Unveiling Hidden Supermassive Black Holes: Galactic Mergers and Gravitational Gluttony

by Tatsuya Nakamura
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Scientists from Newcastle University have conducted groundbreaking research, featured in the Monthly Notices of the Royal Astronomical Society, shedding light on the growth and energetic release of supermassive black holes concealed by dust. The study reveals that these enigmatic entities are more likely to expand and unleash tremendous energy when located within galaxies anticipated to collide with neighboring counterparts.

Nestled at the core of galaxies, including our very own Milky Way, supermassive black holes possess masses millions or even billions of times greater than that of our Sun. Their growth is fueled by the consumption of gas spiraling into them, yet the mechanisms that drive gas close enough for consumption have remained an ongoing enigma.

One intriguing possibility is that when galaxies draw near, gravitational forces compel them to merge, creating a single, larger galaxy. This gravitational dance brings the black holes closer, instigating a remarkable transformation as gas on its final journey into the black hole ignites, generating an immense amount of energy. Conventionally, this energy is detected through visible light or X-rays. However, the team behind this study utilized infrared light to observe the burgeoning black holes. Their analysis incorporated data from various telescopes, including the Hubble Space Telescope and the infrared Spitzer Space Telescope.

To determine the likelihood of galaxies being in close proximity and heading for a collision, the researchers devised a novel technique. Applying this method to hundreds of thousands of galaxies in the distant universe—those formed 2 to 6 billion years after the Big Bang—they sought to unravel the mysteries of the “cosmic noon,” a pivotal period characterized by extensive galaxy and black hole growth.

Understanding the growth of black holes during this era holds significant importance in contemporary galactic research, offering insights into the supermassive black hole residing within the Milky Way and the evolutionary trajectory of our galaxy over time.

The immense distances of cosmic noon galaxies pose a challenge, making it arduous to precisely measure their proximity to one another. The study addresses this issue by presenting a statistical approach that surpasses previous limitations in accurately determining galaxy and supermassive black hole distances during cosmic noon. By employing this statistical methodology, the researchers eliminate the need for spectroscopic distance measurements of individual galaxies, relying instead on images at different wavelengths.

Anticipated data from the upcoming James Webb Space Telescope is expected to revolutionize infrared studies, unveiling further secrets about the growth of these enigmatic, dust-shrouded black holes.

Lead author of the paper, Sean Dougherty, a postgraduate student at Newcastle University, explains, “Our innovative approach employs statistical analysis of hundreds of thousands of distant galaxies, evaluating the likelihood of any two galaxies being in close proximity and heading towards a collision.”

Co-author Dr. Chris Harrison adds, “Finding these supermassive black holes has proven to be a challenge because the X-ray light traditionally used by astronomers is blocked and undetectable by our telescopes. However, these very same black holes can be detected using infrared light emitted by the surrounding hot dust.”

Dr. Harrison further notes, “The difficulty in locating and precisely measuring the distances of these black holes elucidates why pinpointing these distant ‘cosmic noon’ galaxies has been a formidable task. With the James Webb Space Telescope, we anticipate discovering numerous hidden growing black holes. The enhanced capabilities of the JWST will enable us to detect them more effectively, providing us with a wealth of subjects to study, including the most elusive ones. Subsequently, we can deepen our understanding of the surrounding dust and ascertain the prevalence of these hidden giants within distant galaxies.”

Reference: “Obscured AGN enhancement in galaxy pairs at cosmic noon: evidence from a probabilistic treatment of photometric redshifts” by Sean L Dougherty, C M Harrison, Dale D Kocevski and D J Rosario, 8 May 2023, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/stad1300

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