Galaxies in Collision


A Grazing Encounter Between Two Spiral Galaxies


Credit: NASA and The Hubble Heritage Team (STScI)
In the direction of the constellation Canis Major, two spiral galaxies pass by each other like majestic ships in the night. The near-collision has been caught in images taken by NASA's Hubble Space Telescope and its Wide Field Planetary Camera 2.

The larger and more massive galaxy is cataloged as NGC 2207 (on the left in the Hubble Heritage image), and the smaller one on the right is IC 2163. Strong tidal forces from NGC 2207 have distorted the shape of IC 2163, flinging out stars and gas into long streamers stretching out a hundred thousand light-years toward the right-hand edge of the image.

Computer simulations, carried out by a team led by Bruce and Debra Elmegreen, demonstrate the leisurely timescale over which galactic collisions occur. In addition to the Hubble images, measurements made with the National Science Foundation's Very Large Array Radio Telescope in New Mexico reveal the motions of the galaxies and aid the reconstruction of the collision.

The calculations indicate that IC 2163 is swinging past NGC 2207 in a counterclockwise direction, having made its closest approach 40 million years ago. However, IC 2163 does not have sufficient energy to escape from the gravitational pull of NGC 2207, and is destined to be pulled back and swing past the larger galaxy again in the future.

The high resolution of the Hubble telescope image reveals dust lanes in the spiral arms of NGC 2207, clearly silhouetted against IC 2163, which is in the background. Hubble also reveals a series of parallel dust filaments extending like fine brush strokes along the tidally stretched material on the right-hand side. The large concentrations of gas and dust in both galaxies may well erupt into regions of active star formation in the near future.

Trapped in their mutual orbit around each other, these two galaxies will continue to distort and disrupt each other. Eventually, billions of years from now, they will merge into a single, more massive galaxy. It is believed that many present-day galaxies, including the Milky Way, were assembled from a similar process of coalescence of smaller galaxies occurring over billions of years.

This image was created from 3 separate pointings of Hubble. The Wide Field Planetary Camera 2 data sets were obtained by Debra Meloy Elmegreen (Vassar College), Bruce G. Elmegreen (IBM Research Division), Michele Kaufman (Ohio State U.), Elias Brinks (Universidad de Guanajuato, Mexico), Curt Struck (Iowa State University), Magnus Thomasson (Onsala Space Obs., Sweden), Maria Sundin (Goteborg University, Sweden), and Mario Klaric (Columbia, South Carolina).






Galaxy Collision on NGC6745


Credit: NASA and The Hubble Heritage Team (STScI/AURA)
Acknowledgement: Roger Lynds (KPNO/NOAO)

A large spiral galaxy, with its nucleus still intact, has collided with a smaller passing galaxy (nearly out of the field of view at lower right) and the bright blue streak across its arms show the distinct path taken during the smaller galaxy's journey. These galaxies did not merely interact gravitationally as they passed one another, they actually collided.

When galaxies collide, the stars that normally comprise the major portion of the luminous mass of each of the two galaxies will almost never collide with each other, but will pass rather freely between each other with little damage. This occurs because the physical size of individual stars is tiny compared to their typical separations, making the chance of physical encounter relatively small. In our own Milky Way galaxy, the space between our Sun and our nearest stellar neighbor, Proxima Centauri (part of the Alpha Centauri triple system), is a vast 4.3 light-years.

However, the situation is quite different for the interstellar media in the above two galaxies - material consisting largely of clouds of atomic and molecular gases and of tiny particles of matter and dust, strongly coupled to the gas. Wherever the interstellar clouds of the two galaxies collide, they do not freely move past each other without interruption but, rather, suffer a damaging collision. High relative velocities cause ram pressures at the surface of contact between the interacting interstellar clouds. This pressure, in turn, produces material densities sufficiently extreme as to trigger star formation through gravitational collapse. The hot blue stars in this image are evidence of this star formation.

This image was created by the Hubble Heritage Team using NASA Hubble Space Telescope archive data taken with the Wide Field Planetary Camera 2 in March 1996. Members of the science team, which include Roger Lynds (KPNO/NOAO) and Earl J. O'Neil, Jr. (Steward Obs.), used infrared, red, visual and ultravoilet filters to image this galaxy system. Lynds and O'Neil are currently using the Hubble data along with ground-based radio observations to further study the interactions within NGC 6745.






Freewheeling Galaxies Collide in a Blaze of Star Birth


Image Credit: NASA and The Hubble Heritage Team (STScI/AURA)
Acknowledgment: M. Donahue (STScI) and J. Trauger (JPL)

A dusty spiral galaxy appears to be rotating on edge, like a pinwheel, as it slides through the larger, bright galaxy NGC 1275, in this NASA Hubble Space Telescope image.

These images, taken with Hubble's Wide Field Planetary Camera 2 (WFPC2), show traces of spiral structure accompanied by dramatic dust lanes and bright blue regions that mark areas of active star formation. Detailed observations of NGC 1275 indicate that the dusty material belongs to a spiral system seen nearly edge-on in the foreground. The second galaxy, lying beyond the first, is actually a giant elliptical with peculiar faint spiral structure in its nucleus. These galaxies are believed to be colliding at over 6 million miles per hour.

NGC 1275 is about 235 million light-years away in the constellation Perseus. Embedded in the center of a large cluster of galaxies known as the Perseus Cluster, it is also known to emit a powerful signal at both X-ray and radio frequencies. The galaxy collision causes the gas and dust already existing in the central bright galaxy to swirl into the center of the object. The X-ray and radio emission indicates the probable existence of a black hole at the bright galaxy's center.

While the dark dusty material in the Hubble image falls inward, NGC 1275 displays intricate filamentary structures at a much larger scale outside the image. This is a typical feature of bright cluster galaxies. Additional observational evidence of strong interactions between at least two galaxies, and possibly a few smaller galaxies, includes the formation of new stars and large star clusters. Although similar in shape to the old globular clusters in the Milky Way galaxy, NGC 1275's clusters are much younger and contain 100,000 to a million stars each.

This image was created from archived blue and red Hubble WFPC2 data taken in 1995 by John Trauger (JPL) and Jon Holtzman (NMSU). The Hubble Heritage team, along with collaborators Megan Donahue, Jennifer Mack, and Mark Voit (STScI), took follow-up WFPC2 observations at infrared wavelengths in 2001 to help produce this full-color image.






An Abrasive Collision Gives One Galaxy a "Black Eye"


Credit: NASA and The Hubble Heritage Team (AURA/STScI)
Acknowledgment: S. Smartt (Institute of Astronomy) and D. Richstone (U. Michigan)
The full image of M64 was taken in January 1997 with the 0.9-meter telescope of the Kitt Peak National Observatory, Arizona.

A collision of two galaxies has left a merged star system with an unusual appearance as well as bizarre internal motions. Messier 64 (M64) has a spectacular dark band of absorbing dust in front of the galaxy's bright nucleus, giving rise to its nicknames of the "Black Eye" or "Evil Eye" galaxy.

Fine details of the dark band are revealed in this image of the central portion of M64 obtained with the Hubble Space Telescope. M64 is well known among amateur astronomers because of its appearance in small telescopes. It was first cataloged in the 18th century by the French astronomer Messier. Located in the northern constellation Coma Berenices, M64 resides roughly 17 million light-years from Earth.

At first glance, M64 appears to be a fairly normal pinwheel-shaped spiral galaxy. As in the majority of galaxies, all of the stars in M64 are rotating in the same direction, clockwise as seen in the Hubble image. However, detailed studies in the 1990's led to the remarkable discovery that the interstellar gas in the outer regions of M64 rotates in the opposite direction from the gas and stars in the inner regions.

Active formation of new stars is occurring in the shear region where the oppositely rotating gases collide, are compressed, and contract. Particularly noticeable in the image are hot, blue young stars that have just formed, along with pink clouds of glowing hydrogen gas that fluoresce when exposed to ultraviolet light from newly formed stars.

Astronomers believe that the oppositely rotating gas arose when M64 absorbed a satellite galaxy that collided with it, perhaps more than one billion years ago. This small galaxy has now been almost completely destroyed, but signs of the collision persist in the backward motion of gas at the outer edge of M64.

This image of M64 was taken with Hubble's Wide Field Planetary Camera 2 (WFPC2). The color image is a composite prepared by the Hubble Heritage Team from pictures taken through four different color filters. These filters isolate blue and near-infrared light, along with red light emitted by hydrogen atoms and green light from Strömgren y.






Giant Interacting Galaxies NGC 6872 and IC 4970


Credit: ESO
This image shows the spectacular barred spiral galaxy NGC 6872 that is shaped like an "integral sign". It is of type SBb and is accompanied by a smaller, interacting galaxy, IC 4970 of type S0 (just above the center). The bright object to the lower right of the galaxies is a star in the Milky Way whose image has been strongly overexposed and exhibits multiple optical reflections in the telescope and instrument. There are also many other, fainter and more distant galaxies of many different forms in the field.

The upper left spiral arm of NGC 6872 is significantly disturbed and is populated by a plethora of bluish objects, many of which are star-forming regions. This may have been be caused by a recent passage of IC 4970 through it.

This interesting system is located in the southern constellation Pavo (The Peacock). It is comparatively distant, almost 300 million light-years away. It extends over more than 7 arcmin in the sky and its real size from tip to tip is thus nearly 750,000 light-years. It is in fact one of the largest known, barred spiral galaxies.

In order to image all of this extraordinary object within the available field of the FORS1 camera, the instrument was rotated so that the galaxy extends along the diagonal. For this reason, the orientation is such that North is to the upper right and East is to the upper left. The image is a three-color composite reproduced from one blue (B), one green-yellow (V) and one red (R) exposure, obtained with FORS1 at ANTU in the morning of March 29, 1999. The field size is 6.8x6.8 arcmin2.






Two Merging Galaxies


Credit: NASA and the ACS Science Team
The Advanced Camera for Surveys (ACS), the newest camera on NASA's Hubble Space Telescope, has captured a spectacular pair of galaxies engaged in a celestial dance of cat and mouse or, in this case, mouse and mouse.

Located 300 million light-years away in the constellation Coma Berenices, the colliding galaxies have been nicknamed "The Mice" because of the long tails of stars and gas emanating from each galaxy. Otherwise known as NGC 4676, the pair will eventually merge into a single giant galaxy.

The image shows the most detail and the most stars that have ever been seen in these galaxies. In the galaxy at left, the bright blue patch is resolved into a vigorous cascade of clusters and associations of young, hot blue stars, whose formation has been triggered by the tidal forces of the gravitational interaction. Streams of material can also be seen flowing between the two galaxies.

The clumps of young stars in the long, straight tidal tail [upper right] are separated by fainter regions of material. These dim regions suggest that the clumps of stars have formed from the gravitational collapse of the gas and dust that once occupied those areas. Some of the clumps have luminous masses comparable to dwarf galaxies that orbit in the halo of our own Milky Way Galaxy.

Computer simulations by astronomers Josh Barnes (University of Hawaii) and John Hibbard (National Radio Astronomy Observatory, Charlottesville, Va.) show that we are seeing two nearly identical spiral galaxies approximately 160 million years after their closest encounter. The long, straight arm is actually curved, but appears straight because we see it edge-on. The simulations also show that the pair will eventually merge, forming a large, nearly spherical galaxy (known as an elliptical galaxy). The stars, gas, and luminous clumps of stars in the tidal tails will either fall back into the merged galaxies or orbit in the halo of the newly formed elliptical galaxy.

The Mice presage what may happen to our own Milky Way several billion years from now when it collides with our nearest large neighbor, the Andromeda Galaxy (M31).

This picture is assembled from three sets of images taken on April 7, 2002, in blue, orange, and near-infrared filters.






Star Clusters Born in the Wreckage of Cosmic Collisions


Image Credits: NASA, Jayanne English (University of Manitoba), Sally Hunsberger (Pennsylvania State University), Zolt Levay (Space Telescope Science Institute), Sarah Gallagher (Pennsylvania State University), and Jane Charlton (Pennsylvania State University)
Science Credits: Sarah Gallagher (Pennsylvania State University), Jane Charlton (Pennsylvania State University), Sally Hunsberger (Pennsylvania State University), Dennis Zaritsky (University of Arizona), and Bradley Whitmore (Space Telescope Science Institute)

This close-up view of Stephan's Quintet, a group of five galaxies, reveals a string of bright star clusters that sparkles like a diamond necklace. The clusters, each harboring up to millions of stars, were born from the violent interactions between some members of the group. The rude encounters also have distorted the galaxies' shapes, creating elongated spiral arms and long, gaseous streamers.

The NASA Hubble Space Telescope photo showcases three regions of star birth: the long, sweeping tail and spiral arms of NGC 7319 [near center]; the gaseous debris of two galaxies, NGC 7318B and NGC 7318A [top right]; and the area north of those galaxies, dubbed the northern starburst region [top left].

The clusters' bluish color indicates that they're relatively young. Their ages span from about 2 million to more than 1 billion years old.

The brilliant star clusters in NGC 7318B's spiral arm (about 30,000 light-years long) and the northern starburst region are between 2 million and more than 100 million years old. NGC 7318B instigated the starburst by barreling through the region. The bully galaxy is just below NGC 7318A at top right. Although NGC 7318B appears dangerously close to NGC 7318A, it's traveling too fast to merge with its close neighbor. The partial galaxy on the far right is NGC 7320, a foreground galaxy not physically bound to the other galaxies in the picture.

About 20 to 50 of the clusters in the northern starburst region reside far from the coziness of galaxies. The clusters were born about 150,000 light-years from the nearest galaxy.

A galaxy that is no longer part of the group triggered another collision that wreaked havoc. NGC 7320C [not in the photo] plowed through the quintet several hundred million years ago, pulling out the 100,000 light-year-long tail of gaseous debris from NGC 7319. The clusters in NGC 7319's streaming tail are 10 million to 500 million years old and may have formed at the time of the violent collision. The faint bluish object at the tip of the tail is a young dwarf galaxy, which formed in the gaseous debris.

The quintet is in the constellation Pegasus, 270 million light-years from Earth. Spied by Edouard M. Stephan in 1877, Stephan's Quintet is the first compact group ever discovered.

The mosaic picture was taken by Hubble's Wide Field and Planetary Camera 2 on Dec. 30, 1998 and June 17, 1999.






Stellar Fireworks Accompanying Galaxy Collision


Credit: Brad Whitmore (STScI), and NASA
This Hubble Space Telescope image provides a detailed look at a brilliant "fireworks show" at the center of a collision between two galaxies. Hubble has uncovered over 1,000 bright, young star clusters bursting to life as a result of the head-on wreck.

A ground-based telescopic view of the Antennae galaxies (known formally as NGC 4038/4039) - so named because a pair of long tails of luminous matter, formed by the gravitational tidal forces of their encounter, resembles an insect's antennae. The galaxies are located 63 million light-years away in the southern constellation Corvus.

The respective cores of the twin galaxies are the orange blobs, left and right of image center, crisscrossed by filaments of dark dust. A wide band of chaotic dust, called the overlap region, stretches between the cores of the two galaxies. The sweeping spiral-like patterns, traced by bright blue star clusters, shows the result of a firestorm of star birth activity which was triggered by the collision.

This natural-color image is a composite of four separately filtered images taken with the Wide Field Planetary Camera 2 (WFPC2), on January 20, 1996. Resolution is 15 light-years per pixel (picture element).






Close-up of Star Formation in Antennae Galaxy


Credit: Brad Whitmore (STScI), and NASA
These four close-up views are taken from a head-on collision between two spiral galaxies, called the Antennae galaxies, seen at image center. The scale bar at the top of each image is 1,500 light-years across.

[Left images]
The collision triggers the birth of new stars in brilliant blue star clusters, the brightest of which contains roughly a million stars. The star clusters are blue because they are very young, the youngest being only a few million years old, a mere blink of the eye on the astronomical time scale.

[Right images]
These close-up views of the cores of each galaxy show entrapped dust and gas funneled into the center. The nucleus of NGC 4038 (lower right) is obscured by dust which dims and reddens starlight by scattering the shorter, bluer wavelengths. This is also the reason the young star clusters in the dusty regions appear red instead of blue.

This natural-color image is a composite of four separately filtered images taken with the Wide Field Planetary Camera 2 (WFPC2), on January 20, 1996. Resolution is 15 light-years per pixel (picture element).






Intergalactic 'Pipeline' Funnels Matter Between Colliding Galaxies


Credits: NASA, William C. Keel (University of Alabama, Tuscaloosa)
This visible-light picture, taken by NASA's Hubble Space Telescope, reveals an intergalactic "pipeline" of material flowing between two battered galaxies that bumped into each other about 100 million years ago.

The pipeline [the dark string of matter] begins in NGC 1410 [the galaxy at left], crosses over 20,000 light-years of intergalactic space, and wraps around NGC 1409 [the companion galaxy at right] like a ribbon around a package.

Although astronomers have taken many stunning pictures of galaxies slamming into each other, this image represents the clearest view of how some interacting galaxies dump material onto their companions. These results are being presented today at the 197th meeting of the American Astronomical Society in San Diego, CA.

Astronomers used the Space Telescope Imaging Spectrograph to confirm that the pipeline is a continuous string of material linking both galaxies.

Scientists believe that the tussle between these compact galaxies somehow created the pipeline, but they're not certain why NGC 1409 was the one to begin gravitationally siphoning material from its partner. And they don't know where the pipeline begins in NGC 1410. More perplexing to astronomers is that NGC 1409 is seemingly unaware that it is gobbling up a steady flow of material. A stream of matter funneling into the galaxy should have fueled a spate of star birth. But astronomers don't see it. They speculate that the gas flowing into NGC 1409 is too hot to gravitationally collapse and form stars.

Astronomers also believe that the pipeline itself may contribute to the star-forming draught. The pipeline, a pencil-thin, 500 light-year-wide string of material, is moving a mere 0.02 solar masses of matter a year.

Astronomers estimate that NGC 1409 has consumed only about a million solar masses of gas and dust, which is not enough material to spawn some of the star-forming regions seen in our Milky Way. The low amount means that there may not be enough material to ignite star birth in NGC 1409, either.

The glancing blow between the galaxies was enough, however, to toss stars deep into space and ignite a rash of star birth in NGC 1410. The arms of NGC 1410, an active, gas-rich spiral galaxy classified as a Seyfert, are awash in blue, the signature color of star-forming regions. The bar of material bisecting the center of NGC 1409 also is a typical byproduct of galaxy collisions.

Astronomers expect more fireworks to come. The galaxies are doomed to continue their game of "bumper cars," hitting each other and moving apart several times until finally merging in another 200 million years. The galaxies' centers are only 23,000 light-years apart, which is slightly less than Earth's distance from the center of the Milky Way. They are bound together by gravity, orbiting each other at 670,000 miles an hour (1 million kilometers an hour). The galaxies reside about 300 million light-years from Earth in the constellation Taurus.

The Hubble picture was taken Oct. 25, 1999.






Faraway Galaxies Provide a Stunning "Wallpaper" Backdrop for a Runaway Galaxy


Credit: NASA and the ACS Science Team
Against a stunning backdrop of thousands of galaxies, this odd-looking galaxy with the long streamer of stars appears to be racing through space, like a runaway pinwheel firework.

This picture of the galaxy UGC 10214 was taken by the Advanced Camera for Surveys (ACS), which was installed aboard NASA's Hubble Space Telescope in March during Servicing Mission 3B. Dubbed the "Tadpole," this spiral galaxy is unlike the textbook images of stately galaxies. Its distorted shape was caused by a small interloper, a very blue, compact galaxy visible in the upper left corner of the more massive Tadpole. The Tadpole resides about 420 million light-years away in the constellation Draco.

Seen shining through the Tadpole's disk, the tiny intruder is likely a hit-and-run galaxy that is now leaving the scene of the accident. Strong gravitational forces from the interaction created the long tail of debris, consisting of stars and gas that stretch out more than 280,000 light-years.

Numerous young blue stars and star clusters, spawned by the galaxy collision, are seen in the spiral arms, as well as in the long "tidal" tail of stars. Each of these clusters represents the formation of up to about a million stars. Their color is blue because they contain very massive stars, which are 10 times hotter and 1 million times brighter than our Sun. Once formed, the star clusters become redder with age as the most massive and bluest stars exhaust their fuel and burn out. These clusters will eventually become old globular clusters similar to those found in essentially all halos of galaxies, including our own Milky Way.

Two prominent clumps of young bright blue stars in the long tail are separated by a "gap" — a section that is fainter than the rest of the tail. These clumps of stars will likely become dwarf galaxies that orbit in the Tadpole's halo.

The galactic carnage and torrent of star birth are playing out against a spectacular backdrop: a "wallpaper pattern" of 6,000 galaxies. These galaxies represent twice the number of those discovered in the legendary Hubble Deep Field, the orbiting observatory's "deepest" view of the heavens, taken in 1995 by the Wide Field and Planetary Camera 2. The ACS picture, however, was taken in one-twelfth the time it took to observe the original Hubble Deep Field. In blue light, ACS sees even fainter objects than were seen in the "deep field." The galaxies in the ACS picture, like those in the deep field, stretch back to nearly the beginning of time. They are a myriad of shapes and represent fossil samples of the universe's 13-billion-year evolution.

The ACS image is so sharp that astronomers can identify distant colliding galaxies, the "building blocks" of galaxies, an exquisite "Whitman's Sampler" of galaxies, and many extremely faraway galaxies.

ACS made this observation on April 1 and 9, 2002. The color image is constructed from three separate images taken in near-infrared, orange, and blue filters.






Huge Clusters of Stars Formed by Ancient Encounter


Credits for Hubble image: NASA, ESA, R. de Grijs (Institute of Astronomy, Cambridge, UK)
Credits for ground-based picture: N.A. Sharp (Association of Universities for Research in Astronomy, National Optical Astronomy Observatories, National Science Foundation)

This stunningly beautiful image [right] taken with the NASA Hubble Space Telescope shows the heart of the prototypical starburst galaxy M82. The ongoing violent star formation due to an ancient encounter with its large galactic neighbor, M81, gives this galaxy its disturbed appearance.

The smaller picture at upper left shows the entire galaxy. The image was taken in December 1994 by the Kitt Peak National Observatory's 0.9-meter telescope. Hubble's view is represented by the white outline in the center.

In the Hubble image, taken by the Wide Field and Planetary Camera 2, the huge lanes of dust that crisscross M82's disk are another telltale sign of the flurry of star formation. Below the center and to the right, a strong galactic wind is spewing knotty filaments of hydrogen and nitrogen gas.

More than 100 super star clusters -- very bright, compact groupings of about 100,000 stars -- are seen in this detailed Hubble picture as white dots sprinkled throughout M82's central region. The dark region just above the center of the picture is a huge dust cloud. A collaboration of European and American scientists used these clusters to date the ancient interaction between M82 and M81. About 600 million years ago, a region called "M82 B" (the bright area just below and to the left of the central dust cloud) exploded with new stars. Scientists have discovered that this ancient starburst was triggered by the violent encounter with M81.

M82 is a bright (eighth magnitude), nearby (12 million light-years from Earth) galaxy in the constellation Ursa Major (the Great Bear).

The Hubble picture was taken Sept. 15, 1997. The natural-color composite was constructed from three Wide Field and Planetary Camera 2 exposures, which were combined in chromatic order: 4,250 seconds through a blue filter (428 nm); 2,800 seconds through a green filter (520 nm); and 2,200 seconds through a red (820 nm) filter.






Hubble Finds More Evidence of Galactic Cannibalism in NGC 1316


Credit: Carl Grillmair (California Institute of Technology) and NASA
This beautiful, eerie silhouette of dark dust clouds against the glowing nucleus of the elliptical galaxy NGC 1316 may represent the aftermath of a 100 million year old cosmic collision between the elliptical and a smaller companion galaxy.

A number of faint objects are scattered across the image, including both reddish galaxies in the distant background and bluer, point-like star clusters orbiting NGC 1316. These clusters, relatively loosely-knit swarms containing a few thousand stars each, are smaller and fainter than those found in other elliptical galaxies. These clusters are too old to have been created in the collision which produced the dusty debris we see today, and too young to have been torn apart by galactic tidal forces. The clusters may have been born in the course of a still earlier collision, or belonged to the galaxy which most recently fell victim to NGC 1316.

The picture was taken in April of 1996 with the Wide Field Planetary Camera 2. The color rendition was constructed using separate images taken in blue and red light. NGC 1316 is located 53 million light-years away in the constellation Fornax. The field of view shown is about 12,000 light-years across.






A Collision in the Heart of a Galaxy


Credit: Rodger Thompson, Marcia Rieke, Glenn Schneider (University of Arizona) and Nick Scoville (California Institute of Technology), and NASA
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.






NGC 5090 and NGC 5091


Credit: ESO
This pair of galaxies [close-up below], NGC 5090 and 5091 in the southern constellation Centaurus, is an interacting elliptical-spiral system with some evidence of tidal disruption of NGC 5091 (left) by NGC 5090 (right). The velocity of the nucleus of NGC 5091 has been measured as 3429 km/sec, while NGC 5090 has a velocity of 3185 km/sec. NGC 5090 is associated with a strong, double radio source (PKS 1318-43).

NGC 5090 is a typical S0 galaxy with a bright diffuse center, surrounded by a fainter envelope of stars (not resolved in this picture). However, some of the starlike objects seen in this region may be globular clusters (or dwarf galaxies) in orbit around NGC 5090. NGC 5091 is of type Sa (the spiral structure is more developed) and is seen at a steep angle.







A Galaxy's Fatal Plunge


Credit: NASA, W. Keel (U Alabama), F. Owen (NRAO), M. Ledlow (Gemini Obs.), and D. Wang (U Mass.)
These images offer a dramatic look at a spiral galaxy like our Milky Way being ripped apart as it races at 4.5 million miles per hour through the heart of a distant cluster of galaxies. The images, taken over several wavelengths, provide evidence of the "galactic assault and battery," namely, gas being stripped from the doomed galaxy, called C153.

The composite photograph at left was made by combining the four images at right, taken in X-ray, radio, and visible wavelengths as well as the green light of oxygen gas. Astronomers studied the galaxy across several wavelengths to trace how stars, gas, and dust are being tossed around and torn from the fragile galaxy.

The composite image at left shows long streamers of gas flowing from the galaxy as it travels through the cluster, called Abell 2125. Hot gas from the cluster is stretching the galaxy's cooler gas into long streamers. Velocity measurements of several hundred galaxies in the cluster indicate that C153 is moving away from Earth within the cluster. The image, therefore, shows the streaking tail of "stripped gas." The image spans about 1 million light-years across. Abell 2125 is about 3 billion light-years away, on the boundary between the constellations Ursa Minor and Draco.

The visible-light image [above, right], taken by the Wide Field Planetary Camera 2 aboard NASA's Hubble Space Telescope, reveals intricate detail in the structure of stars and dust within C153. The galaxy exhibits evidence of a large-scale disturbance that has left its star-forming regions concentrated to one side of its disk and beyond. Dust features are twisted into chaotic patterns, obscuring any spiral pattern the galaxy once had.

X-ray emission [above, right], imaged by the Chandra X-ray Observatory, shows a gas tail extending from C153, which roughly matches the glowing gas tail observed in visible light. The temperature of the gas tail is cooler than the surrounding gas. This temperature difference is further evidence that gas is being "stripped" from the galaxy. The hotter gas is so diffuse that it cannot be seen in the image.

Radio observations [below, right] depict high-energy particles as they spiral through the galaxy's magnetic field, with some escaping in a perpendicular direction to the galaxy's disk. The particles probably came from an energetic black hole that was fueled by a collision between two galaxy clusters. This emission first marked C153 as unusual, leading scientists to conduct further observations.

The image [below, right], taken by the 4-meter Mayall telescope at Kitt Peak National Observatory, isolated the light from glowing oxygen gas. This view shows a tail forming as gas is pulled from the galaxy and stretched into long streamers that extends for about 200,000 light-years.