Cosmic Fossil

NGC 1132 A Mysterious Elliptical. Credit: Chanda

The NASA/ESA Hubble Space Telescope has captured a new image of the galaxy NGC 1132 which is, most likely, a cosmic fossil - the aftermath of an enormous multi-galactic pile-up, where the carnage of collision after collision has built up a brilliant but fuzzy giant elliptical galaxy far outshining typical galaxies.

The elliptical galaxy NGC 1132, seen in this latest image from Hubble, belongs to a category of galaxies called giant ellipticals. NGC 1132, together with the small dwarf galaxies surrounding it, are dubbed a "fossil group" as they are most likely the remains of a group of galaxies that merged together in the recent past.

In visible light NGC 1132 appears as a single, isolated, giant elliptical galaxy, but this is only the tip of the iceberg. Scientists have found that NGC 1132 resides in an enormous halo of dark matter, comparable to the amount of dark matter usually found in an entire group of tens or hundreds of galaxies.

It also has a strong X-ray glow from an abundant amount of hot gas - an amount normally only found in galaxy groups. Its X-ray glow extends over a region of space ten times larger than the 120,000 light-year radius it has in visible light. An X-ray glow that is equal in size to that of an entire group of galaxies.

The origin of fossil group systems remains a puzzle. The most likely explanation is that they are the end-products of a cosmic feeding frenzy in which a large cannibal galaxy devours all of its neighbours. A less likely explanation is that they may be very rare objects that formed in a region or period of time where the growth of moderate-sized galaxies was somehow suppressed, and only one large galaxy formed.

Many galaxies reside in groups that are gravitationally bound together, including our own Milky Way, which is part of the Local Group. Sometimes gravity makes galaxies collide and eventually merge into one single galaxy. There is strong evidence that the Milky Way is one such cannibal that has snacked on numerous smaller galaxies during its lifetime, inheriting their stars in the process.

Scientists are keenly studying the environment surrounding galaxies such as NGC 1132 using space telescopes like Hubble, and they try to trace the history of the formation these galaxies by analysing their properties.

In this Hubble image, NGC 1132 is seen surrounded by thousands of ancient globular clusters, swarming around the galaxy like bees around a hive. These globular clusters are likely to be the survivors of the disruption of their cannibalised parent galaxies that have been eaten by NGC 1132 and may reveal its merger history. In the background, there is a stunning tapestry of numerous galaxies that are much further away.

Elliptical galaxies are smooth and featureless. They contain hundreds of millions to trillions of stars, and their shapes range from nearly spherical to very elongated in shape. Their overall yellowish colour is a telltale sign of their great age. Because elliptical galaxies do not contain much cool gas they can no longer make new stars.

NGC 1132 is located approximately 320 million light-years away in the constellation of Eridanus, the River. This image of NGC 1132 was taken with Hubble's Advanced Camera for Surveys. Data obtained in 2005 and 2006 through green and near-infrared filters were used to produce a colour composite.
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Cosmic Fingers

Click on Image to Enlarge. Credit: LiveScience

A giant gas finger emanating from two neighbouring galaxies has hooked into the starry disk of the Milky Way and is pulling all three galaxies closer. The hydrogen gas is actually the pointy end of the so-called Leading Arm of gas that streams ahead of two irregular galaxies called the Large and Small Magellanic Clouds.

The fate of these nearby galaxies, which are impacted by the Milky Way's gravity, has been somewhat of a mystery. The new findings suggest that the Magellanic Clouds will eventually merge with the Milky Way rather than zooming past.

Located about 160,000 light-years from Earth, the Large Magellanic Cloud (LMC) is only one-twentieth the diameter of our galaxy and contains one-tenth as many stars. The Small Magellanic Cloud resides 200,000 light-years from Earth and is about 100 times smaller than the Milky Way.

"We can determine exactly where this gas is plowing into the Milky Way," said research team leader Naomi McClure-Griffiths of CSIRO's Australia Telescope National Facility.

Called HVC306-2+230, the gas finger is gouging into our galaxy's starry disk about 70,000 light-years away from Earth. In the night sky, the contact point would be nearest the Southern Cross.

Until last year, astronomers thought the Magellanic Clouds had orbited our galaxy many times. This scenario held a gloomy outlook for the clouds, which were said to be doomed to be ripped apart and swallowed by the gravitational goliath.

But then new Hubble Space Telescope measurements revealed the clouds are paying our galaxy a one-time visit rather than being its lunch.

McClure-Griffiths' results, however, are more in line with the previous tale pegging the Milky Way and the Magellanic Clouds as long-time companions. McClure-Griffiths remarks that this isn't the final word and that both theories are still on the table.

By pointing out the spot of contact between the Leading Arm and our galactic disk, the recent study will help astronomers to predict where the clouds themselves will travel in the future.

"We think the Leading Arm is a tidal feature, gas pulled out of the Magellanic Clouds by the Milky Way's gravity," McClure-Griffiths said. "Where this gas goes, we'd expect the clouds to follow, at least approximately."

In the distant future, the three galaxies could become one.
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Cosmic Interactions

Credit: ESA Cosmic Interactions NGC 7173, 7174 and 7176

ESO's Very Large Telescope (VLT) images triplet of dancing galaxies intertwined in a cosmic dance.

The three galaxies: NGC 7173 (top), 7174 (bottom right) and 7176 (bottom left), are located 106 million light-years away towards the constellation of Piscis Austrinus (the 'Southern Fish').

NGC 7173 and 7176 are elliptical galaxies, while NGC 7174 is a spiral galaxy with quite disturbed dust lanes and a long, twisted tail.

This seems to indicate that the two bottom galaxies are currently interacting, with NGC 7176 providing fresh material to NGC 7174. Matter present in great quantity around the triplet's members also points to the fact that NGC 7176 and NGC 7173 have interacted in the past.

Astronomers have suggested that The three galaxies will finally merge into a giant 'island universe', tens to hundreds of times as massive as our own Milky Way.

The triplet is part of a so-called 'Compact Group', as compiled by Canadian astronomer Paul Hickson in the early 1980s. The group, which is the 90th entry in the catalogue and is therefore known as HCG 90, actually contains four major members. One of them - NGC 7192 - lies above the trio, outside of this image, and is another peculiar spiral galaxy.

Compact groups are small, relatively isolated, systems of typically four to ten galaxies in close proximity to one another. Another striking example is Robert's Quartet. Compact groups are excellent laboratories for the study of galaxy interactions and their effects, in particular the formation of stars.

As the striking image reveals, there are many other galaxies in the field. Some are distant ones, while others seem to be part of the family. Studies made with other telescopes have indeed revealed that the HCG 90 group contains 16 members, most of them much smaller in size than the four members with an entry in the NGC catalogue.
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Orphaned Star Clusters

Hubble: Orphaned Star Clusters - Click on Image to Enlarge

[LEFT] A GALEX ultraviolet image of the interacting galaxies M81 and M82, which lie 12 million light-years away in the constellation Ursa Major. The gravity from each galaxy dramatically affected the other during their last close encounter, 200 million years ago.

Gas density waves rippling around M81 make it a grand design spiral. M82 is undergoing a starburst at its core, creating glowing fingers of hydrogen.

[RIGHT] A Hubble Space Telescope visible light image of bright blue star clusters found along a wispy bridge of gas that was tidally stretched between the two galaxies, and a third companion galaxy not seen in this picture. This is not the place astronomers expect to find star clusters because the density of gas is so low. Turbulence in the gas may have enhanced the density locally to trigger starbirth.

The "blue blobs" are clumped together in a structure called Arp's Loop. Hubble reveals the clusters contain the equivalent of five Orion Nebulae. A Hubble plot of the stellar population in the clusters yields an age of approximately 200 million years, which coincides with the epoch of the collision.
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The Cosmic Bird

Credit: ESO Press Photo 54a/07

Using ESO's Very Large Telescope, an international team of astronomers has discovered a stunning rare case of a triple merger of galaxies. This system, which astronomers have dubbed 'The Bird' - composed of two massive spiral galaxies and a third irregular galaxy.

The galaxy ESO 593-IG 008, or IRAS 19115-2124, was previously merely known as an interacting pair of galaxies at a distance of 650 million light-years. But surprises were revealed by observations made with the NACO instrument attached to ESO's VLT, which peered through the all-pervasive dust clouds, using adaptive optics to resolve the finest details.

Underneath the chaotic appearance of the optical Hubble images - retrieved from the Hubble Space Telescope archive - the NACO images show two unmistakable galaxies, one a barred spiral while the other is more irregular.

The surprise lay in the clear identification of a third, clearly separate component, an irregular, yet fairly massive galaxy that seems to be forming stars at a frantic rate.

Because of the resemblance of the system to a bird, the object was dubbed as such, with the 'head' being the third component, and the 'heart' and 'body' making the two major galaxy nuclei in-between of tidal tails, the 'wings'. The latter extend more than 100,000 light-years, or the size of our own Milky Way.

Subsequent optical spectroscopy with the new Southern African Large Telescope, and archive mid-infrared data from the NASA Spitzer space observatory, confirmed the separate nature of the 'head', but also added further surprises. The 'head' and major parts of the 'Bird' are moving apart at more than 400 km/s (1.4 million km/h!). Observing such high velocities is very rare in merging galaxies. Also, the 'head' appears to be the major source of infrared luminosity in the system, though it is the smallest of the three galaxies.

The 'head' is forming stars violently, at a rate of nearly 200 solar masses per year, while the other two galaxies appear to be at a more quiescent epoch of their interaction-induced star formation history.

The 'Bird' belongs to the family of luminous infrared galaxies, with an infrared luminosity nearly one thousand billion times that of the Sun. This family of galaxies has long been thought to signpost important events in galaxy evolution, such as mergers of galaxies, which in turn trigger bursts of star formation, and may eventually lead to the formation of a single elliptical galaxy.
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Hubblesite presents - Striking Galaxy Encounters
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Death Star Galaxy

Credit: NASA, ESA, and D. Evans (Harvard-Smithsonian Center for Astrophysics)

A powerful jet from a supermassive black hole is blasting a nearby galaxy, according to new data from NASA observatories. This never-before witnessed galactic violence may have a profound effect on planets in the jet's path and trigger a burst of star formation in its destructive wake.

Known as 3C 321, the system contains two galaxies in orbit around each other. Data from NASA's Chandra X-ray Observatory show both galaxies contain supermassive black holes at their centers, but the larger galaxy has a jet emanating from the vicinity of its black hole. The smaller galaxy apparently has swung into the path of this jet.

This "death star galaxy" was discovered through the combined efforts of both space and ground-based telescopes. NASA's Chandra X-ray Observatory, Hubble Space Telescope, and Spitzer Space Telescope were part of the effort. The Very Large Array (VLA) in Socorro, N.M., and the Multi-Element Radio Linked Interferometer Network (MERLIN) telescopes in the United Kingdom also were needed for the finding.

Jets from supermassive black holes produce high amounts of radiation, especially high-energy X-rays and gamma-rays, which can be lethal in large quantities. The combined effects of this radiation and particles traveling at almost the speed of light could severely damage the atmospheres of planets lying in the path of the jet. For example, protective layers of ozone in the upper atmosphere of planets could be destroyed.

Jets produced by supermassive black holes transport enormous amounts of energy far from the black holes and enable them to affect matter on scales vastly larger than the size of the black hole. Learning more about jets is a key goal for astrophysical research.

The effect of the jet on the companion galaxy is likely to be substantial, because the galaxies in 3C 321 are extremely close at a distance of only about 20,000 light-years apart, approximately the same distance as Earth is from the center of the Milky Way galaxy.

A bright spot in the VLA and MERLIN images shows where the jet has struck the side of the galaxy, dissipating some of the jet's energy. The collision disrupted and deflected the jet.

Another unique aspect of the discovery in 3C 321 is how relatively short-lived this event is on a cosmic time scale. Features seen in the VLA and Chandra images indicate that the jet began impacting the galaxy about one million years ago, a small fraction of the system's lifetime. This means that such an alignment is quite rare in the nearby universe, making 3C 321 an important opportunity to study such a phenomenon.

It is possible the event is not all bad news for the galaxy being struck by the jet. The massive influx of energy and radiation from the jet could induce the formation of large numbers of stars and planets after its initial wake of destruction is complete.

For more images and information about 3C 321, visit:
http://chandra.harvard.edu
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The Closest Galaxy

The Closest Galaxy: Canis Major Dwarf. - Illustration Credit & Copyright: R. Ibata (Strasbourg Observatory, ULP) et al., 2MASS, NASA

What is the closest galaxy to the Milky Way? The new answer to this old question is the Canis Major dwarf galaxy. For many years astronomers thought the Large Magellan Cloud (LMC) was closest, but its title was supplanted in 1994 by the Sagittarius dwarf galaxy.

Recent measurements indicate that the Canis Major dwarf is only 42,000 light years from the Galactic center, about three quarters of the distance to the Sagittarius dwarf and a quarter of the distance to the LMC. The discovery was made in data from the 2MASS-sky survey, where infrared light allows a better view through our optically opaque Galactic plane.

The labeled illustration above shows the location of the newly discovered Canis Major dwarf and its associated tidal stream of material in relation to our Milky Way Galaxy. The Canis Major dwarf and other satellite galaxies are slowly being gravitationally ripped apart as they travel around and through our Galaxy
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Astronomers Discover Stars With Carbon Atmospheres from Space Daily
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Astronomers Observe Acidic Milky Way Galaxies from Science Daily
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Weighing the Universe's Mass

Spiral Galaxy NGC1232 Credit: ESO.

This image of the large spiral galaxy NGC 1232 was obtained with the Very Large Telescope (VLT). Its distance from Earth is about 100 million light-years. It is thought to contain more dark matter than visible matter.

The same University of Alabama (UAH) group that in 2002 found what was theorized to be a significant fraction of the "missing mass" that binds together the universe has discovered that some x-rays thought to come from intergalactic clouds of "warm" gas are instead probably caused by lightweight electrons.

If the source of so much x-ray energy is tiny electrons instead of hefty atoms, it is as if billions of lights thought to come from billions of aircraft carriers were found instead to come from billions of extremely bright fireflies.

"This means the mass of these x-ray emitting clouds is much less than we initially thought it was," said Dr. Max Bonamente, an assistant professor in UAH's Physics Department. "A significant portion of what we thought was missing mass turns out to be these 'relativistic' electrons."

Travelling at almost the speed of light (and therefore "relativistic"), these feather weight electrons collide with photons from the cosmic microwave background. Energy from the collisions converts the photons from low-energy microwaves to high-energy x-rays.

In 2002 the UAH team reported finding large amounts of extra "soft" (relatively low-energy) x-rays coming from the vast space in the middle of galaxy clusters. This was in addition to previously-discovered "hot" gas in that space, which emits higher energy "hard" x-rays.

Although the soft x-ray-emitting atoms were thought to be spread thinly through space (less than one atom per cubit meter), they would have filled billions of billions of cubic light years. Their cumulative mass was though to account for as much as ten percent of the mass and gravity needed to hold together galaxies, galaxy clusters and perhaps the universe itself.

When Bonamente and his associates looked at data gathered by several satellite instruments, including the Chandra X-ray Observatory, from a galaxy cluster in the southern sky, however, they found that energy from those additional soft x-rays doesn't look like it should.

"We have never been able to detect spectral emission lines associated with those detections," he explained. "If this 'bump' in the data were due to cooler gas, it would have emission lines."

The best, most logical explanation seems to be that a large fraction of the energy comes from electrons smashing into photons instead of from warm atoms and ions, which would have recognizable spectral emission lines. Finding these electrons, however, is like finding "the tip of the iceberg," said Bonamente, because they would not be limited to emitting only the soft x-ray signal. The signal from these electrons would also make up part of the previously observed harder X-rays, which would reduce the amount of mass thought to make up the hot gas at the center of galaxy clusters.

The energy from these electrons might also "puff up" the cluster. Previously, astrophysicists used the energy coming from inside these clusters to calculate how much mass is needed to reach the equilibrium seen there; too much mass and the cloud would collapse; too little and the hot gas cloud would expand. Since the energy coming from these hot clouds can be accurately measured, it was thought the mass could be calculated with reasonable confidence.

Instead, says Bonamente, if a significant portion of the total x-ray energy comes from fast electrons, "that could trick us into thinking there is more gas than is actually there." It means we need to revise how we calculate both the gas mass and the total mass. If part of the hard x-ray energy comes from electrons and photons, it might also shift what we think is the mix of elements in the universe.

Outside of the excess soft x-rays, the x-ray energy coming from galaxy clusters has emission lines which are especially prominent around iron and other metals. Non-thermal x-rays from electrons colliding with photons might mask those emission lines, like thick snow can mask the height of fence posts. "This is also telling us there is fractionally more iron and other metals than we previously thought," said Bonamente. "Less mass but more metals."
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Missing Mass Theory Revised from Centauri Dreams
Big Chunk Of The Universe Is Missing - Again from Science Daily
Dark Matter & visible Matter in Galaxies from Life in the Universe
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Dark 'Black Eye' Galaxy

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" galaxy.

Fine details of the dark band are revealed in this image of the central portion of M64 obtained with the Hubble Space Telescope. First cataloged in the 18th century by the French astronomer Messier, M64 is located in the northern constellation Coma Berenices, and 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 colour image is a composite from pictures taken through four different colour filters. These filters isolate blue and near-infrared light, along with red light emitted by hydrogen atoms and green light from Strömgren y.
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Modified Gravity in the absence of Dark Matter
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Dancing With The Stars

Arp 87. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

Two galaxies perform an intricate dance in this new Hubble Space Telescope image. The galaxies, containing a vast number of stars, swing past each other in a graceful performance choreographed by gravity.

The pair, known collectively as Arp 87, is one of hundreds of interacting and merging galaxies known in our nearby universe.
Arp 87 was originally cataloged by astronomer Halton Arp in the mid 1960s. Arp's Atlas of Peculiar Galaxies is a compilation of astronomical photographs using the Palomar 200-inch Hale and the 48-inch Samuel Oschin telescopes.

The resolution in the Hubble image shows exquisite detail and fine structure that was not observable when Arp 87 was first cataloged in the 1960s.

The two main players comprising Arp 87 are NGC 3808 on the right (the larger of the two galaxies) and its companion NGC 3808A on the left. NGC 3808 is a nearly face-on spiral galaxy with a bright ring of star formation and several prominent dust arms. Stars, gas, and dust flow from NGC 3808, forming an enveloping arm around its companion. NGC 3808A is a spiral galaxy seen edge-on and is surrounded by a rotating ring that contains stars and interstellar gas clouds. The ring is situated perpendicular to the plane of the host galaxy disk and is called a "polar ring."

As seen in other mergers similar to Arp 87, the corkscrew shape of the tidal material or bridge of shared matter between the two galaxies suggests that some stars and gas drawn from the larger galaxy have been caught in the gravitational pull of the smaller one. The shapes of both galaxies have been distorted by their gravitational interaction with one another.

Interacting galaxies often exhibit high rates of star formation. Many lines of evidence - colours of their starlight, intensity of emission lines from interstellar gas, far-infrared output from heated interstellar dust - support this fact. Some merging galaxies have the highest levels of star formation we can find anywhere in the nearby universe.

A major aspect of this excess star formation could be properly revealed only when Hubble turned its imaging capabilities toward colliding galaxies. Among the observatory's first discoveries was that galaxies with very active star formation contain large numbers of super star clusters - clusters more compact and richer in young stars than astronomers were accustomed to seeing in our galactic neighbourhood.

Arp 87 is in the constellation Leo, the Lion, approximately 300 million light-years away from Earth. These observations were taken in February 2007 with the Wide Field Planetary Camera 2. Light from four isolated wavelength ranges (centred around 450, 555, 656 and 814 nm) blue, green, red, and infrared ranges was composited together to form this colour image.
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NGC 474's Shells

Galaxy NGC 474 - Credit & Copyright: Mischa Schirmer

The multiple layers of emission appear strangely complex and unexpected given the relatively featureless appearance of the elliptical galaxy in less deep images. The cause of the shells is currently unknown, but possibly tidal tails related to debris left over from absorbing numerous small galaxies in the past billion years.

Alternatively the shells may be like ripples in a pond, where the ongoing collision with the spiral galaxy to the right of NGC 474 is causing density waves to ripple though the galactic giant.

Whatever the possible cause, this image dramatically highlights the increasing consensus that the outer halos of most large galaxies are not really smooth but have complexities induced by frequent interactions with - and accretions of - smaller nearby galaxies.

NGC 474 spans about 250,000 light years and lies about 100 million light years distant toward the constellation of the Fish Pisces.
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Akari Galaxy Image M101

M101 is a spiral galaxy twice the size of our own Galaxy, 170 000 light-years in diameter, near the tail of the Great Bear constellation. AKARI’s new observations reveal differing populations of stars spread across its spiral arms.

Using the AKARI space infrared telescope, astronomers have been able to find the warm dust heated by the birth of stars like our Sun in M101, and cooler dust heated by stars more like the present-day Sun.

The image shows the visible light (green), the far-ultraviolet light (cyan) from young stars, the warm dust (red) and the cooler dust (blue). This warm dust is mainly along the spiral arms, with hot spots along the galaxy's outer edge. These hot spots are giant star-forming regions, and it is unusual to find these on the edges of a galaxy.

Commenting on this image Dr Stephen Serjeant from The Open University said, "The evidence points to M101 having experienced a close encounter or near collision with a neighbour or companion galaxy, and it could be that it yanked material out of its neighbour which is now raining down on one side of galaxy and triggering this star formation."

Galaxies near and far from Akari - In Depth Article - from ESA
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Cosmic Mystery Deepens

The difficulties trying to fathom and understand what we detect and observe or see billions of light years away.

Abell 520 in the Constellation of Orion some 2.4 billion light years away, where astronomers have discovered a chaotic scene unlike any witnessed before in a collision between giant galaxy clusters. The results challenge our understanding of the way clusters merge, they possibly make us even reexamine the nature of dark matter itself.

There are three main components to galaxy clusters: individual galaxies composed of billions of stars, hot gas in between the galaxies, and dark matter, a mysterious substance that dominates the cluster mass and can be detected only through its gravitational effects.
[+/-] Click here to expand

Optical telescopes can observe the starlight from the individual galaxies, and can infer the location of dark matter by its subtle light-bending effects on distant galaxies. X-ray telescopes like Chandra detect the multimillion-degree gas.

A popular theory of dark matter predicts that dark matter and galaxies should stay together, even during a violent collision, as observed in the case of the so-called Bullet Cluster.

However, when the Chandra data of the galaxy cluster system known as Abell 520 was mapped along with the optical data from the Canada-France-Hawaii Telescope and Subaru Telescope atop Mauna Kea (Hawaii), a puzzling picture emerged. A dark matter core was found, which also contained hot gas but no bright galaxies.

In addition to the dark matter core, a corresponding "light region" containing a group of galaxies with little or no dark matter was also detected. The dark matter appears to have separated from the galaxies.

In the Bullet Cluster, the hot gas is slowed down during the collision but the galaxies and dark matter appear to continue on unimpeded. In Abell 520, it appears that the galaxies were unimpeded by the collision, as expected, while a significant amount of dark matter has remained in the middle of the cluster along with the hot gas.

While the components of Abell 520 - galaxies, hot gas, and dark matter - are found in unexpected places, the overall amount of these components totals what scientists expect.

The results lead to two possible explanations: one involving how galaxy clusters interact, and the other about the nature of dark matter itself. Both of these explanations would pose uncomfortable problems for current prevailing theories.

The first option is that the galaxies were separated from the dark matter through a complex set of gravitational "slingshots." This explanation is problematic because computer simulations have not been able to produce slingshots that are nearly powerful enough to cause such a separation.

The second option is that dark matter is affected not only by gravity, but also by an as-yet-unknown interaction between dark matter particles. This exciting alternative would require new physics and could be difficult to reconcile with observations of other galaxies and galaxy clusters, such as the aforementioned Bullet Cluster.

Credit:
X-ray: NASA/CXC/UVic./A.Mahdavi et al. Optical/Lensing: CFHT/UVic./A.Mahdavi et al.

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Quad Galaxy Collision

One of the biggest galaxy collisions ever observed is taking place at the centre of this image from Spitzer.
The four white blobs in the middle are large galaxies that have begun to tangle and ultimately merge into a single gargantuan galaxy.


The whitish cloud around the colliding galaxies contains billions of stars tossed out during the messy encounter. Other galaxies and stars appear in yellow, orange and red hues. Blue shows hot gas that permeates this distant region of tightly packed galaxies.

NASA's Spitzer Space Telescope serendipitously spotted the quadruple merger during a routine survey of a distant galaxy cluster, called CL0958+4702, located nearly 5 billion light years away.

Spitzer's infrared eyes observed an unusually large fan-shaped plume of light emerging from a gathering of four elliptical galaxies. Three of the galaxies are about the size of the Milky Way, while the fourth is three times as large.
[+/-] Click here to expand

The plume turned out to be billions of elderly stars ejected and abandoned during the clash. About half of the stars in the plume will later fall back into the galaxies.

Spitzer observations also show that, unlike most known mergers, the galaxies involved in the quadruple collision are bereft of gas, the source material that fuels star birth. As a result, astronomers predict relatively few new stars will be born in the new, combined galaxy.


Artist's concept showing what the night sky might look like from a hypothetical planet around a star tossed out of an ongoing four-way collision between big galaxies.

Credit: NASA/JPL-Caltech/Harvard-Smithsonian CfA

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Galactic Collisions Set Quasars Ablaze from Universe Today
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The Bullet Cluster

Composite image of the Bullet Cluster.

When individual galaxies collide and spiral into one another, they discard trails of hot gas that stretch across space, providing signposts to the mayhem. Recognising the signs of collisions between whole clusters of galaxies, however, is not as easy.

The orbiting X-ray telescopes XXM-Newton and Chandra have caught a pair of galaxy clusters merging into a giant cluster. The discovery adds to existing evidence that galaxy clusters can collide faster than previously thought.

During the collision the hot gas (shown in pink) in each cluster is slowed and distorted by a drag force, similar to air resistance. A bullet-shaped cloud of gas forms in one of the clusters.

The optical image from the Magellan and the Hubble Space Telescope shows galaxies in orange and white in the background. Hot gas, which contains the bulk of the normal matter in the cluster, is shown by the Chandra X-ray image, which shows the hot intracluster gas in pink. Gravitational lensing and the distortion of background images by mass in the cluster, reveals the mass of the cluster may be dominated by dark matter (blue), an exotic form of matter abundant in the Universe, with very different properties compared to normal matter.

Major cluster-cluster collisions are expected to be rare, with estimates of their frequency ranging from less than one in a thousand clusters to one in a hundred. On collision, their internal gas is thrown out of equilibrium and if unrecognised, causes underestimation of its mass by between 5 and 20 percent.

This is important because the masses of the various galaxy clusters are used to estimate the cosmological parameters that describe how the Universe expands. So, identifying colliding systems is extremely important to our understanding of the Universe.

X-ray satellites discover the biggest collisions in the Universe from ESA
Image Credits: X-ray: NASA/CXC/CfA/M.Markevitch, Optical and lensing map: NASA/STScI, Magellan/U.Arizona/D.Clowe, Lensing map: ESO WFI
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Biggest Collisions in the Universe from Universe Today
A Close Stellar Encounter? debris disk around HD 15115 from Centauri Dreams
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Stellar Fireworks

Nearly 12.5 million light-years away, in the dwarf galaxy NGC 4449, stellar fireworks on display have been captured by the Hubble Space Telescope. NGC 4449 belongs to a group of galaxies in the constellation Canes Venatici, ‘the Hunting Dogs’. Astronomers think that NGC 4449’s episode of star formation has been influenced by interactions with several of its neighbours. It is likely that the current widespread starburst was triggered by interaction or merger with a smaller companion.

The NASA/ESA Hubble Space Telescope’s Advanced Camera for Surveys observed NGC 4449 in the visible (blue and green), infrared, and hydrogen-alpha regions of the spectrum.

Hundreds of thousands of vibrant blue and red stars are visible in this new image. Hot bluish white clusters of massive stars are scattered throughout the galaxy, interspersed with numerous dustier reddish regions where star formation is taking place. Massive, dark clouds of gas and dust are silhouetted against starlight.

NGC 4449 has been forming stars for thousands of millions of years, but is currently experiencing star formation at a much higher rate than in the past. This unusual explosive and intense activity qualifies as a starburst, meaning that at the current rate, the gas that feeds stellar production would run out in about a thousand million years.

Starbursts usually occur in the central regions of galaxies, but in NGC 4449 it is more widespread, since the youngest stars are present both in the nucleus and in streams surrounding the galaxy.

A galaxy-wide starburst such as that seen in NGC 4449 resembles primordial star-forming galaxies, which grew by merging with and accreting smaller stellar systems. Since it is close enough to be observed in detail, NGC 4449 is the ideal laboratory for the investigation of what may have occurred during galactic formation and evolution in the early Universe.

Stellar Fireworks Are Ablaze in Galaxy NGC 4449 A Hubble Heritage Release
Stellar fireworks through Hubble’s eyes plusanimation - from ESA
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Black Eyed Galaxy

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 colour image is a composite prepared by the Hubble Heritage Team from pictures taken through four different colour filters. These filters isolate blue and near-infrared light, along with red light emitted by hydrogen atoms and green light from Strömgren y.

Credit: NASA and The Hubble Heritage Team (AURA/STScI)
Acknowledgment: S. Smartt (Institute of Astronomy) and D. Richstone (U. Michigan)
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A Close Neighbour

Andromeda, the nearest major galaxy to the Milky Way, is shown here in this wide-field optical image from Kitt Peak.

Located in the constellation of Andromeda (the Princess), the Andromeda Galaxy is a large spiral galaxy over 65,000 light years in diameter and approximately 2.9 million light years from Earth.
(Credit: NOAO/AURA/NSF/T.Rector & B.A.Wolpa)

The central region of Andromeda is shown in a composite image, with X-rays from NASA's Chandra X-ray Observatory (blue) combined with the optical image. Astronomers believe that Andromeda, also known as Andromeda Galaxy (M31), and the Milky Way will merge in a few billion years.

In the composite image (insert), hot, X-ray bright gas is seen to envelop the middle of Andromeda. Point sources are also prominent, which mostly reveal pairs of stars that are interacting with each other. Many of these double stars are thought to include white dwarfs pulling large amounts of material away from a companion star. When the amount of gas being dumped onto the white dwarf gets too high a thermonuclear explosion occurs on the surface of the white dwarf, emitting bright X-rays. (Credit: NASA/CXC/MPE/W.Pietsch et al)

Andromeda Galaxy (M31):
A New Look at a Close Neighbor from Chandra
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Blackholes in Colliding Galaxies

Adaptive Optics Pinpoints Two Supermassive Black Holes In Colliding Galaxies

Astronomers have used powerful adaptive optics technology at the W. M. Keck Observatory in Hawaii to reveal the precise locations and environments of a pair of supermassive black holes at the center of an ongoing collision between two galaxies 300 million light-years away.

NGC 6240 is an ongoing collision of two gas-rich disk galaxies. Using adaptive optics at the Keck II Telescope, researchers have resolved young star clusters formed because of the merger (small blue dots), and have identified which features within the twin nuclei are associated with the two supermassive black holes known to inhabit the nuclear regions. The green vertical line represents one second of arc, or 1,600 light years at the distance of NGC 6240.
(Credit: C. Max, G. Canalizo, W. de Vries)
[+/-] Click here to expand

The new observations of the galaxy merger known as NGC 6240 reveal that each of the black holes resides at the center of a rotating disk of stars and is surrounded by a cloud of young star clusters formed in the merger, said Claire Max, professor of astronomy and astrophysics at the University of California, Santa Cruz.

"People had observed this pair of colliding galaxies at different wavelengths and seen what they thought were the black holes, but it's been very hard to make sense of how the observations at various wavelengths correspond to each other," Max said. "The adaptive optics results enabled us to tie it all together, so now we can really see it all--the hot dust in the infrared, the stars in the visible and infrared, and the x-rays and radio emissions coming from right around the black holes."

Adaptive Optics (AO) enables astronomers to counteract the blurring effects of turbulence in Earth's atmosphere, which degrades images seen by ground-based telescopes. Max, who directs the Center for Adaptive Optics at UC Santa Cruz, is the lead author of a paper describing the new findings published by the journal Science. Her coauthors are Gabriela Canalizo, who worked with Max as a postdoctoral researcher at Lawrence Livermore National Laboratory (LLNL) and is now at UC Riverside, and Willem de Vries, a physicist with LLNL and UC Davis.

Images of NGC 6240 in visible light from the Hubble Space Telescope show the outer parts of the colliding galaxies distorted by their ongoing merger into long tidal tails of stars, gas, and dust. In the bright central region, two distinct nuclei can be discerned, but clouds of dust obscure much of the visible light from the core. The presence of two supermassive black holes in NGC 6240 was first demonstrated by x-ray observations from NASA's Chandra X-ray Observatory in 2002. Two pointlike radio sources were also detected in the central region.

But trying to match up the data from one instrument with those obtained at different wavelengths by other instruments is very difficult because there are few common reference points in the various wavelength regimes, Max said. The infrared images her group obtained using the AO system on the 10-meter Keck II Telescope provided the high spatial resolution needed to identify features in NGC 6240 that can be seen in different wavelengths.

"With the infrared images we got at Keck, we were able to line up the information from all the different wavelengths to determine which features in the images are the black holes," Max said.

The infrared wavelengths are less affected by dust than visible light, and the Keck infrared images show distinct nuclei with complex substructure surrounded by many faint point sources. The faint point sources are young star clusters produced in a burst of star formation triggered by the collision of the two gas-rich galaxies. Pinpointing which of the features in the infrared images correspond to the positions of the black holes involved several steps and required Keck adaptive optics observations at different infrared wavelengths.

"We uncovered it piece by piece, until we were able to make the correspondence between the black holes and the features seen at different wavelengths, as well as the stuff around them," Max said. "It really shows how powerful the Keck adaptive optics system is. We were also fortunate to have an extraordinarily good observing night."

Galaxy mergers are thought to play a major role in the evolution of galaxies and may help explain many of their properties. For example, astronomers have found that the mass of the black hole at the center of a galaxy is highly correlated with large-scale properties of the galaxy itself. The "coevolution" hypothesis explains this correlation as the result of both the black hole and the galaxy around it growing incrementally in repeated merger events over cosmic timescales.

"The gravitational influence of the black hole is actually limited to a relatively small region right around it, so how can it affect the rest of the galaxy" But if the black hole and the galaxy around it evolved together through the same sequence of merger events, that would explain the correlations," Max said. "That's why people are so excited about understanding galaxy mergers, and here we're seeing it in action."

The two black holes in NGC 6240 will eventually, in 10 million to 100 million years, spiral into each other and merge, producing a powerful burst of gravitational radiation, she said.

Story adapted from University of California - Santa Cruz news release

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X-rays provide a new way to investigate exploding stars from ESA
Slicing the Universe with HARP/ACSIS - A New Look at Orion from SciTech
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Sombrero Galaxy M104

The Sombrero, also known as M104, is one of the largest galaxies in the nearby Virgo cluster, about 28 million light years from Earth. This Great Observatories view of the famous Sombrero galaxy was made using NASA's Chandra X-ray Observatory, Hubble Space Telescope and Spitzer Space Telescope. The main figure shows the combined image from the three telescopes, while the three inset images show the separate observatory views.

The Chandra X-ray image (in blue) shows hot gas in the galaxy and point sources that are a mixture of objects within the Sombrero as well as quasars in the background. The Chandra observations show that diffuse X-ray emission extends over 60,000 light years from the center of the Sombrero. (The galaxy itself spans 50,000 light years across.) Scientists think this extended X-ray glow may be the result of a wind from the galaxy, primarily being driven by supernovas that have exploded within its bulge and disk. The Hubble optical image (green) shows a bulge of starlight partially blocked by a rim of dust, as this spiral galaxy is being observed edge on. That same rim of dust appears bright in Spitzer's infrared image, which also reveals that Sombrero's central bulge of stars.

Chandra featured stories
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If light travels the shortest distance between two points,
even if from the remotest parts of the observable universe
and all things being equal there is nothing between point A & B
what exactly would Carl Sagan et al have us tunneling thru
that would get us to point B from point A - faster than light.

Nova, It's String Theory - The Elegant Universe
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