TORN APART: Streams of stars [blue] have been liberated from the Sagittarius Dwarf Galaxy as it is gradually devoured by the larger Milky Way. The image above comes from supercomputer simulations of the two galaxies' gravitational dance. Image: Erik Tollerud
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The lovely, familiar swirl of the Milky Way, with its symmetric spiral arms winding outward from a central bulge, may be scars from a smaller galaxy punching above its weight. A new computer re-enactment of billions of years of galactic evolution suggests that the Milky Way owes much of its current shape to interactions with a nearby dwarf galaxy.
The Sagittarius Dwarf Galaxy, first discovered in 1994, is a satellite galaxy that is slowly being torn apart and ingested into the larger Milky Way. In the process, however, Sagittarius seems to have been making its presence felt. A group of astrophysicists at the University of Pittsburgh, the University of California, Irvine, and Florida Atlantic University simulated the gravitational infall of Sagittarius over the past few billion years to uncover what effects the dwarf galaxy may have had on the Milky Way. [Read more about the structure of the Milky Way.]
The effects, as it turned out, were strong. In the simulations, described in a study published in the September 15 issue of Nature, Sagittarius stirred up enough ripples to make a smooth, circular, spinning galactic disk evolve into a spiral much like the Milky Way. (Scientific American is part of Nature Publishing Group.) The resulting galactic perturbation also resulted in the development of loose strands of stars at its periphery that resemble an outer Milky Way feature known as the Monoceros ring. [See a video of the simulation below.]
Without the influence of a Sagittarius-type satellite, the simulated galaxy remained a flat, rather uniform disk that little resembled our galaxy. "We just ran the disk in isolation, and it stays pretty much globally smooth," says lead study author Chris Purcell, a University of Pittsburgh astrophysicist. "You certainly don't see any spiral arm formation." Had it not been for Sagittarius, then, the Milky Way might never have taken its familiar, whirlpoolesque form.
The study demonstrates that as large galaxies consume their smaller neighbors in so-called minor mergers that are common throughout the universe, the bully in the galactic interaction does not escape unscathed. "We've known for awhile that minor mergers can have visible effects on their host galaxies," says David Law, an astrophysicist at the University of Toronto's Dunlap Institute for Astronomy and Astrophysics, who did not contribute to the new study. "But this is one of the first times that we've been able to make a good link between a specific minor merger and a specific effect."
The dwarf galaxy's outsize influence stems from the assumption that although Sagittarius today is a mere fraction of the Milky Way's mass, it should once have rested inside a hefty cocoon of dark matter, known as a dark matter halo, some 100 billion times the mass of the sun. (Dark matter is a mysterious, theorized substance thought to account for one quarter of the universe's mass, some five times as much as ordinary matter provides.) Sagittarius's merger with the Milky Way is not a simple collision—the dwarf galaxy has followed a looping, spiraling inward orbit for the past few billion years that has drawn it repeatedly into contact with the Milky Way. As Sagittarius approached the Milky Way, passed through its disk, and circled back again, the dark halo of Sagittarius would have slammed into the plane of the Milky Way twice, knocking the disk askew and stirring up the formation of its spiral arms.
"We have this dramatic perturbation to the entire disk—it's coming straight down onto it in the last two impacts at least," Purcell says of the circuitous path around and through the Milky Way that Sagittarius took in the simulations. "You can't really get away from causing a spiral structure if you have an impact from a galaxy that's as massive as we think Sagittarius was."
It remains to be seen whether spiral galaxies across the universe owe their distinctive shape to similar events, or whether other effects can trigger the formation of spiral arms. "My feeling is that it shouldn't be a necessary condition," Law says. "People are still trying to figure out exactly what drives the evolution of spiral structure. It doesn't seem like on the basis of simulations that you need to have a satellite galaxy impact."
Another potential case study lies just 2.5 million light-years away. Purcell says that he and his colleagues may soon shift their focus to Andromeda, the nearest spiral-galaxy analogue to the Milky Way. "We're interested in knowing how common these events are in the bigger picture," he says. Perhaps, after all, a relatively recent galactic merger is responsible for Andromeda's structure—and the structure of countless other galaxies—as well.
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Add CommentSince more than fifty percent of all galaxies are spirals, does this mean that all of them developed their shape through collisions?
Reply | Report Abuse | Link to thisIf all galaxies a encapsulated in enormous peripheral halos of dark matter which contain at least 5 times the observable galactic mass, wouldn't their gravitational interactions begin with the halos?
Reply | Report Abuse | Link to thisHow could a dwarf galaxy penetrate the massive dark matter halo surrounding the Milky Way without being gravitationally perturbed to the extent that is could no longer slice into the Milky way's disk?
Like actual galactic interactions, this simulation does not appear to me to include any gravitational effects of any dark matter halos in it's results.
To be more specific, http://en.wikipedia.org/wiki/Milky_way states: "It is agreed that the Milky Way is a barred spiral galaxy, 100,000 light years in diameter..." To be consistent with dark matter halo estimates (below), it must be concluded that this statement refers only to visible stars and gas, extending to a radial distance of around 50,000 light years.
Reply | Report Abuse | Link to thishttp://en.wikipedia.org/wiki/Dark_matter_halo#Milky_Way_dark_matter_halo states: "The dark matter halo is the single largest part of the Galaxy as it covers the space between 100,000 light-years to 300,000 light-years from the galactic center. ...It is now believed that about 95% of the Galaxy [mass] is composed of dark matter..." Also, see the referenced published research report.
The simulation video seems to show that the interloping dwarf galaxy is most significantly gravitationally perturbed AFTER it appears to interact with the visible matter of galaxy. By that time it would have had to have penetrated the dark matter halo with a diameter from 2 to six times that of the visible galaxy. While the dark matter within the Milky Way's halo would not have physically contacted any of the matter within the dwarf galaxy, the dwarf galaxy would have passed through the vast majority of the Milky way's gravitational effects.
If this simulation model includes an reasonable representation of the hypothesized enormous dark matter halo, it's not apparent in the video. Perhaps the video should illustrate the dynamics of the dark matter halo as a 'blue cloud' like all those Bullet cluster 'simulators', since it is supposed to represent most of the galactic mass...
#2 and 3
Reply | Report Abuse | Link to thisyou're doing your research through wikipedia, these folks are doing it through telescopes and supercomputers-
who's going to come up with the more valid theories, i wonder?
the things you are asking are the equivalent of asking" if this is an animation, where's snow white and the 7 dwarves?" - sorry dude - it's 're-presentation' for the public, not '100% accurate simulation' for the astrogator
I wish i could feel you were doing more than nit picking
Simulations tend to do what the simulators want done.
Reply | Report Abuse | Link to thisWhen they don't get what they want, they reprogram until they do.
Simple. Keep programming. Even monkeys will eventually get it they way they like.
Since there are large amounts of spiral galaxies, one need reasonably presume that there is some general mechanism at work. I agree that computer simulations should never be regarded as observation results. They are only interesting if they are the outcome of theoretically grounded research, and independently testable. There is something obnoxious, even, about the idea that some people should be paid to entertain themselves in this way. In short, I totally agree with Quinn the Eskimo.
Reply | Report Abuse | Link to this5. Quinn the Eskimo
Reply | Report Abuse | Link to thisSimulations "get what we want"?
If by that you mean they must agree with what we OBSERVE, then you're correct. That doesn't involve random re-programming (as would occur if chimps were doing the sims).
Simulations are tools to allow us to see if our ideas have merit. Suggesting they are more than that is ludicrous.
#2 and 3
Reply | Report Abuse | Link to thisActually, this simulation *does* include dark matter for both the Milky Way and the satellite. That's what makes it different from past simulations of this system (and why earlier simulations didn't see the spiral arm effect). The Video, though, is showing only the stars... and they're deep inside the dark matter halos. If you watched a video instead of the dark matter, you'd see that it clearly is affected by the Milky Way halo even quite a ways further out.
Regardless though, it's well established that this sort of tidal disruption is much stronger when the satellite is closest to the center of the Milky Way, because while the dark matter extends much further than the stars do and its total mass is larger, the stellar material is *denser*. So the tidal effect is strongest when near the Milky Way disk, even though there's a slight (but still noticeable, if you look closely) effect well before it gets close.
That effect, by the way, was first seen in simulations... and then later proven to be true by observations. That's one of the other main things simulations (and indeed, any theory) are good for: they gave us direction for what effects might explain the observations.