Transcript Document

BLACK HOLES
The following images and text descriptions are taken from the Hubble
Space telescope (http://www.stsci.edu).
There are many resources on the web relating to black holes. Here
are 2 other interesting links:
A good introductory tutorial on GR and black holes:
http://casswww.ucsd.edu/public/tutorial/GR.html
Joe Wolfe's web pages discussing the twin paradox and other aspects
of special relativity – linked from the School of Physics web page.
PRESS RELEASE NO.: STScI-PR97-28
Hubble Finds A Bare Black Hole Pouring Out Light
Probing the heart of the active galaxy NGC 6251, NASA's Hubble Space Telescope has provided a never-before-seen view of a warped
disk or ring of dust caught in a blazing torrent of ultraviolet light from a suspected massive black hole. This discovery, which is reported in
the September 10 issue of the Astrophysical Journal Letters, suggests that the environments around black holes may be more varied than
thought previously, and may provide a new link in the evolution of black holes in the centers of galaxies.
"This is a completely new phenomenon which has never before been seen. It blew my mind away," says Dr. Philippe Crane of the
European Southern Observatory, in Garching, Germany. "Before Hubble you could never do this kind of research. We used a lightly
exploited facility of Hubble: its extremely high resolution imaging capability in the near ultraviolet provided by the Faint Object Camera
(FOC), built by the European Space Agency."
Previously, black holes observed by Hubble have been largely hidden from view because they are embedded inside a torus, a donut-shaped
distribution of dust that forms a partial cocoon around the black hole.
In galaxies previously studied, the intense light from super hot gas entrapped by the black hole's powerful gravitational field shines out
from inside the "donut hole" of the torus and is restricted to a narrow beam, like a searchlight.
But this is the first clear example of an "exposed" black hole that illuminates the surrounding disk. Because Hubble sees ultraviolet light
reflected on one side of the disk, astronomers conclude the disk must be warped like the brim of a hat.
Such a warp could be due to gravitational perturbations in the galaxy's nucleus that keep the disk from being perfectly flat, or from
precession of the rotation axis of the black hole relative to the rotation axis of the galaxy. The suspected black hole's mass has not yet been
confirmed through velocity measurements of entrapped material, though yet unpublished Hubble measurements have been made with the
Faint Object Spectrograph (FOS), prior to its replacement during the 1997 Hubble servicing mission.
However, strong circumstantial evidence for the black hole is provided by the powerful 3 million light-year-long jet of radiation and
particles emanating from the black hole's location at the hub of the elliptical galaxy. The galaxy is located 300 million light-years away in
the constellation Ursa Minor.
Hubble's sensitivity to ultraviolet light, combined with the exceptional resolution of the FOC which can see details as small as 50 lightyears across, allowed Crane and his team to look for structure in the hot gas near the black hole at the base of the jet. Crane was surprised
to see a peculiar finger-like object extending from the nucleus, at right angles to the main jet.
Comparing the FOC image to a visible light image taken with Hubble's Wide Field Planetary Camera 2 (WFPC2), Crane realized the
finger-like extension ran parallel to a 1,000 light-year-wide dust disk encircling the nucleus. He concluded that the ultraviolet light must
be reflecting off fine dust particles in a disk, or possibly the back wall of a ring. A ring-like structure would have been shaped by a torrent
of radiation coming from the exposed black hole, which would have ploughed out a cavity around the hole.
The Hubble astronomers are hoping to confirm ideas about scattering by looking at the disk's spectrum with ground-based telescopes.
They will propose to use Hubble to look at several other extragalactic jet sources which have dust.
PRESS RELEASE NO.: STScI-PR97-18
FIREWORKS NEAR A BLACK HOLE IN THE CORE OF SEYFERT GALAXY NGC 4151
The Space Telescope Imaging Spectrograph (STIS) simultaneously records, in unprecedented detail, the
velocities of hundreds of gas knots streaming at hundreds of thousands of miles per hour from the
nucleus of NGC 4151, thought to house a supermassive black hole. this is the first time the velocity
structure in the heart of this object, or similar objects, has been mapped so vividly this close to its central
black hole. The twin cones of gas emission are powered by the energy released from the supermassive
black hole believed to reside at the heart of this Seyfert galaxy. The STIS data clearly show that the gas
knots illuminated by one of these cones is rapidly moving towards us, while the gas knots illuminated by
the other cone are rapidly receding.
The images have been rotated to show the same orientation of NGC 4151. The figures show:
WFPC2 (upper left) -- A Hubble Wide Field Planetary Camera 2 image of the oxygen emission (5007
Angstroms) from the gas at the heart of NGC 4151. Though the twin cone structure can be seen, the
image does not provide any information about the motion of the oxygen gas.
STIS OPTICAL (upper right) -- In this STIS spectral image of the oxygen gas, the velocities of the
knots are determined by comparing the knots of gas in the stationary WFPC2 image to the horizontal
location of the knots in the STIS image.
STIS OPTICAL (lower right) -- In this false colour image the two emission lines of oxygen gas (the
weaker one at 4959 Angstroms and the stronger one at 5007 Angstroms) are clearly visible. The
horizontal line passing through the image is from the light generated by the powerful black hole at the
centre of NGC 4151.
STIS ULTRAVIOLET (lower left) -- This STIS spectral image shows the velocity distribution of the
carbon emission from the gas in the core of NGC 4151. It requires more energy to make the carbon gas
glow (CIV at 1549 Angstroms) than it does to ionise the oxygen gas seen in the other images. This
means we expect that the carbon emitting gas is closer to the heart of the energy source.
PRESS RELEASE NO.: STScI-PR97-17
A COLLISION IN THE HEART OF A GALAXY
The Hubble Space Telescope's Near Infrared Camera and Multi-Object
Spectrometer (NICMOS) has uncovered a collision between two spiral galaxies in
the heart of the peculiar galaxy called Arp 220. The collision has provided the
spark for a burst of star formation. The NICMOS image captures bright knots of
stars forming in the heart of Arp 220. The bright, crescent moon-shaped object is a
remnant core of one of the colliding galaxies. The core is a cluster of 1 billion
stars. The core's half-moon shape suggests that its bottom half is obscured by a
disk of dust about 300 light-years across. This disk is embedded in the core and
may be swirling around a black hole. The core of the other colliding galaxy is the
bright round object to the left of the crescent moon-shaped object. Both cores are
about 1,200 light-years apart and are orbiting each other.
Arp 220, located 250 million light-years away in the constellation Serpens, is the
220th object in Halton Arp's Atlas of Peculiar Galaxies.
The image was taken with three filters. The colors have been adjusted so that, in
this infrared image, blue corresponds to shorter wavelengths, red to longer
wavelengths.
The image was taken April 5, 1997.
PRESS RELEASE NO.: STScI-PR97-12
STIS RECORDS A BLACK HOLE'S SIGNATURE
The colourful "zigzag" on the right is not the work of a flamboyant artist, but the signature of a supermassive black hole
in the centre of galaxy M84, discovered by Hubble Space Telescope's Space Telescope Imaging Spectrograph (STIS).
The image on the left, taken with Hubble's Wide Field Planetary and Camera 2 shows the core of the galaxy where the
suspected black hole dwells. Astronomers mapped the motions of gas in the grip of the black hole's powerful
gravitational pull by aligning the STIS's spectroscopic slit across the nucleus in a single exposure.
The STIS data on the right shows the rotational motion of stars and gas along the slit. The change in wavelength records
whether an object is moving toward or away from the observer. The larger the excursion from the centreline -- as seen
as a green and yellow picture element (pixels) along the centre strip, the greater the rotational velocity. If no black hole
were present, the line would be nearly vertical across the scan.
Instead, STIS's detector found the S-shape at the centre of this scan, indicating a rapidly swirling disk of trapped
material encircling the black hole. Along the S-shape from top to bottom, velocities skyrocket as seen in the rapid,
dramatic swing to the left (blueshifted or approaching gas), then the region in the centre simultaneously records the
enormous speeds of the gas both approaching and receding for orbits in the immediate vicinity of the black hole, and
then an equivalent swing from the right, back to the centre line.
STIS measures a velocity of 880,000 miles per hour (400 kilometres per second) within 26 light-years of the galaxy's
centre, where the black hole dwells. This motion allowed astronomers to calculate that the black hole contains at least
300 million solar masses. (Just as the mass of our Sun can be calculated from the orbital radii and speeds of the planets.)
This observation demonstrates a direct connection between a supermassive black hole and activity (such as radio
emission) in the nucleus of an active galaxy. It also shows that STIS is ideally suited for efficiently conducting a survey
of galaxies to determine the distribution of the black holes and their masses.
Each point on STIS's solid-state CCD (Charge Coupled Device) detector samples a square patch at the galaxy that is 12
light-years on a side. The detection of black holes at the centres of galaxies is about 40 times faster than the earlier Faint
Object Spectrograph. STIS was configured to record five spectral features in red light from glowing hydrogen atoms as
well as nitrogen and sulphur ions in orbit around the centre of M84. At each sampled patch the velocity of the entrapped
gas was measured. Because the patches are contiguous, the astronomers could map the change of velocity in detail.
M84 is located in the Virgo Cluster of galaxies, 50 million light-years from Earth.
PRESS RELEASE NO.: STScI-PR97-01
Massive Black Holes Dwell In Most Galaxies, According To Hubble Census
Announcing the discovery of three black holes in three normal galaxies, an international team of astronomers suggests
nearly all galaxies may harbour supermassive black holes which once powered quasars (extremely luminous nuclei of
galaxies), but are now quiescent. This conclusion is based on a census of 27 nearby galaxies carried out by NASA's Hubble
Space Telescope and ground-based telescopes in Hawaii, which are being used to conduct a spectroscopic and photometric
survey of galaxies to find black holes which have consumed the mass of millions of Sun-like stars.
The findings, being presented today at the 189th Meeting of the American Astronomical Society in Toronto, Canada,
should provide insights into the origin and evolution of galaxies, as well as clarify the role of quasars in galaxy evolution.
The key results are:
•Supermassive black holes are so common, nearly every large galaxy has one.
•A black hole's mass is proportional to the mass of the host galaxy, so that, for example, a galaxy twice as massive as
another would have a black hole that is also twice as massive. This discovery suggests that the growth of the black hole is
linked to the formation of the galaxy in which it is located.
•The number and masses of the black holes found are consistent with what would have been required to power the quasars.
"We believe we are looking at "fossil quasars" and that most galaxies at one time burned brightly as a quasar," says team
leader Doug Richstone of the University of Michigan, Ann Arbor, Michigan. These conclusions are consistent with
previous Hubble Space Telescope observations showing quasars dwelling in a variety of galaxies, from isolated normallooking galaxies to colliding pairs.
Two of the black holes "weigh in" at 50 million and 100 million solar masses in the cores of galaxies NGC 3379 (also
known as M105) and NGC 3377 respectively. These galaxies are in the "Leo Spur", a nearby group of galaxies about 32
million light-years away and roughly in the direction of the Virgo cluster.
Located 50 million light-years away in the Virgo cluster, NGC 4486B possesses a 500-million solar mass black hole. It is a
small satellite of the galaxy M87, a very bright galaxy in the Virgo cluster. M87 has an active nucleus and is known to have
a black hole of about 2 billion solar masses.
Though several groups have previously found massive black holes dwelling in galaxies the size of our Milky Way or larger,
these new results suggest smaller galaxies have lower-mass black holes, below Hubble's detection limit. The survey shows
the black hole's mass is proportional to the host galaxy's mass. Like shoe sizes on adults, the bigger the galaxy, the larger
the black hole.
PRESS RELEASE NO.: STScI-PR97-01
Massive Black Holes Dwell In Most Galaxies, According To Hubble Census (continued)
It remains a challenging puzzle as to why black holes are so abundant, or why they should be proportional to a galaxy's
mass. One idea, supported by previous Hubble observations, is that galaxies formed out of smaller "building blocks"
consisting of star clusters. A massive "seed" black hole may have been present in each of these protogalaxies. The larger
number of building blocks needed to merge and form very luminous galaxies would naturally have provided more seed
black holes to coalesce into a single, massive black hole residing in a galaxy's nucleus.
An alternative model is that galaxies start at some early epoch with a modest black hole (not necessarily approaching
the masses discussed here), but that the black hole consumes some fixed fraction of the total gas shed by the stars in the
galaxy during their normal evolution. If that fraction is around 1 percent, the black holes could easily weigh as much as
they do now, and would naturally track the current luminosity of the galaxy.
Critical ground-based observations to identify candidates were obtained for all three of these objects by John Kormendy
with the Canada-France-Hawaii Telescope (CFHT) on Mauna Kea, Hawaii. The NGC 4486b black hole detection was
also based on CFHT spectra.
Hubble's high resolution then allowed the team to peer deep into the cores of the galaxies with extraordinary resolution
unavailable from ground-based telescopes, and measure velocities of stars orbiting the black hole. A sharp rise in
velocity means that a great deal of matter is locked away in the galaxy's core, creating a powerful gravitational field that
accelerates nearby stars.
The team is confident their statistical search technique has allowed them to pinpoint all the black holes they expect to
see, above a certain mass limit. "However, our result is complicated by the fact that the observational data for the
galaxies are not of equal quality, and that the galaxies are at different distances," says Richstone.
One of the features of the February 1997 servicing mission to the Hubble will be the installation of the Space Telescope
Imaging Spectrograph (STIS). This spectrograph will greatly increase the efficiency of projects, such as this black hole
census, that require spectra of several nearby positions in a single object. This group will be continuing this census with
the refurbished telescope.
PRESS RELEASE NO.: STScI-PR95-47
HUBBLE FINDS A NEW BLACK HOLE -- AND UNEXPECTED NEW MYSTERIES
Confirming the presence of yet another super-massive black hole in the universe, astronomers using the Hubble Space Telescope
have found unexpected new mysteries. The black hole, and a 800 light-year-wide spiral-shaped disk of dust fuelling it, are slightly
offset from the centre of their host galaxy, NGC 4261, located 100 million light-years away in the direction of the constellation Virgo.
This discovery is giving astronomers a ringside seat to bizarre, dynamic processes that may involve a titanic collision and a runaway
black hole. This relatively nearby galaxy could shed light on how far more distant active galaxies and quasars produce their
prodigious amounts of energy.
"I'm delighted by this new finding. It doesn't fit our expectations, and this should lead us to a new understanding of black holes," said
Holland Ford. "The new Hubble observations have moved us beyond the question of whether black holes exist. Now we can work on
the demographics of black holes and address a number of other questions: does every galaxy have a black hole? How do these
extraordinary engines work?"
Predicted by Einstein's general theory of relativity, a black hole is an extremely compact and massive object that has such a powerful
gravitational field that nothing, not even light can escape. This is the second super-massive black hole confirmed by Hubble. By
measuring the speed of gas swirling around the black hole, the team of astronomers was able to calculate its mass to be 1.2 billion
times the mass of our Sun, yet concentrated into a region of space not much larger than our solar system.
The strikingly geometric disk -- which contains enough mass to make 100,000 stars like our Sun -- was first identified in Hubble
observations made in 1992. These new Hubble images reveal for the first time structure in the disk, which may be produced by
waves or instabilities in the disk.
Prior to Hubble observations, astronomers did not think dust was common in elliptical galaxies like NGC 4261, which were thought to
have stopped making stars long ago due to the absence of the requisite raw materials: interstellar gas and dust. However, Hubble is
showing that dust and beautiful disks are common in the centres of elliptical galaxies. The most conventional explanation is that the
disk is the remnant of a smaller galaxy that fell into the core of NGC 4261. The black hole will swallow the gas from the intruder over
the next 100 million years, and in the process produce spectacular fireworks, researchers predict. (Also see:
http://www.damtp.cam.ac.uk/user/gr/public/bh_obsv.html)
PRESS RELEASE NO.: STScI-PR95-47
HUBBLE FINDS A NEW BLACK HOLE -- AND UNEXPECTED NEW MYSTERIES (continued)
Such collisions may have been more common in the past, when the expanding universe was smaller. This
would help explain the abundance of quasars and active galaxies in the distant past. However, according to
theoretical simulations, it's difficult, dynamically, to get an intruder galaxy to plunge directly into a galaxy's core.
Another possibility is that dust ejected from ancient stars in the galaxy has fallen into the core and formed a
disk. But this does not explain why the disk is off-center, which is evidence for a dynamic close encounter.
Presumably, the black hole was at the center of the galaxy, but something has pulled it 20 light-years from the
center, according to the Hubble observations. However, the black hole is so massive it's hard to imagine how it
could have been moved. One exotic idea is that the black hole is self-propelled. The cold, dusty disk serves as
a rocket "fuel tank" by feeding material onto the black hole where gravity compresses and heats it to tens of
millions of degrees. Hot gas exhausts out from the black hole's vicinity producing the radio jets observed by
radio telescopes as twin-lobe structures extending far beyond the galaxy. This exhaust may be pushing the
black hole across space just like a rocket engine which propels an object by rapidly ejecting mass.
Hubble is ideally suited for hunting super-massive black holes in the universe. With the astronomical equivalent
of surgical precision, Hubble's spectrographs can measure the rotation of gas near enough to a suspected
black hole to capture its unmistakable gravitational signature. The speed of gas orbiting a back hole will rapidly
increase toward the center of the disk - just as the planets closer to our Sun orbit faster. To date two other
galaxies have confirmed black holes. Hubble detected a 2.4-billion-solar-mass black hole was identified in the
core of elliptical galaxy M87 in 1994, and later that year, astronomers using a radio telescope array to examine
the dynamics of a thin, warped disk of molecules deep in the core of spiral galaxy NGC 4258 measured a 40million-solar-mass black hole.
HUBBLE OBSERVES SPIRAL GAS DISK IN ACTIVE GALAXY
A NASA Hubble Space Telescope image of a spiral-shaped disk of hot gas in
the core of active galaxy M87. HST measurements show the disk is rotating
so rapidly it contains a massive black hole at its hub. A black hole is an object
that is so massive yet compact nothing can escape its gravitational pull, not
even light. The object at the centre of M87 fits that description. It weighs as
much as three billion suns, but is concentrated into a space no larger than our
solar system. Now that astronomers have seen the signature of the
tremendous gravitational field at the centre of M87, it is clear that the region
contains only a fraction of the number of stars that would be necessary to
create such a powerful attraction. The giant elliptical galaxy M87 is located 50
million light-years away in the constellation Virgo. Earlier observations
suggested the black hole was present, but were not decisive. A brilliant jet of
high-speed electrons that emits from the nucleus (diagonal line across image)
is believed to be produced by the black hole "engine." The image was taken
with HST's Wide Field Planetary Camera 2
HUBBLE MEASURES VELOCITY OF GAS ORBITING BLACK HOLE
A schematic diagram of velocity measurements of a rotating disk of hot
gas in the core of active galaxy M87. The measurement was made by
studying how the light from the disk is redshifted and blueshifted -- as
part of the swirling disk spins in earth's direction and the other side spins
away from earth. The gas on one side of the disk is speeding away from
Earth, at a speed of about 1.2 million miles per hour (550 kilometers per
second). The gas on the other side of the disk is orbiting around at the
same speed, but in the opposite direction, as it approaches viewers on
Earth. This high velocity is the signature of the tremendous gravitational
field at the center of M87. This is clear evidence that the region harbors a
massive black hole, since it contains only a fraction of the number of
stars that would be necessary to create such a powerful attraction. A
black hole is an object that is so massive yet compact nothing can
escape its gravitational pull, not even light. The object at the center of
M87 fits that description. It weights as much as three billion suns, but is
concentrated into a space no larger than our solar system. The
observations were made with HST's Faint Object Spectrograph.
Cygnus X-1, a
Galactic black
hole
Black hole
Artists impression of
the accretion disk
Is there a huge black hole in the Milky Way?
On the 26 October last year, a tiny patch of darkness in the
constellation Sagittarius flashed a brief pulse of X-rays into space,
providing compelling evidence that slap-bang in the middle of our
Galaxy is one of the weirdest objects known to astronomers: a
supermassive black hole.
So many observations point to the existence of black holes that
physicists talk about them as if they are old friends, yet no one has
ever actually verified their presence.
The length and location of the X-ray pulse make it strong evidence of
a black hole at the heart of our Galaxy. "There's nothing in the known
Universe that could be faking this," says Fulvio Melia an X-ray
astronomer at the University of Arizona in Tucson.
The flare was released when something, a comet perhaps, was sucked
violently into a black hole, says Frederick Baganoff at the
Massachussets Institute of Technology. His team spotted it using the
orbiting Chandra X-ray Observatory.
Astronomers already know that the centre of our Galaxy - a region
called Sagittarius A* - weighs about 2.6 million times more than our
Sun. The flare came from here and lasted about three hours, except
for a lull of a crucial 10 minutes.
That the flare disappeared and returned 10 minutes later means that
the X-rays took just 10 minutes to cross the entire span of Sagittarius
A*. In other words, the region is less than 15 million kilometres
across.
According to the astrophysics rule book, general relativity, such a vast
mass squeezed into an area so small can mean only one thing: "This
has to be a black hole," says Baganoff.
Baganoff, F. K. et al. Rapid X-ray flaring from the direction of the supermassive black
hole at the Galactic Centre, Nature, 413, 45 - 48, (2001).
http://www.nature.com/nsu/010906/010906-10.html Also see: http://ca.news.yahoo.com/010909/6/a4iw.html
Energy may escape from a black hole when it is in a strong magnetic field
which exerts a braking effect. This artist's impression illustrates how the
MCG-6-30-15 system may look.
Web site: http://sci.esa.int/content/news/index.cfm?aid=1&cid=1&oid=28779
Schematic diagram illustrating the possible origins
of the iron line in the spectrum of MCG-6-30-15
The XMM-Newton spectrum of MCG-6-30-15
Two lines are present at 6.4 keV: the narrow blue line corresponds to Xrays coming from iron that is far away from the black hole, towards the
outer parts of the accretion disc. The broad yellow line is the new mystery
feature fully revealed by XMM-Newton.
EXTRACTS FROM THE WEB SITE:
The spiral galaxy MCG-6-30-15 is situated 100 million light-years away. The data
obtained has led them to conclude that energy is not only going in to the galaxy's black
hole, but is also escaping.
This graph displays an unusually broad 'line' for the X-ray emission corresponding to the
presence of iron in the accretion disc. This broad line had first been detected in 1995
with the ASCA satellite but we had never seen it so clearly.
Analysis of this iron line suggests that this broad line arises from X-ray emission
stemming from the innermost areas of the accretion disc, just before matter disappears
into the black hole. But the number of photons and their energies measured by XMMNewton far exceed what could be expected from the established models for accretion
discs of supermassive black holes.
This may correspond to a theory proposed over 25 years ago by two Cambridge
University astronomers. Roger Blandford and Roman Znajek had suggested that
rotational energy could escape from a black hole when it is in a strong magnetic field
which exerts a braking effect. This theory fits the physical laws of thermodynamics which
state that energy released should be absorbed by the surrounding gas.
"We have probably seen this electric dynamo effect for the very first time. Energy is
being extracted from the black hole's spin and is conveyed into the innermost parts of
the accretion disc, making it hotter and brighter in X-rays.”