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Obscured Quasars Unveil a Hidden Phase of Galaxy Evolution

A recent study has uncovered a surprising phenomenon: supermassive black holes located at the centers of galaxies, known as quasars, can occasionally be concealed by dense clouds of gas and dust within their host galaxies.

Obscured Quasars Unveil a Hidden Phase of Galaxy Evolution
Artistic illustration of the thick dust torus thought to surround supermassive black holes and their accretion disks. [ESA / V. Beckmann (NASA-GSFC)] Image Credit: Durham University

This finding challenges the conventional notion that quasars are solely obscured by torus-shaped rings of dust located in close proximity to the black hole. Quasars, characterized by their intense brightness, are fueled by the gravitational pull of black holes as they consume surrounding material.

However, their formidable radiation can be obstructed when thick clouds of gas and dust intervene between the quasar and the line of sight.

Astronomers believed that the material obstructing the view of quasars was predominantly located near the quasar, forming a "dusty torus" or donut-shaped structure around it.

However, a team of scientists led by Durham University has now presented evidence suggesting that in certain quasars, the obscuration is primarily caused by the host galaxy where the quasar is situated.

To investigate this, they utilized the Atacama Large Millimeter Array (ALMA) in Chile to observe a group of highly dusty quasars with significant star formation rates. Their observations revealed that many of these quasars are found in compact galaxies, referred to as "starburst galaxies," which are no larger than 3,000 light-years in diameter.

This discovery challenges the conventional understanding of quasar obscuration and highlights the role of the host galaxy in this phenomenon.

These starburst galaxies can generate over a thousand stars, each similar in size to the sun, annually.

Achieving such a high rate of star formation requires a substantial reservoir of gas and dust, which essentially serve as the fundamental elements for the formation of stars. Within these galaxies, the clouds of gas and dust, perturbed by the rapid process of star formation, can accumulate and entirely obscure the quasar within them.

The research was published in the journal Monthly Notices of the Royal Astronomical Society (MNRAS).

It's like the quasar is buried in its host galaxy. In some cases, the surrounding galaxy is so stuffed with gas and dust, not even X-Rays can escape. We always thought the dusty donut around the black hole was the only thing hiding the quasar from view. Now we realize the entire galaxy can join in. This phenomenon only seems to happen when the quasar is undergoing an intense growth spurt.

Carolina Andonie, Study Lead Author and PhD Student, Center for Extragalactic Astronomy, Durham University

The team suggests the host galaxy is the sole cause of obscuration in approximately 10–30% of rapidly star-forming quasars.

The results provide insights into the connection between galaxy development and black hole behavior.

Obscured quasars might signify an initial evolutionary phase when young galaxies contain abundant cold gas and dust, supporting rapid star formation and black hole expansion.

It's a turbulent, messy phase of evolution, when gas and stars collide and cluster in the galaxy's center. The cosmic food fight cloaks the baby quasar in its natal cocoon of dust.

David Alexander, Study Co-Author and Professor, Durham University

Revealing these concealed quasars will aid scientists in comprehending the relationship between galaxies and the supermassive black holes at their centers.

Journal Reference:

Andonie, C., et al. (2023). Obscuration beyond the nucleus: infrared quasars can be buried in extreme compact starbursts. Monthly Notices of the Royal Astronomical Society: Letters.


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