, including the Hubble Space Telescope and the infrared Spitzer Space Telescope.

Researchers have developed a new technique to determine the likelihood that two galaxies are in close proximity and are expected to collide in the future. They applied the new method to hundreds of thousands of galaxies in the distant universe (looking at galaxies that formed between 2 and 6 billion years after the Big Bang), in an attempt to better understand the so-called “cosmic noon,” which is the time period for most of the universe. The growth of galaxies and black holes in the universe is expected to have occurred.

Understanding how black holes have grown during this time is fundamental to modern galactic research, not least because it can give us insight into the supermassive black holes that sit inside our galaxy and how our galaxy has evolved over time.

Because of their great distances, only a handful of cosmic noon galaxies meet the required criteria for their distances to be accurately measured. This makes it difficult to know with high precision whether any two galaxies are very close to each other.

The study proposes a new statistical method to overcome previous limitations in measuring the precise distances of galaxies and supermassive black holes at cosmic noon. It uses a statistical approach to determine galaxy distances using images at different wavelengths and eliminates the need for spectral distance measurements of individual galaxies.

Data from the James Webb Space Telescope in the coming years promises to revolutionize infrared research and reveal more secrets about how these dusty black holes grow.

Sean Dougherty, a postgraduate student at Newcastle University and lead author of the paper, said: “Our new method looks at hundreds of thousands of distant galaxies statistically and asks how likely it is that any two galaxies will come close together, so it is likely It was during a collision.”

Dr Chris Harrison, co-author of the study, said: “Finding these supermassive black holes is very challenging because the X-rays that astronomers normally use to look for these growing black holes are blocked from our telescopes to detect it. But these same black holes can be spotted in infrared light, which is produced by the hot dust surrounding them.”

He added, “The difficulty of finding these black holes and establishing precise distance measurements explains why this result has previously been difficult to pinpoint these distant ‘cosmic noon’ galaxies. With JWST, we look forward to discovering many more of these hidden growing black holes.” JWST will find them better, so we’ll have more things to study, including the hardest ones. From there, we can do more to understand the dust around them, and find out how many are hidden in the distant in the galaxy.”

More information

The study appears in ‘AGN enhancement of ambiguity in galaxy pairs at the cosmic noon: Evidence from probabilistic processing of photometric redshifts‘, Dougherty et al., Monthly Notices of the Royal Astronomical Society, in press. This research was supported by a UK Research and Innovation (UKRI) Future Leaders Fellowship (MR/V022830/1).

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