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ING research news release
26 April, 2023

Astronomers Solve the 60-Year Mystery of How Quasars Are Ignited

Scientists have unlocked one of the biggest mysteries of quasars – the brightest, most powerful objects in the Universe – by establishing that they are ignited by mergers between galaxies. First discovered 60 years ago, quasars can shine as brightly as a trillion stars packed into a volume the size of our Solar System. In the decades since they were first observed, it has remained a mystery what could trigger such powerful activity. New work led by scientists at the Universities of Sheffield and Hertfordshire and the Instituto de Astrofisica de Canarias has now revealed that they are the consequence of galaxies merging.

The mergers were discovered when researchers, using the deep imaging capabilities of the Wide Field Camera on the Isaac Newton Telescope and the PF-QHY camera on the William Herschel Telescope, observed the presence of distorted structures in the outer regions of the galaxies that are home to quasars.

Most galaxies have supermassive black holes at their centres. They also contain substantial amounts of gas, but most of the time this gas is orbiting at large distances from the galaxy centres, out of reach of the black holes. When two galaxies merge, gravitational forces drive the gas towards the black hole at the centre of the remnant galaxy system; just before the gas is consumed by the black hole, it releases extraordinary amounts of energy in the form of radiation, resulting in the characteristic quasar brilliance.

The ignition of a quasar can have dramatic consequences for entire galaxies – it can drive the rest of the gas out of the galaxy, which prevents it from forming new stars for billions of years into the future.

This is the first time that a sample of quasars of this size has been imaged to such a high level of sensitivity. By comparing observations of 48 quasars and their host galaxies with images of over 100 non-quasar galaxies, researchers concluded that galaxies hosting quasars are approximately three times as likely to be interacting or colliding with other galaxies. The study has provided a significant step forward in our understanding of how these powerful objects are triggered and fuelled.

Example WFC/INT images of quasars taken as part of this study, showing distorted structures in the outer parts of the systems that are characteristic of galaxy mergers. Credit: G. Pérez, IAC. Large format: JPG.

Professor Clive Tadhunter, from the University of Sheffield's Department of Physics and Astronomy, said: "Quasars are one of the most extreme phenomena in the Universe, and what we see is likely to represent the future of the Milky Way when it collides with the Andromeda galaxy in about five billion years. It's exciting to observe these events and finally understand why they occur – but thankfully Earth won't be anywhere near one of these apocalyptic episodes for quite some time."

Quasars are important to astrophysicists because, due to their brightness, they stand out at large distances and therefore act as beacons to the earliest epochs in the history of the Universe. Dr Jonny Pierce, Post-Doctoral Research Fellow at the University of Hertfordshire, explains:

"It's an area that scientists around the world are keen to learn more about – one of the main scientific motivations for NASA's James Webb Space Telescope was to study the earliest galaxies in the Universe, and Webb is capable of detecting light from even the most distant quasars, emitted nearly 13 billion years ago. Quasars play a key role in our understanding of the history of the Universe, and possibly also the future of the Milky Way".

About the William Herschel and Isaac Newton Telescopes

The William Herschel (WHT) and the Isaac Newton Telescope (INT) are operated on the island of La Palma by the Isaac Newton Group of Telescopes (ING) in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias (IAC). The ING is funded by the Science and Technology Facilities Council (STFC-UKRI) of the United Kingdom, the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) of the Netherlands, and the IAC in Spain. IAC's contribution to ING is funded by the Spanish Ministry of Science, Innovation and Universities.

Journal article

J. C. S. Pierce, C. Tadhunter, C. Ramos Almeida, P. Bessiere, J. V. Heaton, S. L. Ellison, G. Speranza, Y. Gordon, C. O'Dea, L. Grimmett and L. Makrygianni, 2023, "Galaxy interactions are the dominant trigger for local type 2 quasars", MNRAS, 522, 1736. Paper: stad455.


Clive Tadhunter
University of Sheffield’s Department of Physics and Astronomy, UK

Jonny Pierce
Post-Doctoral Research Fellow at the University of Hertfordshire, UK

Cristina Ramos Almeida
Instituto de Astrofísica de Canarias, Spain

Javier Méndez (ING PR Officer)

Additional links

"Resuelven el misterio de cómo se encienden los cuásares", IAC press release, 26th April 2023.

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Last modified: 27 April 2023