Category: galaxies

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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

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When you stir cream in your coffee or tea, does the swirl stay the same or does it change as it spins in your cup? As galaxies form and swirl, the motions and eddies may actually cause stars to move within the galaxy. A long-standing scientific belief holds that stars tend to hang out in the same general part of a galaxy where they originally formed. But some astrophysicists have recently questioned whether that is true, and now new simulations show that, at least in galaxies similar to our own Milky Way, stars such as the sun can migrate great distances. If this is true, it could change the entire notion that there are parts of galaxies – so-called habitable zones – that are more conducive to supporting life than other areas.

“Our view of the extent of the habitable zone is based in part on the idea that certain chemical elements necessary for life are available in some parts of a galaxy’s disk but not others,” said Rok RoÅ¡kar, a doctoral student in astronomy at the University of Washington. “If stars migrate, then that zone can’t be a stationary place.”

RoÅ¡kar is lead author of a paper describing the findings from the simulations, published in the Sept. 10 edition of the Astrophysical Journal Letters. If the idea of habitable zone doesn’t hold up, it would change scientists’ understanding of just where, and how, life could evolve in a galaxy, he said.

Using more than 100,000 hours of computer time on a UW computer cluster and a supercomputer at the University of Texas, the scientists ran simulations of the formation and evolution of a galaxy disk from material that had swirled together 4 billion years after the big bang. Watch a simulation video.

The simulations begin with conditions about 9 billion years ago, after material for the disk of our galaxy had largely come together but the actual disk formation had not yet started. The scientists set basic parameters to mimic the development of the Milky Way to that point, but then let the simulated galaxy evolve on its own.

If a star, during its orbit around the center of the galaxy, is intercepted by a spiral arm of the galaxy, scientists previously assumed the star’s orbit would become more erratic in the same way that a car’s wheel might become wobbly after it hits a pothole.

However, in the new simulations the orbits of some stars might get larger or smaller but still remain very circular after hitting the massive spiral wave. Our sun has a nearly circular orbit, so the findings mean that when it formed 4.59 billion years ago (about 50 million years before the Earth), it could have been either nearer to or farther from the center of the galaxy, rather than halfway toward the outer edge where it is now.

Migrating stars also help explain a long-standing problem in the chemical mix of stars in the neighborhood of our solar system, which has long been known to be more mixed and diluted than would be expected if stars spent their entire lives where they were born. By bringing in stars from very different starting locations, the sun’s neighborhood has become a more diverse and interesting place, the researchers said.

The findings are based on a few runs of the simulations, but the scientists plan to run a range of simulations with varying physical properties to generate different kinds of galactic disks, and then determine whether stars show similar ability to migrate large distances within different types of disk galaxies.

Source: University of Washington

What a great way to start the day, with a gorgeous image like this one of the galaxy Messier 83, adorned with what looks like rubies on the spiral arms. This shot was captured by the Wide Field Imager at ESO’s La Silla Observatory, located high in the dry desert mountains of the Chilean Atacama Desert. Messier 83 lies roughly 15 million light-years away towards the southern constellation of Hydra. To make this image, the WFI stared at M83 for roughly 100 minutes through a series of specialist filters, allowing the faint detail of the galaxy to reveal itself. The brighter stars in the foreground are stars in our own galaxy, and behind M83 the darkness is peppered with the faint smudges of distant galaxies.

M83 stretches over 40,000 light-years, making it roughly 2.5 times smaller than our own Milky Way. However, in some respects, Messier 83 is quite similar to our own galaxy. Both the Milky Way and Messier 83 possess a bar across their galactic nucleus, the dense spherical conglomeration of stars seen at the centre of the galaxies.

The red, ruby like features are in fact huge clouds of glowing hydrogen gas. Ultraviolet radiation from newly born, massive stars is ionizing the gas in these clouds, causing the great regions of hydrogen to glow red. These star forming regions are contrasted dramatically in this image against the ethereal glow of older yellow stars near the galaxy’s central hub. The image also shows the delicate tracery of dark and winding dust streams weaving throughout the arms of the galaxy.

Messier 83 was discovered by the French astronomer Nicolas Louis de Lacaille in the mid 18th century. Decades later it was listed in the famous catalogue of deep sky objects compiled by another French astronomer and famous comet hunter, Charles Messier.

Source: ESO