CfA: Milky Way Sidelined in Galactic Tug of War

[bystander]

Milky Way Sidelined in Galactic Tug of War
Harvard-Smithsonian Center for Astrophysics | 29 Sept 2010

[size=90]This plot shows the simulated gas distribution of the Magellanic System resulting from the tidal encounter between the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) as they orbit our home Milky Way Galaxy. The entire sky is plotted in galactocentric coordinates of longitude and latitude. The Magellanic Stream is the pronounced tail of material that stretches 150 degrees across the southern sky. The solid line shows the calculated path of the LMC and the dotted line is the path of the SMC. The color range from dark to light shows the density (lower to higher) of the hydrogen gas making up the Magellanic Stream and the Bridge that connects the two dwarf galaxies. [i](Credit: Plot by G. Besla, Milky Way background image by Axel Mellinger)[/i][/size])

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The Magellanic Stream is an arc of hydrogen gas spanning more than 100 degrees of the sky as it trails behind the Milky Way's neighbor galaxies, the Large and Small Magellanic Clouds. Our home galaxy, the Milky Way, has long been thought to be the dominant gravitational force in forming the Stream by pulling gas from the Clouds. A new computer simulation by Gurtina Besla (Harvard-Smithsonian Center for Astrophysics) and her colleagues now shows, however, that the Magellanic Stream resulted from a past close encounter between these dwarf galaxies rather than effects of the Milky Way.

"The traditional models required the Magellanic Clouds to complete an orbit about the Milky Way in less than 2 billion years in order for the Stream to form," says Besla. Other work by Besla and her colleagues, and measurements from the Hubble Space Telescope by colleague Nitya Kallivaylil, rule out such an orbit, however, suggesting the Magellanic Clouds are new arrivals and not long-time satellites of the Milky Way.

This creates a problem: How can the Stream have formed without a complete orbit about the Milky Way?

To address this, Besla and her team set up a simulation assuming the Clouds were a stable binary system on their first passage about the Milky Way in order to show how the Stream could form without relying on a close encounter with the Milky Way.

The team postulated that the Magellanic Stream and Bridge are similar to bridge and tail structures seen in other interacting galaxies and, importantly, formed before the Clouds were captured by the Milky Way.

"While the Clouds didn't actually collide," says Besla, "they came close enough that the Large Cloud pulled large amounts of hydrogen gas away from the Small Cloud. This tidal interaction gave rise to the Bridge we see between the Clouds, as well as the Stream."

"We believe our model illustrates that dwarf-dwarf galaxy tidal interactions are a powerful mechanism to change the shape of dwarf galaxies without the need for repeated interactions with a massive host galaxy like the Milky Way."

While the Milky Way may not have drawn the Stream material out of the Clouds, the Milky Way's gravity now shapes the orbit of the Clouds and thereby controls the appearance of the tail.

"We can tell this from the line-of-sight velocities and spatial location of the tail observed in the Stream today," says team member Lars Hernquist of the Center.

Simulations of the Magellanic Stream in a First Infall Scenario - G Besla et al

• Astrophysical Journal Letters 721(2) l97 (01 Oct 2010) DOI: 10.1088/2041-8205/721/2/L97
  arXiv.org > astro-ph > arXiv:1008.2210 > 12 Aug 2010

[neufer]

The bas-relief of the [b][color=#0000FF]Churning of the Sea of Milk[/color][/b] shows Vishnu in the centre, his turtle avatar Kurma below, and apsaras & Indra above. This is a tug of war between the asuras & demons. (Angkor Wat, Cambodia)

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<<There is no specific time and place in history to define the origin of the game of Tug of War. The contest of pulling on the rope originates from ancient ceremonies and rituals. Evidence is found in countries like Egypt, India, Myanmar, New Guinea... The origin of the game in India has strong archaeological roots going back at least to the 12th Century AD in the area what is today the State of Orissa on the east coast. The famous Sun Temple of Konark has a stone relief on the west wing of the structure clearly showing the game of Tug of War in progress.

Tug of war was once royal sport was practiced in ancient Egypt and China, where it was held in legend that the Sun and Moon played Tug of War over the light and darkness.

On October 25, 1997, a massive Tug-of-War event was held at a park along the Keelung River in Taipei in celebration of Retrocession Day (the 52nd anniversary of the end of the Japanese colonial rule). 1,600+ participants exerted over 80,000+ kg of force on a 2 inch nylon rope that could only withstand a maximum of 26,000 kg. The rope immediately snapped and the sheer rebounding force of the broken rope tore off two men's arms. Both men were immediately taken to a nearby hospital where their arms were successfully reattached.

A retired warden lost four fingers of his left hand during an annual tug-of-war between residents of two counties on September 25, 2010, in Nova Scotia, Canada. The man is recovering in hospital after surgery which reattached the four fingers >>

[Beyond]

Just goes to show that ANY war is dangerous!!

[bystander]

Stolen: Magellanic Clouds – Return to Andromeda
Universe Today | Astronomy | Galaxies | 19 Oct 2010

Click to view full size image

The Magellanic Clouds are an oddity. Their relative velocity is suspiciously close to the escape velocity of the Milky Way system making it somewhat difficult for them to have been formed as part of the system. Additionally, their direction of motion is nearly perpendicular to the disk of the galaxy and systems, especially ones as large as the Magellanic Clouds, should show more orientation to the plane if they formed along side. Their gas content is also notably different than other satellite galaxies of our galaxy. The combination of these features suggests to some, that the Magellanic Clouds aren’t native to the Milky Way and were instead intercepted.

But where did they come from? Although the suggestion is not entirely new, a recent paper, accepted to the Astrophysical Journal Letters, suggests they may have been captured after a past merger in the Andromeda Galaxy (M31).

To analyze this proposition, the researchers, Yang (from the Chinese Academy of Sciences) and Hammers (of the University of Paris, Diderot), conducted simulations backtracking the positions of the Magellanic Clouds. While this may sound straightforward, the process is anything but. Since galaxies are extended objects, their three dimensional shapes and mass profiles must be worked out extremely well to truly account for the path of motion. Additionally, the Andromeda galaxy is certainly moving and would have been in a different position that it is observed today. But exactly where was it when the Magellanic Clouds would have been expelled? This is an important question, but not easy to answer given that observing the proper motions of objects so far away is difficult.

But wait. There’s more! As always, there’s a significant amount of the mass that can’t be seen at all! The presence and distribution of dark matter would greatly have affected the trajectory of the expelled galaxies. Fortunately, our own galaxy seems to be in a fairly quiescent phase and other studies have suggested that dark matter halos would be mostly spherical unless perturbed. Furthermore, distant galaxy clusters such as the Virgo supercluster as well as the “Great Attractor” would have also played into the trajectories.

These uncertainties take what would be a fairly simple problem and turn it into a case in which the researchers were instead forced to explore the parameter space with a range of reasonable inputs to see which values worked. In doing so, the pair of astronomers concluded “it could be the case, within a reasonable range of parameters for both the Milky Way and M31.” If so, the clouds spent 4 – 8 billion years flying across intergalactic space before being caught by our own galaxy.

But could there be further evidence to support this? The authors note that if Andromeda underwent a merger event of such scale would likely have induced vast amounts of star formation. As such, we should expect to see an increase in numbers of stars with this age. The authors do not make any statements as to whether or not this is the case. Regardless, the hypothesis is interesting and reminds us how dynamic our universe can be.

Could the Magellanic Clouds be tidal dwarves expelled from a past-merger event occurring in Andromeda?

• Y Yang, F Hammer > arXiv.org > astro-ph > arXiv:1010.2748 > 13 Oct 2010

[bystander]

Andromeda: born out of a massive collision?
Discover Blogs | Bad Astronomy | 24 Nov 2010

Click to play embedded YouTube video.

Was the Andromeda Galaxy, the largest and most massive galaxy in our local neighborhood, shaped into its current structure due to a monstrous collision over 6 billion years ago? According to a new study by some French astronomers, the answer is oui.

They created a lovely animation based on the model. It shows the collision of the two galaxies and how they interact:

Wow! You can see how the galaxies get disrupted, and perhaps get something of a feel for just how violent and incredible an event on this scale can be.

Using a sophisticated computer code that models the gravitational and fluid (pedantic: hydrodynamical) interaction between stars, gas, dust, and dark matter, they found that an ancient and massive collision between a galaxy a bit bigger than our Milky Way, and a smaller one about 1/3 the mass, reproduces a large amount of the structure we see in Andromeda today. That includes "…the large thin disk including its giant ring of gas and dust, the massive central bulge, the gigantic thick disk, the giant stream of old stars, as well as many other stellar streams discovered in the galaxy halo" according to the press release (the paper itself is in French).

Not only that, but it may solve another big mystery: what the heck are the Magellanic Clouds? These are two small, irregular galaxies that apparently orbit the Milky Way; they are the closest galaxies to us at a distance of about 180,000 light years each. Recent studies have hinted that these galaxies may not actually be orbiting us as always thought, but might be passing us by at nearly a million kilometers per hour. This new model of Andromeda shows that during the collision, a long streamer of gas would’ve been drawn out and ejected. This is called a tidal tail, and is common in large collisions. In this case, the model indicates the tail may have formed the two Magellanic galaxies and sent them heading our way!

The most basic aspect of a good scientific model is that it should solve some issue you see. For example, why does the Andromeda galaxy have a ring of gas circling its center? If your model explains that, then great! But if it also explains lots of other structures, then that means the model has a much better chance of being right, or at least being mostly right. This new model seems to do just that.

So this new study is pretty cool. We know that there have been lots of little collisions over the eons; we think all big galaxies like the Milky Way and Andromeda built up to their current size by eating and absorbing smaller galaxies. But those are usually dinky dwarf galaxies; this new study may show that Andromeda also had at least one really, really big collision long ago.

And regular readers know as well that there’s another one coming: in a billion years or two, the Milky Way and Andromeda may themselves collide. Both galaxies are among the largest in the near Universe, so when this happens it’ll be a spectacular and amazing event. I wonder if alien astronomers hundreds of million of light years away, and billions of years hence, will get images of the ensuing merger and gasp (assuming they breathe) in awe as I do when I see such a magnificent tableau?

Collisions of two galaxies may have formed Andromeda
PhysOrg | Astronomy | 26 Nov 2010

New computer simulations suggest the nearest galactic neighbors to our own Milky Way, the Magellanic Clouds and the Andromeda Galaxy, may have been shaped by the massive collision of two galaxies billions of years ago.

The Milky Way lies in a region known to astronomers as the “Local Group,” which consists of almost 40 galaxies, including the two biggest: Andromeda Galaxy and the Milky Way. Both are massive spiral galaxies.

Astronomers have thought the Andromeda Galaxy (Messier 31), which is around 2.5 million light-years away from Earth, was formed by a merging of two smaller galaxies or a combination of many smaller mergers, but the ideas had never been tested or dated before. So a team of researchers in France and China decided to model how the galaxy might have evolved.
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The collision must have been extremely violent to produce the massive angular momentum required to form the Andromeda Galaxy. It would have been the most dramatic and important event in the history of the Local Group.

The computer simulation also predicted the collision would have led to the formation of massive streams (called tidal tails) of long, thin areas of stars and interstellar gas with a total mass around a third that of the Milky Way.

The researchers think one such tail may have formed the Large and Small Magellanic Clouds, which are small, irregular, satellite galaxies apparently passing the Milky Way or orbiting at a distance of about 180,000 light years. If they did originate in the collision, they would have been ejected towards the Milky Way at around a million kilometers per hour (620,000 mph).
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The results support the hypothesis that spiral galaxies are mostly formed by collisions between galaxies, and the hypothesis that dwarf galaxies often originate from tidal tails formed by mergers. The findings may also affect the estimations of the amount of dark matter in galaxies.

Does M31 Result from an Ancient Major Merger? - F Hammer et al

• Astronomical Journal 725(1) 542 (10 Dec 2010) DOI: 10.1088/0004-637X/725/1/542
  arXiv.org > astro-ph > arXiv:1010.0679 > 04 Oct 2010 (v1), 21 Oct 2010 (v2)

Could the Magellanic Clouds be Tidal Dwarfs Expelled from a Past-merger Event Occurring in Andromeda? - Y Yang, F Hammer

• Astronomical Journal Letters 725(1) L24 (10 Dec 2010) DOI: 10.1088/2041-8205/725/1/L24
  arXiv.org > astro-ph > arXiv:1010.2748 > 13 Oct 2010 (v1), 19 Oct 2010 (v2)

[bystander]

Milky Way's Galactic Neighbourhood Puzzles Astronomers
Technology Review | The Physics arXiv Blog | 21 Dec 2010

It looks as if the Milky Way and its nearest neighbours make up one of the rarest configurations in the local Universe. Now astronomers are wondering why.

Click to view full size image

There's something odd about our galactic neighbourhood, which Sidney van den Bergh at the Herzberg Institute of Astrophysics in Canada highlights today in a short paper.

Astronomers have long known that the Milky Way's two closest neighbours are the Large and Small Magellanic Clouds, giant clouds of stars, gas and dust called irregular galaxies.

This is strange for two reasons. These galaxies are much younger than ours and may have even formed together. It looks as if they may just be passing by, on their way to somewhere else. Most other galaxies like ours, such as Andromeda, don't have a single companion like this, so having two seems rather fortunate.

But there's something else as well. The Large Magellanic cloud is unusually luminous. In fact, there are only two other irregular galaxies in the entire local universe that come close. "In other words the Large Magellanic Cloud seems to be close to the upper luminosity limit for irregular galaxies," says van den Bergh. That's unusual too.

In recent years, astronomers have begun to work out just how rare this is. Sky surveys such as the Sloan Digital Sky Survey allow astronomers to work out the distribution of various types of galaxy. They've looked at 22581 galaxies like the Milky Way and found that 81% have no satellite galaxies as bright as the Magellanic Clouds, 11% have one such satellite, and only 3.5% host two such satellite galaxies.

That makes the Milky Way very unusual. As van den Bergh puts it: "That the Galaxy should have an irregular companion as luminous as the Large Magellanic Cloud is almost a miracle."

One of the central tenets of cosmology is the Copernican principle: that we live on an ordinary planet, in an average galaxy, in a mediocre part of the Universe.

But it's beginning to look as if the Milky Way, or at least its neighbourhood, doesn't follow that rule at all. So the question for astronomers and cosmologists is why this has come about and what its significance should be. An interesting conundrum.

A Strange Mènage Á Trois - Sidney van den Bergh

• arXiv.org > astro-ph > arXiv:1012.3492 > 15 Dec 2010

[bystander]

Maser to predict Milky Way's fate
New Scientist | Space | 2011 Jan 28

WILL the Milky Way slam into its giant neighbour, Andromeda, in a few billion years? A laser-like spot of light in the galaxy hints at an answer.

The speed at which Andromeda is moving towards the Milky Way can be determined from the Doppler shift of the light it emits. But the galaxy is too spread out for its subtle sideways motion in the sky to be detected. If it moves fast enough in this direction it may miss the Milky Way altogether.

Now Loránt Sjouwerman of the National Radio Astronomy Observatory in Socorro, New Mexico, and colleagues have glimpsed a bright, laser-like spot of microwave radiation, called a maser, in Andromeda that could help determine its sideways motion.^*

Detected with the newly upgraded Very Large Array of telescopes in New Mexico, the maser appears when interstellar methanol molecules get heated up by nearby stars. Tracking the motion of this bright spot precisely should be easier than the galaxy as a whole, say the team. However, they must first find other masers in Andromeda, to confirm that the maser motion reflects Andromeda's path overall.

Discovery of the first methanol (CH₃OH) maser in the Andromeda galaxy (M31) - LO Sjouwerman et al

• Astrophysical Journal Letters 724(2) L158 (2010 Dec 01) DOI: 10.1088/2041-8205/724/2/L158
  arXiv.org > astro-ph > arXiv:1010.4818 > 22 Oct 2010