Galactic collisions found to be trigger behind quasars

Ignition of a quasar can have dramatic consequences for entire galaxies

An artist’s rendering of the most distant quasar This artist’s impression shows how ULAS J1120+0641, a very distant quasar powered by a black hole with a mass two billion times that of the Sun, may have looked | Credit: ESO/M. Kornmesser

Scientists have discovered quasars - the brightest, most powerful objects in the Universe - to be ignited by collision of galaxies. Quasars can shine as brightly as a trillion stars packed into a volume the size of our Solar System.

The scientists at the Universities of Sheffield and Hertfordshire, UK, say they have unlocked the mystery surrounding the trigger behind such powerful activity, first observed 60 years ago.

They have published their findings in the journal Monthly Notices of the Royal Astronomical Society.

The galactic collisions were discovered when researchers, using deep imaging observations from the Isaac Newton Telescope in La Palma (Spain), observed the presence of distorted structures in the outer regions of the galaxies home to quasars.

They said this is the first time that a sample of quasars of this size has been imaged with this 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.

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.

Collisions between galaxies drive the gas towards the black hole at the galaxy centre; 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.

"Quasars are one of the most extreme phenomena in the Universe, and what we see is likely to represent the future of our own Milky Way galaxy 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," said Clive Tadhunter, professor at the University of Sheffield's Department of Physics and Astronomy.