This graph shows the distribution of about
20,000 luminous Sloan Digital Sky Survey quasars in the two-dimensional
space of broad line width versus FeII strength, color-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. A larger
version is available here.
Pasadena, CA—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. 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.
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, 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 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.
“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.
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Support for this research was provided by NASA’s Hubble Fellowship,
awarded by the Space Telescope Science Institute, operated by the
Association of Universites for Research in Astronomy Inc. for NASA, the
Kavli Foundation, Peking University, and the Chinese Academy of Science
through a grant from the Strategic Priority Research Program.
Source: Carnegie Institution for Science
