New Black Hole Census Reveals Major Undercount and Great Extinction
Scientists using NASA's Chandra X-ray Observatory and other
world-class facilities have discovered that the total number of
massive black holes in the Universe is 75 percent more than what was
previously estimated. This new black hole census, a compilation of
five years of observations, dramatically alters theories of how
galaxies form and evolve.
Among the new, surprising findings: quasar activity did not peak in
the early Universe but rather more recent; the largest black holes
formed before the smaller ones; most black hole activity cannot be
detected by optical telescopes; and a period of great extinction
occurred about six billion years ago, when, like the dinosaurs, the
most voracious black holes mysteriously disappeared.
A team led by Dr. Amy Barger of the University of Hawaii in Honolulu
presents these results in an upcoming issue of The Astronomical
Journal. These results are supported by several independent science
teams, whose results will soon be published.
These new results show that scientists underestimated just
about everything concerning black hole evolution -- that is, when and
how they form, how many there are, and how energetic they are. X-ray
telescopes showing phenomena missed by
even the most powerful optical and radio observatories.
An artist's rendition of a galaxy with a supermassive black hole at its core. Such a galaxy is called an Active Galactic Nucleus (AGN). (Illustration: NASA)
"This result represents a major paradigm shift," said Dr. Richard
Mushotzky of NASA Goddard Space Flight Center in Greenbelt, Md., a
co-author. "The what, when, why, where, and how -- we had it all
Barger's team discovered a multitude of Active Galactic Nuclei (AGN).
These are galaxies, such as quasars, with
exceedingly bright cores. The central engine creating this light is
a supermassive black hole with the mass of millions to billions of
suns compressed within a region no larger than our Solar System.
The team compared Chandra X-ray observations with
optical and ultraviolet observations from the Hubble Space Telescope
and other telescopes. The side-by-side comparison revealed that
Chandra was detecting sources that Hubble and others couldn't see.
Optical surveys had looked nearby and found but a few supermassive
black holes. Deeper optical surveys of regions of space more than
six billion light years away also didn't reveal many black holes.
This is because the light has been snuffed out by dust and gas around the
black holes. What scientists had been seeing, apparently, were only
the brightest AGN from afar, such as quasars, and relatively small
supermassive black holes nearby.
But X-rays can penetrate the obscuring material. Thus, X-ray observatories
-- namely Chandra and the European Space Agency's XMM-Newton, both
launched within the last five years -- are detecting more black hole
activity. Scientists are now beginning to understand what has stymied
all previous measurements of black hole populations.
The image on the left is a deep X-ray Chandra image of the Lockman Hole, a patch of sky that avoids most of the X-ray absorbing gas of the Milky Way. Virtually each of these dots - with the red objects usually cooler than the blue objects - represents a supermassive black hole. (Credit: ASA/CXC/U. Wisconsin/A. Barger et al.)
The image on the right is a Digitized Sky Survey (DSS) of the Lockman Hole. The field of view is slightly larger than the Chandra image. This image shows the Lockman Hole in visible light. (Credit: Pal.Obs. DSS)
The largest black holes, with the mass of billions of suns, are most
easily visible to optical telescopes, perhaps because they have
burned off obscuring material. Slightly smaller black holes, with
the mass of about 10 million to 100 million suns, are visible to
optical and radio telescopes only when they are very close to us --
that is, close enough for these telescopes to detect the weak signal
getting through obscuring material.
These Chandra data show this underrepresented
'middle class' of massive black holes. This
population makes up the clear majority of black holes, which was
not known until these observations.
This black hole activity peaks at about 6 billion years ago (at z=1,
a redshift measurement related to distance). This was a major
finding of the census. Optical studies had revealed a peak much
earlier, at about 11 billion years ago (or redshift 2.5). This
implies that the largest black holes seen optically "turned on" long
before the slightly smaller ones.
The Chandra observations next revealed a great and rapid extinction
starting about 6 billion years ago. That's when most of these black
holes "turned off" -- that is, stopped accreting matter and giving
off copious amounts of light. Perhaps this is because they had
pulled in all the matter that they could from the region.
"Only the smaller black holes, which turned on later, appear to
actively pull in matter today," said Dr. Lennox Cowie of the
University of Hawaii, another team member. "A period six billion
years ago marks the beginning of the end of the most massive ones."
The team noted that the recently launched Spitzer Space Telescope may
support these results through observations of infrared light
reflected from dust around black holes.
Other members of the science team are W. Wang and Peter Capak, also
at the University of Hawaii; Yuxuan Yang of the University of
Maryland, College Park; and Aaron Steffen of the University of