Super Galaxies and Quasars


Odd Couple Widely Separated by Time and Space


Image Credit: NASA and The Hubble Heritage Team (STScI/AURA)
Acknowledgment: R. Knacke (Penn State Erie)

Appearances can be deceiving. In this NASA Hubble Space Telescope image, an odd celestial duo, the spiral galaxy NGC 4319 [center] and a quasar called Markarian 205 [upper right], appear to be neighbors. In reality, the two objects don't even live in the same city. They are separated by time and space.

NGC 4319 is 80 million light-years from Earth. Markarian 205 (Mrk 205) is more than 14 times farther away, residing 1 billion light-years from Earth. The apparent close alignment of Mrk 205 and NGC 4319 is simply a matter of chance. Astronomers used two methods to determine the distances to these objects. First, they measured how their light has been stretched in space due to the universe's expansion. Then they measured how much the ultraviolet light from Mrk 205 dimmed as it passed through the interstellar gas of NGC 4319.

The Hubble Wide Field and Planetary Camera 2 image shows the inner region of NGC 4319. In addition to the galaxy's inner spiral arms, an outer arm is faintly visible at lower left. The unusually dark and misshapen dust lanes in the galaxy's inner region are evidence of a disturbance, probably caused by an earlier interaction with another galaxy, NGC 4291, which is not in the photograph.

At a distance of 1 billion light-years, Mrk 205 is a relatively nearby quasar. Many quasars reside much farther away. Quasars, once known only as mysterious point-like objects, are now known to be distant galaxies that have extremely bright cores. These powerhouses of light are probably fueled by massive black holes. With powerful telescopes like Hubble, it is often possible to see the quasar's surrounding halo of faint starlight, as is clearly visible around Mrk 205.

Mrk 205 has a companion, a compact galaxy just below it. The objects appear to be interacting. The compact galaxy may be responsible for the structure in Mrk 205's halo.

The Hubble image shows that interacting galaxies and disturbances within galaxies are a rich source of information about galaxy structure and evolution.






M104, The Sombrero Galaxy


Credit: ESO
The "Sombrero" is located in the constellation Virgo, at a distance of about 50 million light-years. This galaxy is notable for its dominant nuclear bulge, composed primarily of mature stars, and its nearly edge-on disk composed of stars, gas, and intricately structured dust. The complexity of this dust, and the high resolution of this image, is most apparent directly in front of the bright nucleus, but is also very evident as dark absorbing lanes throughout the disk. A significant fraction of the galaxy disk is even visible on the far side of the source, despite its massive bulge.

The radio properties of Messier 104 are unusual for a spiral galaxy - it has a variable core. The optical spectrum of the central region displays emission lines from hot gas (of the "LINER" type, Low Ionization Nuclear Emission line Region). This points to Messier 104 harboring a weak Active Galactic Nucleus (AGN). Although more commonly known from the much more luminous and distant quasars and powerful radio galaxies, the weak AGN in this galaxy lies at the opposite extreme: the most likely explanation being a central black hole accreting circumnuclear matter at a slow pace.

A large number of small and slightly diffuse sources can be seen as a swarm in the halo of Messier 104. Most of these are globular clusters, similar to those found in our own Galaxy. Measurements reveal a steep increase in the mass-to-light ratio and increasing stellar speeds near the nucleus of Messier 104. This is also indicative of the presence of a massive black hole at the center, estimated at about 109 solar masses.

The lower image shows the eastern area, with the pronounced dust bands and many background galaxies. A great amount of fine detail is revealed, from the structures in the pronounced dust band in the equatorial plane, to many faint background galaxies that shine through the outer regions. The overall "sharpness" of this color image corresponds to about 0.7 arcsec which translates into a resolution of about 170 light-years at that distance.

The color image was made by a combination of three CCD images from the FORS1 multi-mode instrument on VLT ANTU, obtained by Peter Barthel from the Kapteyn Institute (Groningen, The Netherlands) during an observing run at the Paranal Observatory. He and Mark Neeser, also from the Kapteyn Institute, produced the composite images. The galaxy fits perfectly into the 6.8 x 6.8 arcmin2 field-of-view of the FORS1 camera.

Messier 104 is the 104th object in the famous catalogue of nebulae by French astronomer Charles Messier (1730 - 1817). It was not included in the first two editions (with 45 objects in 1774; 103 in 1781), but Messier soon thereafter added it by hand in his personal copy as a "very faint nebula". The recession velocity, about 1000 km/sec, was first measured by American astronomer Vesto M. Slipher at the Lowell Observatory in 1912; he was also the first to detect the galaxy's rotation.






Hubble Captures an Extraordinary and Powerful Active Galaxy


Credits: NASA, Andrew S. Wilson (University of Maryland); Patrick L. Shopbell (Caltech); Chris Simpson (Subaru Telescope); Thaisa Storchi-Bergmann and F. K. B. Barbosa (UFRGS, Brazil); and Martin J. Ward (University of Leicester, U.K.)
Resembling a swirling witch's cauldron of glowing vapors, the black hole-powered core of a nearby active galaxy appears in this colorful NASA Hubble Space Telescope image. The galaxy lies 13 million light-years away in the southern constellation Circinus.

This galaxy is designated a type 2 Seyfert, a class of mostly spiral galaxies that have compact centers and are believed to contain massive black holes. Seyfert galaxies are themselves part of a larger class of objects called Active Galactic Nuclei or AGN. AGN have the ability to remove gas from the centers of their galaxies by blowing it out into space at phenomenal speeds. Astronomers studying the Circinus galaxy are seeing evidence of a powerful AGN at the center of this galaxy as well.

Much of the gas in the disk of the Circinus spiral is concentrated in two specific rings -- a larger one of diameter 1,300 light-years, which has already been observed by ground-based telescopes, and a previously unseen ring of diameter 260 light-years.

In the Hubble image, the smaller inner ring is located on the inside of the green disk. The larger outer ring extends off the image and is in the plane of the galaxy's disk. Both rings are home to large amounts of gas and dust as well as areas of major "starburst" activity, where new stars are rapidly forming on timescales of 40 - 150 million years, much shorter than the age of the entire galaxy.

At the center of the starburst rings is the Seyfert nucleus, the believed signature of a supermassive black hole that is accreting surrounding gas and dust. The black hole and its accretion disk are expelling gas out of the galaxy's disk and into its halo (the region above and below the disk). The detailed structure of this gas is seen as magenta-colored streamers extending towards the top of the image.

In the center of the galaxy and within the inner starburst ring is a V-shaped structure of gas. The structure appears whitish-pink in this composite image, made up of four filters. Two filters capture the narrow lines from atomic transitions in oxygen and hydrogen; two wider filters detect green and near-infrared light. In the narrow-band filters, the V-shaped structure is very pronounced. This region, which is the projection of a three-dimensional cone extending from the nucleus to the galaxy's halo, contains gas that has been heated by radiation emitted by the accreting black hole. A "counter-cone," believed to be present, is obscured from view by dust in the galaxy's disk. Ultraviolet radiation emerging from the central source excites nearby gas causing it to glow. The excited gas is beamed into the oppositely directed cones like two giant searchlights.

Located near the plane of our own Milky Way Galaxy, the Circinus galaxy is partially hidden by intervening dust along our line of sight. As a result, the galaxy went unnoticed until about 25 years ago. This Hubble image was taken on April 10, 1999 with the Wide Field Planetary Camera 2.

The research team, led by Andrew S. Wilson of the University of Maryland, is using these visible light images along with near-infrared data to further understand the dynamics of this powerful galaxy.











Radio Galaxy 1938+666


Credit: NASA
The Hubble Space Telescope picture of the distant galaxy 1938+666 which has been imaged into an Einstein ring by an intervening galaxy. The intervening galaxy shows up as the bright spot in the center of the ring. The picture was taken in the infra-red region of the spectrum and the computer-generated color of the image has been chosen simply for ease of viewing.

The MERLIN radio picture of the radio source 1938+666 embedded in the distant galaxy. The incomplete ring (or arc) shows that the radio source is not perfectly aligned with the lens galaxy and the Earth. The lens galaxy does not contain a radio source and hence does not show up in this picture. The colors are computer-generated and represent different levels of radio brightness.









NGC 5128, Centaurus A




Photo credit: NOAO, National Optical Astronomy Observatories
NGC 5128 is one of the most luminous and massive galaxies known and is a strong source of both radio and X-ray radiation. The photo was taken by the Cerro Toloto 4-meter telescope in 1975 and the image at the right combines the optical photo with the radio-wave emission spectrum obtain by the Very Large Array (VLA).












Turbulent Cauldron of Starbirth in NGC 5128


Credit: E.J. Schreier, (STScI) and NASA; Ground-based image (upper left): NOAO
NASA's Hubble Space Telescope offers a stunning unprecedented close-up view of a turbulent firestorm of starbirth along a nearly edge-on dust disk girdling Centaurus A, the nearest active galaxy to Earth.

A ground-based telescopic view (upper left insert) shows that the dust lane girdles the entire elliptical galaxy. This lane has long been considered the dust remnant of a smaller spiral galaxy that merged with the large elliptical galaxy. The spiral galaxy deposited its gas and dust into the elliptical galaxy, and the shock of the collision compressed interstellar gas, precipitating a flurry of star formation. Resembling looming storm clouds, dark filaments of dust mixed with cold hydrogen gas are silhouetted against the incandescent yellow-orange glow from hot gas and stars behind it.

Brilliant clusters of young blue stars lie along the edge of the dark dust rift. Outside the rift the sky is filled with the soft hazy glow of the galaxy's much older resident population of red giant and red dwarf stars.

The dusty disk is tilted nearly edge-on, its inclination estimated to be only 10 or 20 degrees from our line-of-sight. The dust lane has not yet had enough time since the recent merger to settle down into a flat disk. At this oblique angle, bends and warps in the dust lane cause us to see a rippled "washboard" structure.

The picture is a mosaic of two Hubble Space Telescope images taken with the Wide Field Planetary Camera 2, on Aug. 1, 1997 and Jan. 10, 1998. The approximately natural color is assembled from images taken in blue, green and red light. Details as small as seven light-years across can be resolved. The blue color is due to the light from extremely hot, newborn stars. The reddish-yellow color is due in part to hot gas, in part to older stars in the elliptical galaxy and in part to scattering of blue light by dust -- the same effect that produces brilliant orange sunsets on Earth.


A Close-up of the Central Region of NGC 5128






M87, A Cosmic Searchlight


Credit: NASA and The Hubble Heritage Team (STScI/AURA)
Streaming out from the center of the galaxy M87 like a cosmic searchlight is one of nature's most amazing phenomena, a black-hole-powered jet of electrons and other sub-atomic particles traveling at nearly the speed of light. In this NASA Hubble Space Telescope image, the blue of the jet contrasts with the yellow glow from the combined light of billions of unseen stars and the yellow, point-like globular clusters that make up this galaxy.

At first glance, M87 (also known as NGC 4486) appears to be an ordinary giant elliptical galaxy; one of many ellipticals in the nearby Virgo cluster of galaxies. However, as early as 1918, astronomer H.D. Curtis noted a "curious straight ray" protruding from M87. In the 1950s when the field of radio was blossoming, one of the brightest radio sources in the sky, Virgo A, was discovered to be associated with M87 and its jet.

After decades of study, prompted by these discoveries, the source of this incredible amount of energy powering the jet has become clear. Lying at the center of M87 is a supermassive black hole, which has swallowed up a mass equivalent to 2 billion times the mass of our Sun. The jet originates in the disk of superheated gas swirling around this black hole and is propelled and concentrated by the intense, twisted magnetic fields trapped within this plasma. The light that we see (and the radio emission) is produced by electrons twisting along magnetic field lines in the jet, a process known as synchrotron radiation, which gives the jet its bluish tint.

M87 is one of the nearest and is the most well-studied extragalactic jet, but many others exist. Wherever a massive black hole is feeding on a particularly rich diet of disrupted stars, gas, and dust, the conditions are right for the formation of a jet. Interestingly, a similar phenomenon occurs around young stars, though at much smaller scales and energies.

At a distance of 50 million light-years, M87 is too distant for Hubble to discern individual stars. The dozens of star-like points swarming about M87 are, instead, themselves clusters of hundreds of thousands of stars each. An estimated 15,000 globular clusters formed very early in the history of this galaxy and are older than the second generation of stars, which huddle closer to the center of the galaxy.

The data were collected with Hubble's Wide Field Planetary Camera 2 in 1998 by J.A. Biretta, W.B. Sparks, F.D. Macchetto, and E.S. Perlman (STScI). The Hubble Heritage team combined these exposures of ultraviolet, blue, green, and infrared light in order to create this color image.






Close-up Look at M87's Jet


Credit: National Radio Astronomy Observatory/National Science Foundation
[top left] - This radio image of the galaxy M87, taken with the Very Large Array (VLA) radio telescope in February 1989, shows giant bubble-like structures where radio emission is thought to be powered by the jets of subatomic particles coming from the the galaxy's central black hole. The false color corresponds to the intensity of the radio energy being emitted by the jet. M87 is located 50 million light-years away in the constellation Virgo.

Credit: NASA and John Biretta (STScI/JHU)
[top right] - A visible light image of the giant elliptical galaxy M87, taken with NASA Hubble Space Telescope's Wide Field Planetary Camera 2 in February 1998, reveals a brilliant jet of high-speed electrons emitted from the nucleus (diagonal line across image). The jet is produced by a 3-billion-solar-mass black hole.

Credit: National Radio Astronomy Observatory/Associated Universities, Inc.
[bottom] - A Very Long Baseline Array (VLBA) radio image of the region close to the black hole, where an extragalactic jet is formed into a narrow beam by magnetic fields. The false color corresponds to the intensity of the radio energy being emitted by the jet. The red region is about 1/10 light-year across. The image was taken in March 1999.






Hubble Identifies Source of Ultraviolet Light in an Old Galaxy


Credit: NASA and Thomas M. Brown, Charles W. Bowers, Randy A. Kimble, Allen V. Sweigart (NASA Goddard Space Flight Center) and Henry C.Ferguson (Space Telescope Science Institute).
Hubble Space Telescope's exquisite resolution has allowed astronomers to resolve, for the first time, hot blue stars deep inside an elliptical galaxy. The swarm of nearly 8,000 blue stars resembles a blizzard of snowflakes near the core (lower right) of the neighboring galaxy M32, located 2.5 million light-years away in the constellation Andromeda.

Hubble confirms that the ultraviolet light comes from a population of extremely hot helium-burning stars at a late stage in their lives. Unlike the Sun, which burns hydrogen into helium, these old stars exhausted their central hydrogen long ago, and now burn helium into heavier elements.

The observations, taken in October 1998, were made with the camera mode of the Space Telescope Imaging Spectrograph (STIS) in ultraviolet light. The STIS field of view is only a small portion of the entire galaxy, which is 20 times wider on the sky. For reference, the full moon is 70 times wider than the STIS field-of-view. The bright center of the galaxy was placed on the right side of the image, allowing fainter stars to be seen on the left side of the image.

Thirty years ago, the first ultraviolet observations of elliptical galaxies showed that they were surprisingly bright when viewed in ultraviolet light. Before those pioneering UV observations, old groups of stars were assumed to be relatively cool and thus extremely faint in the ultraviolet. Over the years since the initial discovery of this unexpected ultraviolet light, indirect evidence has accumulated that it originates in a population of old, but hot, helium-burning stars. Now Hubble provides the first direct visual evidence.

Nearby elliptical galaxies are thought to be relatively simple galaxies comprised of old stars. Because they are among the brightest objects in the Universe, this simplicity makes them useful for tracing the evolution of stars and galaxies.






Hubble Views Home Galaxy of Record-Breaking Explosion


Credit: Andrew Fruchter (STScI) and NASA
A NASA Hubble Space Telescope view of the rapidly fading visible-light fireball from the most powerful cosmic explosion recorded to date. For a brief moment the light from the blast was equal to the radiance of 100 million billion stars. The initial explosion began as an intense burst of gamma-rays which happened on Jan. 23, 1999.

The blast had already faded to one four-millionth of its original brightness when Hubble made observations on February 8 and 9. Space Telescope captured the fading fireball embedded in a galaxy located 2/3 of the way to the horizon of the observable universe.

Hubble’s resolution shows the galaxy is not the classic spiral or elliptical shape. It appears as finger-like filaments extending above the bright white blob of the fireball. The galaxy might be distorted by a collision with another galaxy. This would induce rapid starbirth as gas clouds were heated and compressed, precipitating millions of newborn stars.

The presence of this so-called starburst activity is strongly supported by Hubble and Keck telescope images that show the host galaxy is exceptionally blue. This means it contains a large number of blue newborn stars.

Space Telescope’s observations further support the idea that these mysterious powerful explosions happen where vigorous star formation takes place. Gamma-ray bursts may be created by the mergers of a pair of neutron stars or black holes, or a hypernova, a theorized type of exceptionally violent exploding star.

Gamma-ray bursts go off at about one per day. The armada of telescopes now looking for them is allowing astronomers to learn more details of the explosion to refine models for explaining these mysterious events.









Lensed Quasar MG0414+0534


Credit: ESO
This is an infrared color composite of the quadruply lensed quasar system MG0414+0534 made by combining 20 min ISAAC J (1.25µm) and Ks (2.16µm) exposures. This complex of images is only about 2 arcsec across. At the center is the red galaxy at redshift z = 0.96 which is responsible for the four (of which two are not completely resolved) gravitationally lensed images of a z = 2.64 quasar plus a faint arc.












A Distant Quasar's Brilliant Light and the Missing Hydrogen


Credit : WIYN Telescope at Kitt Peak National Observatory in Arizona. The telescope is owned and operated by the University of Wisconsin, Indiana University, Yale University, and the National Optical Astronomy Observatories.
For the past decade astronomers have looked for vast quantities of hydrogen that were cooked up in the Big Bang but somehow managed to disappear in the empty blackness of space. Now, NASA's Hubble Space Telescope has uncovered this long-sought missing hydrogen. This gas accounts for nearly half of the "normal" matter in the universe -- the rest is locked up in galaxies. The confirmation of this missing hydrogen will shed new light on the large-scale structure of the universe. The detection also confirms fundamental models of how so much hydrogen was manufactured in the first few minutes of the universe's birth in the Big Bang.

The arrow in this image, taken by a ground-based telescope, points to a distant quasar, the brilliant core of an active galaxy residing billions of light-years from Earth. As light from this faraway object travels across space, it picks up information on galaxies and the vast clouds of material between galaxies as it moves through them. The Space Telescope Imaging Spectrograph aboard NASA's Hubble Space Telescope decoded the quasar's light to find the spectral "fingerprints" of highly ionized (energized) oxygen, which had mixed with invisible clouds of hydrogen in intergalactic space. The quasar's brilliant beam pierced at least four separate filaments of the invisible hydrogen laced with the telltale oxygen. The presence of oxygen between the galaxies implies there are huge quantities of hydrogen in the universe.






Lyman-alpha Companions and Extended Nebulosity Around a Quasar


Credit: ESO
In current theories of galaxy formation, luminous galaxies we see today were built up through repeated merging of smaller protogalactic clumps. Quasars, prodigious sources pouring out 100 to 1000 times as much light as an entire galaxy, have been used as markers of galaxy formation activity and have guided astronomers in their hunting of primeval galaxies and large-scale structures at high redshift. A supermassive black-hole, swallowing stars, gas and dust, is thought to be the engine powering a quasar and the interaction of the galaxy hosting the black-hole with neighboring galaxies is expected to play a key role in "feeding the monster".

At intermediate redshift, a large fraction of radio-loud quasars and radio galaxies inhabit rich clusters of galaxies, whereas radio-quiet quasars are rarely found in very rich environments. Furthermore, tidal interaction between quasars and their nearby companions is also the favored explanation for the presence of large gaseous nebulosities associated with radio-loud quasars and radio galaxies. At high redshift, searches for Lyman-alpha quasar companions and emission-line nebulosities show strong similarities with those seen at lower redshift, although the detection rate is lower.

The upper image is a false-color reproduction of a B-band image of the field around the radio-weak quasar J2233-606 in the Hubble Deep Field South (HDF-S).

The lower image represents emission from the same direction at a wavelength that corresponds to Lyman-alpha emission at the redshift (z = 2.2) of the quasar. Three Lyman-alpha candidate companions are indicated with arrows. Note also the extended nebulosity around the quasar.

A search for Lyman-alpha companions to the radio-weak quasar J2233-606 in the Hubble Deep Field South (HDF-S) was conducted during the VLT UT1 SV program in a small field of 1.2 x 1.3 arcmin2, centered on the quasar. Candidate Lyman-alpha companions were identified by subtracting a broad-band B (blue) image, that traces the galaxy stellar populations, from a narrow-band image, spectrally centered on the redshifted, narrow Lyman-alpha emission line of the quasar (z = 2.2).

Three Lyman-alpha candidate companions were discovered at angular distances of 15 to 23 arcsec, or 200 to 300 kpc (650,000 to 1,000,000 light-years) at the distance corresponding to the quasar redshift. The emission lines are very strong, relative to the continuum emission of the galaxies - this could be a consequence of the strong ionizing radiation field of the quasar. These companions to the quasar may trace a large-scale structure which would extend over larger distances beyond the observed, small field.

Even more striking is the presence of a very extended nebulosity whose size (120 kpc x 160 kpc) and Lyman-alpha luminosity (3 x 1044 erg/cm2/s) are among the largest observed around radio galaxies and radio-loud quasars, but rarely seen around a radio-weak quasar. Tidal interaction between the northern, very nearby companion and the quasar is clearly present: the companion is embedded in the quasar nebulosity, most of its gas has been stripped and lies in a tail westwards of the galaxy.

The scientists involved in this study are: Jacqueline Bergeron (ESO), Stefano Cristiani, Stephane Arnouts, Gianni Fasano (Padova, Italy) and Patrick Petitjean (Institut d'Astrophysique, Paris, France).






Quasar PG1115+080 and Gravitational Lens


Credit: Christopher D. Impey (University of Arizona)
Left: The light from the single quasar PG 1115+080 is split and distorted in this infrared image. PG 1115+080 is at a distance of about 8 billion light years in the constellation Leo, and it is viewed through an elliptical galaxy lens at a distance of 3 billion light years. The NICMOS frame is taken at a wavelength of 1.6 microns and it shows the four images of the quasar (the two on the left are nearly merging) surrounding the galaxy that causes the light to be lensed. The quasar is a variable light source and the light in each image travels a different path to reach the Earth. The time delay of the variations allows the distance scale to be measured directly. The linear streaks on the image are diffraction artifacts in the NICMOS instrument (NASA/Space Telescope Science Institute).

Right: In this NICMOS image, the four quasar images and the lens galaxy have been subtracted, revealing a nearly complete ring of infrared light. This ring is the stretched and amplified starlight of the galaxy that contains the quasar, some 8 billion light years away. (NASA/Space Telescope Science Institute).






Hubble Resolves Quasars' Host Galaxies


Credit: Dr. John Hutchings,Dominion Astrophysical Observatory, NASA
This Hubble Space Telescope image (right) reveals the faint host galaxy that a bright quasar dwells within. The wealth of new detail in this picture helps solve a three-decade old mystery about the true nature of quasars, the most distant and energetic objects in the universe.

The HST image shows clearly that the quasar, called 1229+204, lies in the core of a galaxy that has a common shape consisting of two spiral arms of stars connected by a bar-like feature. The host galaxy is in a spectacular collision with a dwarf galaxy. The collision apparently fuels the quasar "engine" at the galaxy center - presumably a massive black hole -- and also triggers many sites of new star-formation.

The image is one of a pair of relatively nearby quasars that were selected as early targets to test the resolution and dynamic range of HST's newly-installed Wide Field and Planetary Camera, which contains special optics to correct for a flaw in Hubble's primary mirror. The observations were made by Dr. John Hutchings of Dominion Astrophysical Observatory in Victoria, British Columbia. "The project was impossible from ground-based telescopes," says Dr. Hutchings, who has been researching quasars for many years. " The sharpness of the Hubble pictures is leading to major new discoveries almost anywhere you point it in the sky."

Quasars are the most distant objects in the universe, and so are among the earliest objects known to have formed in the young universe, more than 12 billion years ago. The most widely accepted notion is that quasars are in galaxies with active, supermassive black holes at their centers. However, because of their enormous distance, the `host' galaxies appear very small and faint, and are very hard to see against the much brighter quasar light at the center. Though a quasar might no be much larger than our solar system it releases as much energy as billions of stars.

Though a previous ground based observation using the Canada-France-Hawaii Telescope (at 0.5 arcsec resolution) first identified the barred spiral galaxy in 1229+204, Hubble shows clearly the galaxy's structure and reveals details of the collision.

Hubble reveals that an extended blue feature on one side of the galaxy is really a string of knots, which are probably massive young star clusters. The star clusters were most likely formed as a result of a collision between the host galaxy and a small gas-rich companion. HST also reveals shell-like structures along the bar that might be produced by gravitational tidal resonance forces between the spiral and its companion.






Probing a Quasar's Home


Credit: John Bahcall (Institute for Advanced Study, Princeton) and NASA
Images taken by the Hubble Space Telescope have allowed astronomers to clearly see the link between quasars and their companion galaxies. Some quasars, such as the one in this two-panel image, have been caught in the act of merging or colliding with their companion galaxies.

The image on the left reveals the huge, thin tidal arms of a galaxy associated with the luminous quasar, which is 1.5 billion light-years from Earth. The odd-shaped arms suggest an encounter between the quasar and a companion galaxy. The thick bright line above the quasar is an edge-on background galaxy.

In the right-hand panel, the same image is shown at a different contrast level, which enables astronomers to peer closer into the galaxy's nucleus. Only 11,000 light-years separate the quasar and the companion galaxy (located just above the quasar). This galaxy is similar in size and brightness to the Large Magellenic Cloud galaxy near our Milky Way. The galaxy is closer to the quasar's center than our sun is to the center of our galaxy. The quasar and galaxy are drawn together by strong gravitational forces. Eventually, the galaxy will fall into the quasar's engine, the black hole. Black holes are believed to power the compact, energetic quasars. The black hole will gobble up this companion galaxy in no more than 10 million years.

The quasar in these images appears large, but actually, it is a compact, yet powerful light source. The quasar is so bright that it created diffraction spikes on these telescope images.

The images were taken with the Hubble Space Telescope's Wide Field and Planetary Camera 2.






Quasars: "The Light Fantastic"

Credits: NASA (Press Release)
Quasars have been so elusive and mysterious that the hunt to define them would have taxed even the superior analytical skills of detective Sherlock Holmes.

Since their discovery in 1963, astronomers have been trying to crack the mystery of how these compact dynamos of light, which lie at the outer reaches of the universe, produce so much energy. Quasars are no larger than our solar system but outshine galaxies of hundreds of billions of stars. These light beacons have left trails of evidence and plenty of clues, but scientists have only just begun to understand their behavior.

For centuries, quasars went undetected, appearing in the sky as faint stars. No one suspected anything more. In the 1940s, however, radio scientists discovered that celestial objects emitted radio waves, giving birth to radio astronomy.

Soon astronomers began examining the sky to find as many objects as possible that radiated radio waves and tried to link them to optical objects. Their search led to several radio sources whose positions coincided with blue starlike objects.

Astronomers Allan Sandage and Thomas Matthews attempted to uncloak these mysterious objects in 1960. They were puzzled to find a strange source of radio emission that, in visible light, looked like a faint star. But this object was emitting more intense radio waves and ultraviolet radiation than a typical star.

In 1962 British radio astronomer Cyril Hazard took a crack at solving the mystery. He had an ingenious method of using the moon as a marker to pinpoint a radio source. When the moon would pass in front of a particular radio source (called an occultation), he noted the precise instant the radio signal stopped and then reemerged.

But he almost didn't get the opportunity to record the information. Hazard was at the University of Sydney and had arranged to make the observation at Parkes Radio Telescope, located several hundred miles away in the Australian outback. The night the occultation occurred, Hazard took the wrong train and missed the observation. Luckily, observatory director John Bolton and other astronomers made the observation. But Bolton also had his own problems. He couldn't tip the telescope over far enough to record the observation. So, he cut down a bunch of trees. Then he removed the telescope's safety bolts and tilted the several thousand ton telescope over enough to catch the occultation.

The astronomers observed a radio source that could be traced to a single starlike object, then known as 3C 273, in the constellation Virgo. Although this object looked like an ordinary star, it exhibited peculiar behavior. The object was emitting a tremendous quantity of radio signals. An optical analysis of the object's spectrum was unlike anything previously encountered.

What was it? In 1963 Maarten Schmidt at Mount Palomar observatory deciphered the code. The spectrum contained a few strange wide emission lines, which at first seemed pretty confusing. Schmidt soon realized he was looking at normal spectral lines for hydrogen. The lines, however, were shifted toward the red end of the visible spectrum, making them almost unrecognizable. There was only one explanation: This object was receding away from Earth at almost 30,000 miles per second, which meant that it was 3 billion light-years away. Astronomers quickly dubbed the objects quasi-stellar radio sources.

"Once Maarten Schmidt cracked the safe, all of a sudden things began to fall into place," says John Bahcall, who was a young physics professor at Caltech when Schmidt made his discovery.

Now the hunt was on to define these objects. How were they formed? What fueled them? Do they reside in galaxies? How can such tiny objects only a few light-months across emit so much radiation?

"Hunting for quasars became a major sport," says astronomer Mike Disney, who decided to become an astronomer after reading a story about the discovery of quasars. "It was an exciting thing. Clearly, it was a new physical phenomenon."

Astronomers rushed to discover more of these objects. Since then, thousands of them have been identified. Sandage found quasars that don't emit any radio waves. These "radio quiet" quasars now comprise about 99 percent of the population.

Theories also abounded about the nature of these objects. One theory questioned the definition of red shift, the measure of an object's recession velocity. The farther away an object is, the longer and hence redder the wavelength. Maybe quasars - which had high red shifts because they were far away - weren't so far away. Perhaps red shift is a measure of something else. But most astronomers quickly rejected this theory.

Russian scientist Yakov Zeldovich proposed yet another theory, which astronomers still believe today: Black holes provide the power that turns on quasars. A black hole is created when a giant star collapses to a tiny point of infinite density. Astronomers believe that a quasar turns on when a black hole at a galaxy's core feeds on gas and stars. As matter falls into the black hole, intense radiation is emitted.

Astronomers were making considerable progress towards explaining quasars, but there still were many questions they couldn't answer. Quasars were so bright that they drowned out everything around them. What objects were near quasars?

Did quasars reside in galaxies? Ground-based telescopes didn't give astronomers many clues. Some ground-based images hinted that quasars reside in galaxies, but astronomers couldn't see the surroundings clearly.

Finding the answers to many pressing quasar questions was a major reason for the push for a space telescope. Bahcall was a principal advocate for the telescope, telling Congress in 1978 that "one needs to observe quasars with the space telescope to find out whether or not these bright point-objects, quasars, are surrounded by fainter, more diffuse light of galaxies."

Bahcall was right. Spectacular images from the Hubble Space Telescope have shown that quasars indeed reside in galaxies. But the images have revealed other surprising information: Quasars live in a variety of galaxies. Some galaxies are quite normal, others are colliding with their neighbors.

Although Hubble has yielded more clues, many questions still remain unanswered about these enigmatic objects.

"I'd like to know what turns on a quasar," Bahcall says. "We've got some clues, but not definitive answers."

Disney wants to figure out whether quasars are "light bulbs or lighthouses. We don't know if they are pointing their energy at us or pointing it in all directions. And how long are they turned on? Are they short-lived or long-lived? We're only just getting some ideas to these questions."