 'Death Spiral'
Around a Black Hole Yields Tantalizing Evidence of an Event Horizon
NASA PRESS RELEASE NO.:
STScI-PR01-03
Black
holes and Galaxy Formation
Black holes and
micro-lensing
NASA's Hubble Space Telescope may have, for the first
time, provided direct evidence for the existence of black holes by
observing the disappearance of matter as it falls beyond the "event
horizon."
Joseph F. Dolan, of NASA's Goddard Space Flight Center
in Greenbelt, MD, observed pulses of ultraviolet light from clumps of
hot gas fade and then disappear as they swirled around a massive,
compact object called Cygnus XR-1. This activity is just as would have
been expected if the hot gas had fallen into a black hole.
"We are trying to establish the existence of black
holes by obtaining observational evidence that rules out more exotic
things, just as previous observations of black hole candidates have
ruled out less exotic things," says Dolan, who is presenting his
findings today at the American Astronomical Society meeting in San
Diego, CA.
An event horizon is the mysterious region surrounding a
black hole that forever traps light and matter straying nearby. By
definition, no astronomical object other than a black hole can possess
an event horizon.
Black holes have been inferred by observing the furious
whirlpool motion of trapped gas and estimating how much mass is crammed
into the tiny region of space the black hole occupies.
Also, previous X-ray observations have offered evidence
for an event horizon by surveying black hole candidates that seem to be
swallowing nearly a hundred times as much energy as they radiate. Those
results imply that trillion-degree gas is falling over the brink of an
event horizon, like water over the edge of a waterfall.
But no one has ever seen what actually happens to a
piece of matter swirling into the event horizon, like water down a
drain. The secret was tucked away in nearly decade-old Hubble data that
took meticulous analysis.
Dolan cautions that his Hubble black hole observations
see only two infall events. This means there is a finite chance the
signature could simply be a statistical fluke that mimics the behavior
of matter near a black hole. But Dolan emphasizes the results are
consistent with what astronomers would expect to see if matter were
really falling into a black hole.
The discovery comes from a detailed statistical analysis
of a 1992 observation of one of the first black holes ever discovered,
Cygnus XR-1, which lies 6,000 light-years from Earth in the summer
constellation Cygnus the Swan.
Hubble didn't see the event horizon -- it is too small
and too far away - but instead measured chaotic fluctuations in
ultraviolet light from seething gas trapped in orbit around the black
hole. Hubble found two examples of a so-called "dying
pulse train," the rapidly decaying, precisely sequential
flashes of light from a hot blob of gas spiraling into the black hole.
This signature matches theories of what scientists would
predict to see when matter is falling so close to the event horizon that
its light rapidly dims as it is stretched by gravity to ever-longer
wavelengths. Without an event horizon, the blob of gas would have
brightened as it crashed onto the surface of the accreting body.
Instead, the gas crossed over into a twilight-zone realm when time and
space no longer have any practical meaning. Because of the gravitational
stretching of light (an effect called redshift), the fragment
disappeared from Hubble's view before it ever actually reached the event
horizon. The pulsation of the blob - an effect caused by the black
hole's intense gravity -- also shortened as it fell closer to the event
horizon.
Finding the signature wasn't an easy task. Hubble's
high-speed photometer (a very fast light meter) sampled light at the
rate of 100,000 measurements per second, during three separate Hubble
orbits, each executed in June, July, and August of 1992. The observation
yielded 1 billion data-points, which, if printed out on a chart
recorder, would stretch 600 miles! Hubble's ultraviolet capability gave
it the ability to see the faint flicker of material within 1,000 miles
of the event horizon.
Dolan "mined" the enormous database on and off
for years. "Looking for the decaying pulse train was like looking
for the proverbial needle-in-a haystack," he says. "Put
another way, it was like listening for a specific word in a many
hours-long transmission of Morse code."
He found two examples of infall events. One event had
six decaying pulses; the other had seven pulses. The pulses spanned an
interval of merely 0.2 seconds before the blob forever disappeared from
view.
Death Spiral Animation
Dynamical models predict that gas from Cygnus XR-1's
companion star continuously falls into the black hole. The gas can't
directly fall in, but instead swirls into a flattened pancake called an
accretion disk. The viscosity in the accretion disk causes the gas to
spiral down toward the event horizon. About 1,000 miles above the event
horizon (in the case of stellar-mass black holes) the disk vanishes
because gas can no longer maintain a stable orbit. This is due to the
dragging of space-time by the black hole's intense gravitational field.
Instead, blobs of hot gas break off from the inner rim of the disk, like
icebergs off an ice shelf. The blob then spirals down to the event
horizon. Because of gravitational effects on light near the black hole,
the blob appears to pulsate as it makes thousands of orbits around the
black hole every second. When it falls inside the accretion disk, the
light quickly stretches to longer and longer wavelengths because of the
distortion of space-time by the black hole's intense gravity.
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Although one can not see
a black hole directly, it causes immense distortion of the gravitational field
and can affect the way light travels to us from far away stars. The black
hole acts as a gravitational "lens" for the light from a star behind
it. The Hubble Space Telescope was able to resolve the two microlensed
images that only appeared as a single star to ground-based telescopes. NASA
link to this phenomenon. (PRESS RELEASE NO.: STScI-PR00-03)
Astronomers are concluding that monstrous black holes weren't
simply born big but instead grew on a measured diet of gas and stars controlled
by their host galaxies in the early formative years of the universe.
These results, gleaned from a NASA Hubble Space Telescope census
of more than 30 galaxies with its powerful "black hole hunting"
spectrograph, are painting a broad picture of a galaxy's evolution and its long
and intimate relationship with its giant central black hole.
Though much more analysis remains, an initial look at Hubble
evidence favors the idea that titanic black holes did not precede a galaxy's
birth but instead co-evolved with the galaxy by trapping a surprisingly exact
percentage (0.2%) of the mass of the bulbous hub of stars and gas in a galaxy. (diagram
of galaxy/black hole relationship)
This means that black holes in small galaxies went relatively
undernourished, weighing in at a mere few million solar masses. Black holes in
the centers of giant galaxies, some tipping the scale at over one billion solar
masses, were so engorged with infalling gas that they once blazed as quasars,
the brightest objects in the cosmos.
The bottom line is that the final mass of a black hole is not
primordial; it is determined during the galaxy formation process. "This
supports the original theory of why black holes are important and how they got
their masses. It suggests that the major events that made a galaxy and the ones
that made its black hole shine as a quasar were the same events," says John
Kormendy of the University of Texas at Austin. "These results are a
catalyst that help to tie together many lines of investigation on galaxy
formation into a more believable and coherent picture."
These results are being reported at the 196th meeting of the
American Astronomical Society in Rochester, New York, by Kormendy, Karl Gebhardt
(Lick Observatory), Douglas Richstone (University of Michigan), and an
international team of collaborators.
Though this secret relationship between a black hole and its
host galaxy has been suspected for the past several years, it is bolstered by
the Hubble discovery of 10 more supermassive black holes in galaxy centers,
raising the total to more than 30 black holes now available for study. "For
the first time we can put strong constraints on the relationship between galaxy
formation and black hole formation and growth," says Kormendy.
The results now show a close relationship between the black hole
mass and the stars that comprise an elliptical galaxy or the central bulge stars
of a spiral galaxy. But surprisingly, an even tighter correlation is found.
"Other observations of the entire stellar mass of the bulge show a very
tight relationship between a black hole's mass and the depth of the
gravitational potential well as measured by the magnitude of random velocities
of stars in the galaxy's hub. This bolsters the conclusion that the mass
correlation is real," says Gebhardt.
In most cases the black holes not only bulked up through the
accretion of gas in isolated galaxies, but also through the mergers of galaxies
where pairs of black holes combined.
"Hierarchical clustering and merging are an integral part
of the picture that we advocate, and to the extent that no new stars get formed,
they will in any case preserve the correlation between black hole mass and bulge
size," says Kormendy. "This theory has the advantage that it also
accounts for quasar activity. The black hole feeding that makes the black hole's
mass grow is also what makes the quasar shine. A quasar is the brilliant
signature of the fueling and building of the central black hole."
The results also explain why galaxies with small bulges, like
our Milky Way, have diminutive central black holes of a few million solar
masses, while giant elliptical galaxies house billion-solar-mass black holes,
some still smoldering from their days as quasars. Disk galaxies without a
central bulge of stars (like the neighboring galaxy Messier 33) either have no
black hole or have only tiny black holes that are well below Hubble's detection
limit.
An alternative but less favored idea is that black holes came
first, all packaged in a standard size, namely 0.2 percent of the mass of the
first galaxy fragments that formed. Then mergers of small galaxies made bigger
galaxies, and the standard black hole mass fraction was preserved because, when
two galaxies merge, their black holes merge too. This idea is not favored by the
new observations.
The results do not shed light on how seed black holes originate.
They are just required to be in place early in the galaxy formation process so
that they can grow and shine as quasars. Nor do astronomers know why the galaxy
formation process makes a black hole with such a precisely correlated mass.
Evidently, the process that decides how much mass gets fed to black holes
produces almost the same result, largely independent of the details of galaxy
formation.
Hubble is astronomy's preeminent "black hole hunter"
because of its unique ability to use its Space Telescope Imaging Spectrograph (STIS)
camera to precisely measure the speed of gas and stars around a black hole.
Hubble is the best way to find lots of black holes without selection biases.
The findings reported at the AAS meeting are based on two types
of Hubble observations. Several teams measured the black holes' masses by
recording the whirling speeds of disks of gas trapped around the black holes,
like water swirling around a drain. Other teams measured the motions of stars
around the galaxies' hubs like a swarm of bees hovering around a beehive. The
more massive the bulge, the greater the speed of the stars.
Speculation about giant black holes in galaxies began with the
discovery of quasars in the 1960s. Astronomers soon realized that the
extraordinary gravitational field of a large black hole was the necessary
"engine" for generating the prodigious amounts of energy blasted into
space by a quasar.
But astronomers realized that the light from a quasar
represented only 1/10 of the mass that must be devoured by the black hole. Where
was this vast fuel supply coming from? The discovery of many different kinds of
active galactic nuclei in the 1970s also required a process of black hole
feeding early in a galaxy's life. This meant that black hole accretion is not an
incidental process in the life of a galaxy.
But black holes could not be confidently detected in a broad
sample of galaxies until Hubble came along with the precise accuracy to measure
the velocity of matter trapped close into the black hole and "weigh"
its mass.
The research team's next step is to look for the smallest
nuclear black holes that can be detected with Hubble. This information will help
astronomers understand the mechanism for the seed black holes that grew so
rapidly during galaxy formation.
The Space Telescope Science Institute is operated by the
Association of Universities for Research in Astronomy, Inc. for NASA, under
contract with NASA's Goddard Space Flight Center, Greenbelt, MD. The Hubble
Space Telescope is a project of international cooperation between NASA and the
European Space Agency.
