Monster black holes come out of hiding

Monster black holes come out of hiding

A group of supermassive black holes that had been lurking behind a veil of dust have had their hiding places revealed by an infrared survey. All galaxies contain a supermassive black hole at their centre, but not all of them like to announce their presence.


An artist's concept of a quasar at the heart of a galaxy
An artist’s concept of a quasar at the heart of a galaxy. Credit: ESO/M. Kornmesser

Black holes don’t emit any light, but if they suck in enough matter an accretion disc will form around the black hole. This disc of material will then emit radiation, and this radiation can be blazingly bright. Collectively, these are known as active galactic nuclei (AGN) and we see different types of AGN depending on how they are orientated.

A quasar is a type of AGN that is incredibly luminous. The newly discovered group of quasars are beasts, being particularly luminous and massive for quasars at this epoch in the Universe. Yet they are completely undetectable in the visible region of the spectrum as they are encapsulated in a thick shell of dust.

“The radiation from the supermassive black hole has to penetrate through this dust before reaching us and the dust preferentially obscures bluer light making the object extremely red,” Manda Banerji from the University of Cambridge tells Skymania News. “Only by looking at infrared wavelengths can we locate these dustiest black holes, which are actually seen to be some of the most massive black holes known, yet they have been undiscovered so far.”

Infrared image of the giant reddened quasar ULASJ1234+0907. Credit: UKIDSS/WISE
Infrared image of the giant reddened quasar ULASJ1234+0907. Credit: UKIDSS/WISE

Banerji and her group used the UKIRT Infrared Deep Sky Survey (UKIDSS) to find the reddened quasars.

The most extreme quasar of the group is dubbed ULASJ1234+0907, which is the reddest, most luminous and most massive black hole of the lot. This black hole has a mass 10 billion times that of the Sun and is 10,000 times more massive than the supermassive black hole that resides in the centre of the Milky Way.

Most of the new quasars have black hole masses and luminosities that supersede other AGN nearby. “One possibility is that we are detecting these quasars just as they reach their maximum mass. i.e. these objects represent the maximum in luminosity possible,” says Richard McMahon, co-author on the paper.

The dust enveloping these black holes was most likely produced in the chaos that ensues after a galaxy collision. When galaxies collide, it can trigger intense star formation which provides a source of fuel for the black hole. The star formation makes the galaxies very bright, and in the early stages of this process these are known as ultra-luminous infrared galaxies (ULIRGs).

Star formation also causes the dust that obscures the black hole as it feeds on new stars and grows more massive. This dusty phase of the galaxy collision is represented by the red quasars. The dust will eventually settle, unveiling quasars that are bright at visible wavelengths. The detection of the reddened quasars bridges a gap between ULIRGS and optically bright quasars for the first time.

The paper will be published in Monthly Notices of the Royal Astronomical Society and a preprint can be found here.

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