In an article posted June 10 to the
Astrophysical Journal Letters website,
astrophysicists at Stanford report spotting a
black hole so massive that it`s more than 10
billion times the mass of our sun. More
important, this heavyweight is so far away that
the scientists think it formed when the universe
first began to light up with stars and galaxies,
so it may provide a window into our cosmological
origins.
“In cosmology, it turns out that `a galaxy a long
time ago` and `far, far away` really do go
together,” says Associate Professor Roger
Romani, who with graduate student David
Sowards-Emmerd and Professor Peter Michelson of
Stanford, and radio astronomer Lincoln Greenhill
of the Harvard-Smithsonian Center for Astrophysics,
spotted one of the oldest supermassive black holes
yet found. The scientists collaborate at the Kavli
Institute for Particle Astrophysics and Cosmology at
Stanford. “In this case, we`re looking at [a black
hole] far enough away that it`s within a billion
years of the origin of it all, the Big Bang.”
The supermassive black hole sits in the center
of a galaxy. A disk of stars and gas swirl
around the black hole and eventually get sucked
in. “That generates enormous amounts of power,
enormous amounts of energy,” Romani says.
“It`s far more efficient even than nuclear
fusion. These gravity-powered sources are the
most powerful sources in the universe.”
As black holes go, this one is a messy eater. It`s
Jabba the Hutt, in fact, gobbling up its galaxy so
quickly that not everything is making it down its
throat past the point of no return -that place,
called the “event horizon,” where not even light
can escape gravity`s strongest pull. The matter
that doesn`t make it past the event horizon is
spewing back up in the form of accelerated high-
energy particles.
If a black hole amid a galaxy shoots out high-energy
particles in narrow jets that just happen to be
aimed at Earth, astrophysicists give the whole thing
a special name – “blazar.” Amazingly, these blazars
can be detected at nearly all energies, even at the
high energy of gamma rays. In fact, distant blazars
seem to dominate the gamma-ray sky and can obscure
other objects of interest. Pulsars, spinning neutron
stars nearby in our own galaxy, can also emit gamma
rays, but far fewer of them are known. Romani, whose
main interest is pulsars, wanted to identify and
discard blazars so he could concentrate on the neutron
stars.
“I got started working on the blazars as a way
of culling the wheat from the chaff,” Romani says.
“But then the chaff proved just as interesting.”
In preparation for a mission that is scheduled to
launch in 2007, the co-authors have surveyed
200 blazars; eventually they hope to survey
2,000. The mission, led by Michelson, will use
the Gamma Ray Large Area Space Telescope (GLAST)
to study high-energy sources of radiation in the
universe, such as supermassive black holes,
merging neutron stars and hot streams of gas
moving at nearly the speed of light. It is funded
by NASA, the U.S. Department of Energy
and government agencies in France, Italy, Japan
and Sweden.
“Something really new is waiting to be found
in the gamma-ray sky,” Romani says. “If we
could identify all the blazars, tag the pulsars –
the things that are left over, that`s where the
really new discoveries will be.”
Blazar hunting
In photographs, blazars look just like stars. So
how do scientists spot them? The co-authors
first identified gamma rays seen by the
Energetic Gamma Ray Experiment Telescope
(EGRET), a GLAST precursor initiated by Stanford
physics Professor Robert Hofstadter in the 1970s
and subsequently directed by Michelson.
Greenhill led the effort to obtain radio images
of the blazar jet using the Very Long Baseline
Array (VLBA). Funded by the National Science
Foundation and operated by the National Radio
Astronomy Observatory, the VLBA is essentially a
radio camera. It consists of 10 dish antennas –
25 meters wide and distributed from Hawaii
across the United States to St. Croix – slaved
together with computers to create a composite
image with a resolution Greenhill calls
“comparable to what they would get with a
single antenna about as large as a continent.”
To find out how far away the blazar was, Romani
and Sowards-Emmerd used the Hobby-Eberly
Telescope (HET), an optical instrument in a
remote part of Texas, to obtain spectral
patterns of visible and infrared light. HET is
a joint project of the University of Texas at
Austin, Pennsylvania State University, Stanford,
Ludwig-Maximilians-Universitat Munchen and
Georg-August-Universitat Gottingen.
Spectroscopy reveals signatures of elements in a
galaxy`s gases. Elements such as hydrogen,
nitrogen, carbon and oxygen radiate at specific
energies, or equivalently at specific
wavelengths. A consequence of cosmic expansion
is that those wavelengths get shifted to the red
part of the spectrum, or “red-shifted,” if an
object is extremely far away.
The red shift corresponds to age. “The higher
that number, the smaller the universe was when
the light was emitted – hence, the earlier
you`re talking about,” Romani explains.
The Hobby-Eberly Telescope told the researchers
that the red shift of their blazar was 5.5. This
high number told them this was not just some
star in our backyard; it was an enormous source
of energy shining from way across the universe.
“It`s amazing to find something so interesting
and unique in a relatively small survey,” says
Sowards-Emmerd, who re-analyzed EGRET data to
select the targets examined by HET and analyzed
the optical data.
“We immediately realized that a high-redshift
blazar and gamma-ray source would allow us to
test our understanding of relativistic radio
jets and their interaction with the cosmic
microwave background leftover from the Big
Bang,” Greenhill says.
“It`s a searchlight that`s set so far away that
it illuminates matter and radiation all the way
between us, between time one billion years after
the Big Bang and now,” Romani says. “If you
can detect it with a gamma-ray telescope, you
have a handle on the birth of stars and galaxies
between then and now that you never had before.”
Scientists are currently stymied about how a
black hole could have gotten so big so fast. How
do you take something big enough to hold 1,000
solar systems and as heavy as all of the stars
in our Milky Way galaxy put together, and
quickly crunch-collapse it?
Scientists think the universe formed 13.7
billion years ago with the Big Bang. The
distance of the blazar indicates it formed a
billion years after that.
“What`s interesting about a billion years after
the Big Bang is that this marks the end of the
`Dark Age,”` Romani says. “The universe first
formed with an enormous flash of light and heat
– that`s the Big Bang – and then cooled off. And
everything`s dark for about a billion years. And
toward the end of that period, the first stars
and black holes and galaxies start collapsing
and forming and turning on. We talk about that
as the end of the Dark Age. So it`s very
interesting, and this is one of the big pushes
in cosmology, to find objects back in the tail
end of the Dark Age, when things are first
lighting up, and then to use those to figure out
how everything we have in the universe formed.”
Extreme physics
In the next year, the scientists hope to use the
VLBA to take a better picture of the jet
detected with radio waves and then observe its
X-ray spectrum. This will help illuminate the
matter between the supermassive black hole and
Earth, clarify the black hole`s size and
characterize the jet`s material as it moves away
from the black hole at nearly the speed of light.
“Studying these things gives us a window into
the sort of physical processes that we can`t yet
control here on Earth,” Romani says. “They`re
the extremes of physics.”
Those extremes fascinate Romani. “Pulsars are,
I think, the most extreme objects in our
universe,” he says. These cores of dead stars
have collapsed, but not far enough to form an
event horizon, so they are just short of turning
into black holes. They are the densest things in
the measurable universe. They have the strongest
magnetic fields. Their surfaces have extremely
high temperatures. They are cosmic accelerators
that speed particles to the highest energies
known.
So far, scientists have found only a handful of
gamma-ray pulsars, and Romani is particularly
excited about GLAST as a means of hunting down
more in the Milky Way.
“I`m particularly interested in ways in which
you could find extreme physics out there in the
cosmos and get a handle on physics of the 22nd
or 23rd century by seeing what`s going on in the
sky.”
