They are old but not forgotten. Nearby “retired” quasar galaxies,
billions of years past their glory days as the brightest beacons in
the Universe, may be the current source of rare, high-energy cosmic
rays, the fastest-moving bits of matter known and whose origin has
been a long-standing mystery, according to scientists at NASA and
Princeton University.
The scientists have identified four elliptical galaxies that may have
started this second career of cosmic-ray production, all located
above the handle of the Big Dipper and visible with backyard
telescopes. Each contains a central black hole of at least 100
million solar masses that, if spinning, could form a colossal battery
sending atomic particles, like sparks, shooting off towards Earth at
near light speed.
These findings are discussed today in a press conference at the joint
meeting of the American Physical Society and the High Energy
Astrophysics Division of the American Astronomical Society in
Albuquerque, N.M. The team includes Dr. Diego Torres of Princeton
University and Drs. Elihu Boldt, Timothy Hamilton and Michael
Loewenstein of NASA’s Goddard Space Flight Center in Greenbelt, Md.
Quasar galaxies are thousands of times brighter than ordinary
galaxies, fueled by a central black hole swallowing copious amounts
of interstellar gas. In galaxies with so-called quasar remnants, the
black hole nucleus is no longer a strong source of radiation.
“Some quasar remnants might not be so lifeless after all, keeping
busy in their later years,” said Torres. “For the first time, we see
the hint of a possible connection between the arrival directions of
ultra-high energy cosmic rays and locations on the sky of nearby
dormant galaxies hosting supermassive black holes.”
Ultra high-energy cosmic rays represent one of astrophysics’ greatest
mysteries. Each cosmic ray — essentially a single sub-atomic
particle such as a proton traveling just shy of light speed — packs
as much energy as a major league baseball pitch, over 40 million
trillion electron volts. (The rest energy of a proton is about a
billion electron volts.) The particles’ source must be within 200
million light years of Earth, for cosmic rays from beyond this
distance would lose energy as they traveled through the murk of the
cosmic microwave radiation pervading the Universe. There is
considerable uncertainty, however, over what kinds of objects within
200 million light years could generate such energetic particles.
“The very fact that these four giant elliptical galaxies are
apparently inactive makes them viable candidates for generating ultra
high-energy cosmic rays,” said Boldt. Drenching radiation from an
active quasar would dampen cosmic-ray acceleration, sapping most of
their energy, Boldt said.
The team concedes it cannot determine if the black holes in these
galaxies are spinning, a basic requirement for a compact dynamo to
accelerate ultra-high energy cosmic rays. Yet scientists have
confirmed the existence of at least one spinning supermassive black
hole, announced in October 2001. The prevailing theory is that
supermassive black holes spin up as they accrete matter, absorbing
orbital energy from the infalling matter.
Ultra-high-energy cosmic rays are detected by ground-based
observatories, such as the Akeno Giant Air Shower Array near
Yamanashi, Japan. They are extremely rare, striking the Earth’s
atmosphere at a rate about one per square kilometer per decade.
Construction is underway for the Auger Observatory, which will cover
3,000 square kilometers (1,160 square miles) on an elevated plain in
western Argentina. A proposed NASA mission called OWL (Orbiting
Wide-angle Light-collectors) would detect the highest-energy cosmic
rays by looking down on the atmosphere from space.
Loewenstein joins NASA Goddard’s Laboratory for High Energy
Astrophysics as a research associate with the University of Maryland,
College Park. Hamilton, also a member of the Lab, is a National
Research Council fellow.
For images of the “retired” quasar galaxies, refer to:
http://universe.gsfc.nasa.gov/press/images/cosmic_ray/.
