Black Holes, Quasars, Galaxies, Accretion Disks and the Winds that Blow Them

Rochester Institute of Technology scientists have for the first time observed the winds of gas that shoot from the center of quasar galaxies and that are associated with supermassive black holes and, in fact, regulate the black hole's growth.

It has been theorized for a long time that winds of gas from quasar galaxies are launched from and powered by accretion disks around black holes and that they regulate the growth of black holes. But since quasars and accretion disks are undetectable in visible light, no confirming evidence has ever been collected to prove the theory. Until now.

Scientists Stuart Young, David Axon and Andrew Robinson from the Rochester Institute of Technology, along with colleagues James Hough and James Smith from the University of Hertfordshire in England used the William Herschel Telescope on the Canary Islands to examine quasar galaxy PG 1700+518, located 3 billion light years from Earth, by using a technique called polarimetry.

The Herschel Telescope has capabilities for both optical and infrared imaging. Polarimetry is a technique specialized in at the University of Hertfordshire and is defined as the measurement of the polarization of light, x-rays or radio waves (electromagnetic radiation) as they pass through or refract off of a gas or liquid or surface. In the case of quasar galaxies, it would be light passing through gases associated with accretion disks and explusion jets of gas also called quasars around the black hole in the center of the quasar galaxy: polarized light or other electromagnetic radiation takes different directions while exhibiting different properties and specialized imaging can detect this.

To show that winds from the accretion disk fuel the quasar jet that regulates the growth of the black hole, the RIT scientists studied the light polarization of quasar galaxy PG 1700+518 and found that gas is moving in relation to the accretion disk in two directions. Firstly, gas is moving in accord with the rotation of the disk at a speed similar to that of the disk. Secondly, gas is moving vertically away from the disk. This offers proof that indeed the accretion disk is launching a wind.

Robinson, who is an associate professor of physics at RIT and one of the authors of the study, said: "We can't actually see the accretion disk," the rapidly spinning disk of gas that forms around a black hole, "[but] we can see its radiation, [even though] we can't see its structure. In

Robinson further explained, "In quasars, like PG1700+518, we believe that light emitted by the accretion disk becomes polarized [going in opposing directions] because it is scattered by electrons in the wind...."

Polarimetry has advanced the study of accretion disks, quasars and black holes because the analysis of polarization of light, such as that emitted from the accretion disk, opens new information about the underlying structure that produces the light polarization, thus revealing information about internal structure and the attendant motions of the gas that is being emitted and scattered.

Polarimetry has contributed much over the past 20 years to scientific understanding of active galactic nuclei and helped differentiate among a confusing array of different types of galaxies.

"RIT Study Confirms Supermassive Black Holes Produce Powerful Galaxy-Shaping Winds," Rochester Institute of Technology.