BLACK HOLES

1. 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.

3. 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 light-years 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.

5. 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.

7. 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.

9. 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.

11. 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 normal-looking 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.

12. 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.

14. 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)

15. 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 40-million-solar-mass black hole.

17. 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

19. 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.

20. Cygnus X-1, a Galactic black hole Black hole Artists impression of the accretion disk

22. 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 FulvioMelia 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

23. 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

24. Schematic diagram illustrating the possible origins of the iron line in the spectrum of MCG-6-30-15

25. The XMM-Newton spectrum of MCG-6-30-15 Two lines are present at 6.4 keV: the narrow blue line corresponds to X-rays 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.

26. 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 XMM-Newton 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.”