Category: galaxies

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Dive right in to this image that contains a sea of distant galaxies! The Very Large Telescope has obtained the deepest ground-based image in the ultraviolet band, and here, you can see this patch of the sky is almost completely covered by galaxies, each one, like our own Milky Way galaxy, and home of hundreds of billions of stars. A few notable things about this image: galaxies were detected that are a billion times fainter than the unaided eye can see, and also in colors not directly observable by the human eye. In this image, a large number of new galaxies were discovered that are so far away that they are seen as they were when the Universe was only 2 billion years old! Also…

This image contains more than 27 million pixels and is the result of 55 hours of observation, made primarily with the Visible Multi Object Spectrograph (VIMOS) instrument. To get the full glory of this image, here’s where you can download the full resolution version. It’s worth the wait while it downloads. Or click here to be able to zoom around the image.

In this sea of galaxies – or island universes as they are sometimes called – only a very few stars belonging to the Milky Way are seen. One of them is so close that it moves very fast on the sky. This “high proper motion star” is visible to the left of the second brightest star in the image. It appears as a funny elongated rainbow because the star moved while the data were being taken in the different filters over several years.

The VLT folks describe this image as a “uniquely beautiful patchwork image, with its myriad of brightly coloured galaxies.” It shows the Chandra Deep Field South (CDF-S), one of the most observed and best studied regions in the entire sky. The CDF-S is one of the two regions selected as part of the Great Observatories Origins Deep Survey (GOODS), an effort of the worldwide astronomical community that unites the deepest observations from ground- and space-based facilities at all wavelengths from X-ray to radio. Its primary purpose is to provide astronomers with the most sensitive census of the distant Universe to assist in their study of the formation and evolution of galaxies.

The image encompasses 40 hours of observations with the VLT, just staring at the same region of the sky. The VIMOS R-band image was obtained co-adding a large number of archival images totaling 15 hours of exposure.

Source: ESO

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Halloween means its time for ghost stories and here’s a mysterious astronomical story about the “Ghost of Mirach.” When viewed in visible light, as in the picture above on the left, the galaxy called NGC 404 appears as just a white blob. Mirach is a red giant star that looms large in visible light, hiding NGC 404 in its glare, and therefore was nicknamed “The Ghost of Mirach” But when the Galaxy Evolution Explorer (GALEX) spied the galaxy in ultraviolet light, a spooky ring materialized and the region came to “life.” This ring, seen in blue in the picture on the right, contains new stars — a surprise considering that the galaxy was previously thought to be dead. “We thought this celestial ghost was essentially dead, but we’ve been able to show that it has an extended ring of new stars. The galaxy has a hybrid character in which the well-known, very old stellar population tells only part of the story,” said David Thilker of Johns Hopkins University in Baltimore. “It’s like the living dead.”

Oooh, spooky!

The eerie creature, called NGC 404, is a type of galaxy known as “lenticular.” Lenticular galaxies are disk-shaped, with little ongoing star formation and no spiral arms. NGC 404 is the nearest example of a lenticular galaxy, and therefore of great interest. But, usually it lies hidden in the glare from a red giant star Mirach.

Thilker and members of the Galaxy Evolution Explorer team spotted the Ghost of Mirach in images taken during the space telescope’s all-sky survey, where it is scanning the entire visible sky in ultraviolet light, a job never before accomplished.

Long exposure observations of this region show that the NGC 404 is surrounded by a clumpy, never-before-seen ring of stars.

What is this mysterious ultraviolet ring doing around an otherwise nondescript lenticular galaxy? As it turns out, previous imaging with the National Science Foundation’s Very Large Array radio telescope in New Mexico had discovered a gaseous ring of hydrogen that matches the ultraviolet ring observed by the Galaxy Evolution Explorer. The authors of this Very Large Array study attributed the gas ring to a violent collision between NGC 404 and a small neighboring galaxy 900 million years ago.

The ultraviolet observations demonstrate that, when the hydrogen from the collision settled into the plane of the lenticular galaxy, stars began to form in a ghostly ring. Young, relatively hot stars forming in stellar clusters sprinkled throughout NGC 404’s ring give off the ultraviolet light that the Galaxy Evolution Explorer was able to see.

“Before the Galaxy Evolution Explorer image, NGC 404 was thought to contain only very old and evolved red stars distributed in a smooth elliptical shape, suggesting a galaxy well into its old age and no longer evolving significantly,” said Mark Seibert of the Observatories of the Carnegie Institution of Washington in Pasadena, Calif. “Now we see it has come back to life, to grow once again.”

“The Ghost of Mirach has been lucky enough to get a new lease on life through the rejuvenating, chance merger with its dwarf companion,” added Thilker.

The findings indicate that the evolution of lenticular galaxies might not yet be complete. They may, in fact, continue to form stars in a slow, piecemeal fashion as they suck the raw, gaseous material for stars from small, neighboring galaxies. It seems the Ghost of Mirach might act more like a vampire than a ghost.

The fields of view are identical in both pictures, and spans 55,000 light years across. The Ghost of Mirach is located 11 million light-years from Earth. The star Mirach is very close in comparison — it is only 200 light-years away and is visible with the naked eye.

Source: NASA

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It’s a violent universe out there! Yesterday we ran an article about galaxies colliding and forming fireballs. Today, there’s more evidence for galactic collisions, and it’s not good news for potential stars. While this image is stunning, such collisions could spell doom for future star formation. A deep new image of the Virgo cluster has revealed huge tendrils of ionized hydrogen gas 400,000 light-years long connecting the elliptical galaxy M86 and the disturbed spiral galaxy NGC 4438. This image, taken by the 4-meter telescope at Kitt Peak National Observatory, provides striking evidence of a previously unsuspected high-speed collision between the two galaxies. “Our data show that this system represents the nearest recent collision between a large elliptical galaxy and a large spiral,” said Jeffrey Kenney of Yale University, “This discovery provides some of the clearest evidence yet for high-speed collisions between large galaxies, and it suggests that the consequences of such collisions are a plausible alternative to black holes in trying to explain the mystery of what process turns off star formation in the biggest galaxies.”

Astronomers have been trying to understand the mystery of what causes the biggest galaxies in the Universe—which are primarily ellipticals, like M86—to stop forming stars. “Something needs to heat up the gas so it doesn’t cool and form stars,” Kenney says. “A number of recent studies suggest that energy from active galactic nuclei associated with supermassive black holes may do this, (see Universe Today articles here and here) but our new study shows that gravitational interactions may also do the trick.”

The Virgo cluster is located approximately 50 million light-years from Earth. Previous studies had noticed disturbed H-alpha gas around each of the two galaxies, but scientists didn’t think the two had a connection. Indeed, some results have suggested that NGC 4438 collided with the small lenticular galaxy NGC 4435, but NGC 4435 has a much higher line-of-sight velocity as seen from Earth and appears undisturbed.

Spectroscopy of selected regions along the filament between M86 and NGC 4438 shows a fairly smooth velocity gradient between the galaxies, supporting the collision scenario. And here’s the kicker: there are no obvious stars in the filaments.

As in most elliptical galaxies, most of the gas within M86 is extremely hot, and therefore radiates X-rays. The X-ray distribution in M86 is irregular and sports a long plume, which had previously been interpreted as a tail of gas which is being stripped by ram pressure as M86 falls into the intracluster medium of the Virgo cluster. The new H-alpha image from Kitt Peak suggests that most of the disturbances to the interstellar medium in M86 are instead due to the collision with NGC 4438.

Low-velocity collisions, especially between small- to medium-sized galaxies, often cause an increase in the local star formation rate, as the collisions tend to cause gas to concentrate in the galaxy centers. But in high velocity collisions (which happen naturally between large galaxies, since their large gravity pulls mass inward much faster), the kinetic energy of the collision can cause the gas to heat up so much that it cannot easily cool and form stars.

While not many galaxies suffer such extreme collisions as M86, most galaxies experience minor mergers and gas accretion events, and these may play a significant role in heating the galaxy’s gas. These more common but modest events are very hard to study, since their observational signatures are weak.

“The same physical processes occur in both strong and weak encounters, and by studying the observable effects in extreme cases like M86 we can learn about the role of gravity in the heating of galaxy gas, which appears to be quite significant,” Kenney adds.

Kenney is the lead author of a paper to be published in a November 2009 issue of Astrophysical Journal Letters.

Source: NOAO

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During routine observations of the Coma Cluster of galaxies using the Subaru Telescope in Hawaii, astronomers discovered a thread-like structure stretching from one of the galaxies. The astronomers determined this filament was about 260 thousand light years long, and spectral analysis of the filament suggested a younger age toward the outer edge of the filament. The filament also has many young stars surrounded by ionized gas that look like projectiles flying out from the galaxy. So what happened in this chaotic area of space? Astronomers determined a speeding galaxy rammed into the Coma Cluster, stripping gas from the galaxy and creating fireball-like projectiles.

Galaxies evolve over time, and astronomers do not yet understanding how they change in shape, size, and color. Galaxy Clusters, which are dense populations of galaxies, rich with hot intergalactic gas, accompanied by strong gravitational forces are some of the best locations to observe galactic evolution.

A team of researchers from the National Astronomical Observatory of Japan and the University of Tokyo used Suprime-cam on the Subaru Telescope to observe the Coma Cluster of galaxies. The Coma Cluster contains over 1,000 galaxies and is fairly close to Earth at about 300 million light years away.
During observations in 2006 and 2007, the astronomers saw the filament extending from Galaxy RB199 and several of the “fireballs.” Detailed study identified several bright knots connected by blue filamentary structures, and the knots are actually the clusters of young stars weighing 10 million times our Sun, contained in an area about 3000 to 6000 light years across. Because the knots are accompanied by ionized gas, active star formation is going on in the fireballs where usually far less star formation would be expected. The team noted that the size and the mass of the fireballs indicate they could develop into dwarf galaxies.

Because the inside of the cluster is crowded with galaxies, they pass by each other and crash into each other. The team thought that the tidal forces during such encounters could strip gas or stars from the galaxies. They also postulated that as a galaxy falls into the center of the cluster the gravitational forces of the cluster could remove the gas and stars from that galaxy. Both scenarios are possible, however, the research team found that these mechanisms could hardly explain the characteristics of the fireballs. The team then realized that ram pressure stripping occurs when superheated gas (several tens of million Kelvin) in the cluster and the galaxies collide at high speeds. Previous X-ray observation shows the presence of large amounts of hot ionized gas in the middle of the Coma Cluster while RB199 crashes into the center at a speed of 1200 miles per second, causing strong friction with this hot gas. As such, the team concluded that the ram pressure has enough power to strip the gas from the galaxy AND create the fireballs.

While there are several reports indicating ram pressure stripping in nearby galaxy clusters, the identification of fireballs in this study is the first to demonstrate the stripped gas turns into stars while traveling through remote space far away from its source. Similar phenomena have been observed in galaxy clusters much further away at several billions light years, however, those distant cases were interpreted through witnessing the transitional phase of galaxies changing their morphology or colors as they fall into a cluster. The fireballs discovered by this team of Japanese astronomers provide the first sample of such structures in a nearby cluster. Principal investigator, Dr. Michitoshi Yoshida, said “the team is confident that our study of these phenomena leads to a better understanding of the gas stripping processes in galaxy clusters, and the effect of clusters on the evolution of individual galaxies”.

Source: Subaru Press release