Signatures of Black Holes
Hubble Measures Velocity of Gas Orbiting a Black Hole
The schematic diagram of velocity measurements of a rotating disk of hot gas shows a plot of the spectrographic evidence for a supermassive black hole at the center of the elliptical galaxy M87. Gas near the center of the galaxy is moving at 550 km/sec, thus a very large mass must exists to keep the gas from flying off.
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. The object at the center of M87 weights as much as three billion suns, but is concentrated into a space no larger than our solar system.
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 observations were made with HST's Faint Object Spectrograph. The image was taken with HST's Wide Field Planetary Camera 2.
STIS Records a Black Hole's Signature in M84
The colorful "zigzag" on the right is the signature of a supermassive black hole in the center 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 centerline -- as seen as a green and yellow picture element (pixels) along the center 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 center 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 center 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 center line.
STIS measures a velocity of 880,000 miles per hour (400 kilometers per second) within 26 light-years of the galaxy's center, 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 centers 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 sulfur ions in orbit around the center 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.
Feasting Black Hole Blows Bubbles
A monstrous black hole's rude table manners include blowing huge bubbles of hot gas into space. At least, that's the gustatory practice followed by the supermassive black hole residing in the hub of the nearby galaxy NGC 4438. Known as a peculiar galaxy because of its unusual shape, NGC 4438 is in the Virgo Cluster, 50 million light-years from Earth.
These NASA Hubble Space Telescope images of the galaxy's central region clearly show one of the bubbles rising from a dark band of dust. The other bubble, emanating from below the dust band, is barely visible, appearing as dim red blobs in the close-up picture of the galaxy's hub (the colorful picture at right). The background image represents a wider view of the galaxy, with the central region defined by the white box.
These extremely hot bubbles are caused by the black hole's voracious eating habits. The eating machine is engorging itself with a banquet of material swirling around it in an accretion disk (the white region below the bright bubble). Some of this material is spewed from the disk in opposite directions. Acting like high-powered garden hoses, these twin jets of matter sweep out material in their paths. The jets eventually slam into a wall of dense, slow-moving gas, which is traveling at less than 223,000 mph (360,000 kph). The collision produces the glowing material. The bubbles will continue to expand and will eventually dissipate. Compared with the life of the galaxy, this bubble-blowing phase is a short-lived event.
The bubble is much brighter on one side of the galaxy's center because the jet smashed into a denser amount of gas. The brighter bubble is 800 light-years tall and 800 light-years across.
The observations are being presented June 5 at the American Astronomical Society meeting in Rochester, N.Y. Both pictures were taken March 24, 1999 with the Wide Field and Planetary Camera 2. False colors were used to enhance the details of the bubbles. The red regions in the picture denote the hot gas.
Centaurus A: The Inside Story
Astronomers have used NASA's Hubble Space Telescope to probe the core of the nearest active galaxy to Earth, Centaurus A.
[UPPER LEFT] - A close-up high resolution Wide Field Planetary Camera 2 image of the dramatic dust disk which is thought to be the remnant of a smaller spiral galaxy that merged with the large elliptical galaxy. The shock of the collision compressed interstellar gas, precipitating a flurry of star formation and giving the material a fleecy pattern. Dark filaments of dust mixed with cold hydrogen gas are silhouetted against the incandescent yellow-orange glow from stars behind it.
[LOWER RIGHT] - Hubble's Near Infrared Camera and Multi-Object Spectrometer was used to peer past the dust to discover a tilted disk of hot gas at the galaxy's center (white bar running diagonally across image center). This 130 light-year diameter disk encircles a suspected black hole which may be one billion times the mass of our Sun. The disk feeds material to presumably an inner, unresolved accretion disk that is made up of gas entrapped by the black hole. The red blobs near the disk are glowing gas clouds which have been heated up and ionized by the powerful radiation from the active nucleus.
The false-color NICMOS image was taken on Aug. 11, 1997 at a wavelength of 1.87 microns ("Paschen alpha"), characteristic of ionized Hydrogen.
A Lighthouse Beacon from the Core of an Active Galaxy
Courtesy: Allan Sandage, Carnegie Observatories
[Right] - A NASA Hubble Space Telescope (HST) view of the core of the barred spiral Seyfert galaxy NGC 5728 reveals a spectacular bi-conical beam of light that is ionizing the gas in the central region of the galaxy.
This image was presented at the American Astronomical Society meeting in Berkeley, California by Dr. Andrew Wilson of the Space Telescope Science Institute (STScI), Baltimore, Maryland.
Because NGC 5728 is an active galaxy, the core might contain a super massive black hole surrounded by a disk of gas, according to astronomers. This hot disk glows with ultraviolet light. However, a dense ring of gas blocks Hubble's view of the black hole and glowing accretion disk. The visible and ultraviolet light escapes along the open ends of the gas "donut" to several thousands of light-years from the nucleus. The ring thus shapes the escaping ultraviolet light into two lighthouse beacon style "ionization cones."
The image was made September 4, 1992 with the Wide Field and Planetary Camera (WFPC) in PC mode. Exposures were obtained in the light of doubly-ionized oxygen and neutral hydrogen.
[Left] - A ground based image of the barred spiral galaxy NGC 5728, located 125 million light-years away in the constellation Libra.
“Cool” Black Hole at the Heart of Andromeda
This X-ray image shows the central portion of M31, the Andromeda Galaxy. The blue dot in the center of the image is a "cool" million degree X-ray source where a supermassive black hole with the mass of 30 million suns is located. The X-rays are produced by matter funneling toward the black hole. Numerous other hotter X-ray sources are also apparent. Most of these are probably due to X-ray binary systems, in which a neutron star or black hole is in a close orbit around a normal star.
The image was made by the Chandra X-ray camera with the Advanced CCD Imaging Spectrometer (ACIS)
A full-field view of M31 is shown at the right, imaged by a ground-based telescope. It is often called our sister galaxy, having the same approximate size and shape of our own. At under 3 million light-years away, it is the nearest major galaxy to the Milky Way. Only the Magellanic Clouds are closer.
Dust Disk Around a Massive Black Hole
Resembling a gigantic hubcap in space, a 3,700 light-year-diameter dust disk encircles a 300 million solar-mass black hole in the center of the elliptical galaxy NGC 7052.
The disk, possibly a remnant of an ancient galaxy collision, will be swallowed up by the black hole in several billion years.
Because the front end of the disk eclipses more stars than the back, it appears darker. Also, because dust absorbs blue light more effectively than red light, the disk is redder than the rest of the galaxy (this same phenomenon causes the Sun to appear red when it sets in a smoggy afternoon).
This NASA Hubble Space Telescope image was taken with the Wide Field and Planetary Camera 2, in visible light. Details as small as 50 light-years across can be seen.
Hubble's Faint Object Spectrograph (replaced by the STIS spectrograph in 1997) was used to observe hydrogen and nitrogen emission lines from gas in the disk. Hubble measurements show that the disk rotates like an enormous carousel, 341,000 miles per hour (155 kilometers per second) at 186 light-years from the center.
The rotation velocity provides a direct measure of the gravitational force acting on the gas by the black hole. Though 300 million times the mass of our Sun, the black hole is still only 0.05 per cent of the total mass of the NGC 7052 galaxy. Despite its size, the disk is 100 times less massive than the black hole. Still, it contains enough raw material to make three million sun-like stars.
The bright spot in the center of the disk is the combined light of stars that have crowded around the black hole due to its strong gravitational pull. This stellar concentration matches theoretical models linking stellar density to a central black hole's mass.
NGC 7052 is a strong source of radio emission and has two oppositely directed `jets' emanating from the nucleus. (The jets are streams of energetic electrons moving in a strong magnetic field and unleashing radio energy).
Because the jets in NGC 7052 are not perpendicular to the disk, it may indicate that the black hole and the dust disk in NGC 7052 do not have a common origin. One possibility is that the dust was acquired from a collision with a small neighboring galaxy, after the black hole had already formed.
NGC 7052 is located in the constellation of Vulpecula, 191 million light-years from Earth.
Black Hole in Galaxy NGC 4261
Left: Ground Based Composite Visual/Radio View: The giant elliptical galaxy NGC 4261 is one of the twelve brightest galaxies in the Virgo cluster, located 45 million light-years away. Photographed in visible light (white) the galaxy appears as a fuzzy disk of hundreds of billions of stars. A radio image (orange) shows a pair of opposed jets emanating from the nucleus and spanning a distance of 88,000 light-years.
Right: HST Image of NGC 4261: A giant disk of cold gas and dust fuels a possible black hole at the core of the galaxy. Estimated to be 300 light-years across, the disk is tipped enough (about 60 degrees) to provide astronomers with a clear view of the bright hub, which presumably harbors the black hole. The dark, dusty disk represents a cold outer region which extends inwards to an ultra-hot accretion disk with a few hundred million miles from the suspected black hole. This disk feeds matter into the black hole, where gravity compresses and heats the material. Hot gas rushes from the vicinity of the black hole's creating the radio jets. The jets are aligned perpendicular to the disk, like an axle through a wheel. This provides strong circumstantial evidence for the existence of a black hole "central engine" in NGC 4261.
Hubble Discovers Black Holes in Unexpected Places
Image Credits for G1: NASA and Michael Rich (UCLA)
Science Credits: NASA, Roeland Van Der Marel and Joris Gerssen (Space Telescope Science Institute), Puragra Guhathakurta and Ruth Peterson (University of California Observatories/Lick Observatory), Carlton Pryor (Rutgers University), Michael Rich (UCLA), Karl Gebhardt (University of Texas), and Luis Ho (Carnegie Institution of Washington)
These two globular star clusters, M15 and G1, harbor hundreds of thousands of stars. But deep within their dense cores is an unexpected guest: a class of intermediate-sized black holes. Black holes are invisible, but the probing eye of NASA's Hubble Space Telescope found them by measuring the velocities of stars whirling around the crowded cores. Using spectral observations, astronomers discovered that the stars orbiting the cores of M15 and G1 moved at a much faster rate, which suggested the presence of unseen massive bodies. These previously undiscovered black holes provide an important link that sheds light on the way in which black holes grow.
The new findings promise a better understanding of how galaxies and globular clusters first formed billions of years ago. Globular star clusters contain the oldest stars in the universe. If these clusters have black holes now, then they most likely had black holes when they formed billions of years ago.
The black hole in M15 [left] is 4,000 times more massive than our Sun. G1 [right], a much larger globular cluster, harbors a heftier black hole, about 20,000 times more massive than our Sun.
The globular star cluster M15 resides 32,000 light-years away in the constellation Pegasus. M15 is one of nearly 150 known globular clusters that form a vast halo surrounding our Milky Way galaxy. G1, located 2.2 million light-years away in the neighboring Andromeda galaxy (also known as M31), has a total mass of 10 million suns, making it one of the most massive globular clusters known.
The Hubble telescope photograph of M15 was taken December 1998 by the Wide Field and Planetary Camera 2. Hubble's Wide Field and Planetary Camera 2 also snapped the image of G1, in July 1994.
The members of the G1 research team are Michael Rich (University of California, Los Angeles/UCLA), Karl Gebhardt (University of Texas at Austin), and Luis Ho (Carnegie Institute of Washington). The members of the M15 research team are Roeland Van Der Marel and Joris Gerssen (Space Telescope Science Institute), Karl Gebhardt, Puragra Guhathakurta and Ruth Peterson (UCO/Lick Observatory, University of California at Santa Cruz), and Carlton Pryor (Rutgers University).
Massive Black Holes in Galaxies NGC 3377, NGC 3379 and NGC 4486B
These three galaxies are believed to contain central, supermassive black holes. The galaxy NGC 4486B (lower-left) shows a double nucleus (lower-right). The images of NGC 3377 and NGC 4486B are 2.7 arcseconds on a side, and for NGC 3379 the size is 5.4 arcseconds; the lower-right is a blow-up of the central 0.5 arcseconds of NGC 4486B.
Fireworks Near a Black Hole in 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 color 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 center 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 ionize 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.
Black Holes: One Size Doesn't Fit All
This comparison of the hearts of four elliptical galaxies shows that the more massive a galaxy's central bulge of stars, the heftier its black hole. The galaxies are part of a census of 30 galaxies conducted by astronomers using NASA's Hubble Space Telescope. Black holes are dense, compact objects possessing such strong gravitational forces that not even light can escape them.
The column of black-and-white pictures at left, taken by ground-based telescopes, shows the galaxies. The inset boxes define the central regions of stars. Close-up images of these regions, as seen by Hubble's Wide Field and Planetary Camera 2, are in the middle column. The column at right lists the masses of the black holes and illustrates the respective diameters of the event horizons. An event horizon defines a black hole's boundary. Any material that crosses that boundary becomes ensnared in a black hole's grasp and cannot escape. The event horizons cannot be seen in the Hubble images because they are 25 million times smaller than the scale of the pictures.
Astronomers determined the mass of each black hole by measuring the motion of stars swirling around it: the closer the stars approach the black hole, the faster their velocity. Only through observations with Hubble's superior vision could astronomers probe to the core of the galaxy where these effects are easily measured. They discovered a remarkable new correlation between a black hole's mass and the average speed of the stars in a galaxy's central bulge. The faster the stars are moving, the more massive the black hole. This information suggests that the galaxy and the black hole grew simultaneously.