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Black holes offer new cosmic ruler
By Nadia Drake
Astronomers have a new gadget in their
cosmic toolbox capable of measuring
distances to very faraway objects. The
method uses active galactic nuclei, the
bright, violent regions at the centers of
galaxies, to gauge distances farther than
existing cosmic yardsticks can reach.
Such a range finder is crucial for understanding how space, time and matter
behave at cosmic scales and could help
solve mysteries such as the nature of the
dark energy believed to be accelerating
the expansion of the universe.
For four decades, astronomers have
tried to turn active galactic nuclei into
cosmic mile markers. Now, scientists at
the University of Copenhagen’s Dark Cos-
mology Centre and their collaborators
think they’ve worked it out. The bright-
ness of an active nucleus is tightly related
to the radius of a region of hot gases sur-
rounding a central black hole. Knowing
that radius, scientists can predict how
intrinsically bright the nucleus should
be. They then compare that value with
how bright it appears, which depends on
distance. Objects whose properties can be
used to determine their intrinsic bright-
ness are known as “standard candles.”
“It’s the radius-luminosity relationship
that allows us to assume that these active
galactic nuclei are standard candles,” says
Kelly Denney, a coauthor of the study, to
appear in the Astrophysical Journal.
The stable of standard candles already
houses phenomena known as type 1a
supernovas and Cepheid variable stars,
which have predictable luminosities but
are good for measuring distances only to
objects present when the universe was
nearly 4 billion years old. Active galactic nuclei would extend that capability
to objects at distances corresponding to
when the universe was only 1. 5 billion
years old.
“Right now we rely so much on super-
novas, it would be really nice to have
independent verification of cosmological
parameters,” says astrophysicist Bradley
Peterson of Ohio State University. “I’m
really excited about this result.”
A technique called reverberation map-
ping measures how long it takes photons
kicked out of a black hole’s immediate
neighborhood to reappear after they’ve
traversed the hot, gassy maelstrom sur-
rounding the black hole. Because light
travels at a constant speed, astronomers
can determine the gassy region’s radius.
Then, the luminosity of the active galactic
nucleus can be calculated.
Until now, tightening the relationship
between radius and luminosity has been
tricky. Among other reasons, starlight
from a host galaxy contaminated the
brightness measurements of its active
nucleus. But the team had data from
astrophysicist Misty Bentz of Georgia
State University that corrected for the
effects of the starlight, plus Denney’s
own precise measurements of radii.
Mercury’s puzzling pits
Hot and heavy little Mercury is warming to NASA’s MESSENGER probe and
revealing true planetary colors—such as the bizarre blue landforms (shown
in enhanced color) tucked inside impact craters on the planet’s surface.
David Blewett of the Johns Hopkins University Applied Physics Laboratory in
Laurel, Md., and colleagues report these puzzling scarlike pits in the Sept. 30
Science. The pits resemble sunken Swiss cheese holes — smooth, rimless
depressions several meters to a few kilometers across. Irregularly shaped,
the clustered hollows don’t look purely volcanic but make some areas appear
as though the planet’s innards are leaking away, just what scientists speculate
might be happening. They propose that impacts exposed unstable material
from Mercury’s interior to the planet’s harsh surface. Whether baked off by
the sun, blasted away by the solar wind, blown skyward by volcanoes or simply
burped out, the stuff disappeared, leaving these pits behind. — Nadia Drake
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November 19, 2011 | SCIENCE NEWS | 17
