Mysterious quasar sequence explained

A growing black hole, called a Quasar, can be seen at the center of a faraway galaxy in this artist's concept. 

Credit: NASA/JPL-Caltech

Quasars are supermassive black holes that live at the center of distant massive galaxies.

They shine as the most luminous beacons in the sky across the entire electromagnetic spectrum by rapidly accreting matter into their gravitationally inescapable centers.

Yue Shen

New work from Carnegie's Hubble Fellow Yue Shen and Luis Ho of the Kavli Institute for Astronomy and Astrophysics (KIAA) at Peking University solves a quasar mystery that astronomers have been puzzling over for 20 years.

Their work, published in the September 11 issue of Nature, shows that most observed quasar phenomena can be unified with two simple quantities: one that describes how efficiently the hole is being fed, and the other that reflects the viewing orientation of the astronomer.

Luis Ho

Quasars display a broad range of outward appearances when viewed by astronomers, reflecting the diversity in the conditions of the regions close to their centers, but despite this variety, quasars have a surprising amount of regularity in their quantifiable physical properties, which follow well-defined trends (referred to as the "main sequence" of quasars) discovered more than 20 years ago.

Shen and Ho solved a two-decade puzzle in quasar research: What unifies these properties into this main sequence?

Using the largest and most-homogeneous sample to date of over 20,000 quasars from the Sloan Digital Sky Survey (SDSS), combined with several novel statistical tests, Shen and Ho were able to demonstrate that one particular property related to the accretion of the hole, called the Eddington ratio, is the driving force behind the so-called main sequence.

The Eddington Ratio

The Eddington ratio describes the efficiency of matter fueling the black hole, the competition between the gravitational force pulling matter inward and the luminosity driving radiation outward.

This push and pull between gravity and luminosity has long been suspected to be the primary driver behind the so-called main sequence, and their work at long last confirms this hypothesis.

Of additional importance, they found that the orientation of an astronomer's line-of-sight when looking down into the black hole's inner region plays a significant role in the observation of the fast-moving gas innermost to the hole, which produces the broad emission lines in quasar spectra.

This changes scientists' understanding of the geometry of the line-emitting region closest to the black hole, a place called the broad-line region: the gas is distributed in a flattened, pancake-like configuration.

Going forward, this will help astronomers improve their measurements of black hole masses for quasars.

New work solves a quasar mystery that astronomers have been puzzling over for 20 years. 

It shows that most observed quasar phenomena can be unified with two simple quantities: one that describes how efficiently the hole is being fed, and the other that reflects the viewing orientation of the astronomer.

This graph shows the distribution of about 20,000 luminous Sloan Digital Sky Survey (SDSS) quasars in the two-dimensional space of broad line width versus FeII strength, colour-coded by the strength of the narrow [OIII] line emission.

The strong horizontal trend is the main sequence of quasars driven by the efficiency of the black hole accretion, while the vertical spread of broad line width is largely due to our viewing angle to the inner region of the quasar. 

Credit: Yue Shen

"Our findings have profound implications for quasar research. This simple unification scheme presents a pathway to better understand how supermassive black holes accrete matter and interplay with their environments," Shen said.

"And better black hole mass measurements will benefit a variety of applications in understanding the cosmic growth of supermassive black holes and their place in galaxy formation," Ho added.

More information: The diversity of quasars unified by accretion and orientation, Nature, dx.doi.org/10.1038/nature13712

New type of black-hole quasar discovered

Illustration of another theory, showing the gas of a quasar rotating around a black hole. Credit: York University

Like our Milky Way, every known large galaxy has at its center a supermassive black hole, some of which are surrounded by a super-bright disk of hot gas called a quasar—but now a research team that includes Penn State astronomers has discovered a surprising new class of quasars in distant galaxies that even the most current theories had not predicted.

"The gas in this new type of quasar is moving in two directions: some is moving toward Earth but most of it is moving at high velocities away from us, possibly toward the quasar's black hole," said study co-author Niel Brandt, Distinguished Professor of Astronomy and Astrophysics at Penn State University.

"Just as you can use the Doppler shift for sound to tell if an airplane is moving away from you or toward you, we used the Doppler shift for light to tell whether the gas in these quasars is moving away from Earth or toward these distant black holes, which have a mass from millions to billions of times that of the Sun." Brandt explained.

Matter around these black holes forms a quasar disc that is bigger than Earth's orbit around the Sun and hotter than the surface of the Sun. These quasars generate enough light to be seen across the observable universe.

The international research team, led by Patrick Hall of York University in Toronto, Canada, discovered the unusual quasars with data from a large sky survey, the Sloan Digital Sky Survey (SDSS-III).

"Matter falling into black holes may not sound surprising," says Hall, "but what we found is, in fact, quite mysterious and was not predicted by current theories."

Such gas is found in only about 1 out of 10,000 quasars, and only seventeen cases now are known.

This discovery is detailed in a peer-reviewed paper in this month's issue of the journal Monthly Notices of the Royal Astronomical Society, published by Oxford University Press.

In addition to Brandt, the Penn State coauthors of this paper include Graduate Student Nurten Filiz Ak and Distinguished Professor of Astronomy and Astrophysics Donald Schneider, who also is the Survey Coordinator and Scientific Publication Coordinator for SDSS-III.

The two specific research components of the Sloan Digital Sky Survey that were used to make this discovery are the SDSS Legacy survey and the SDSS-III Baryon Oscillation Spectroscopic Survey.

"The gas in the disc must eventually fall into the black hole to power the quasar, but what is often seen instead is gas blown away from the black hole by the heat and light of the quasar, heading toward us at velocities up to 20 per cent of the speed of light," Hall said.

"If the gas is falling into the black hole, then we don't understand why it's so rare to see infalling gas. There's nothing else unusual about these quasars. If gas can be seen falling into them, why not in other quasars?"

Hall said there is one other possible explanation for these objects. "It could be that the gas moving away from us is not falling into the black hole but is orbiting around it, just above the disc of hot gas, and is very gradually being pushed away from the black hole. A wind like that will show gas moving both toward us and away from us.

To make an analogy: imagine an ant on a spinning merry-go-round, crawling from the center to the edge. You will see the ant moving toward you about half the time and away from you about half the time.

The same idea could apply to the gas in these quasars. In either case, the gas in these quasars is moving in an unusual fashion."

Models of quasars and their winds will have to be revised to account for these objects.

To help understand what revision is needed, the research team is observing these quasars further using Canadian and American access to the Gemini-North telescope in Hawaii.

More information: Paper: mnras.oxfordjournals.org/content/434/1/222.abstract