Just four percent of galaxies have neighbors like the Milky Way

May 23, 2011, National Science Foundation
This image, taken from a visualization created by the Advanced Visualization Laboratory at the National Center for Supercomputing Applications (NCSA), shows the formation of the Milky Way galaxy at 16 million to 13.7 billion years old. Brian O'Shea of Michigan State University (formerly of Los Alamos National Laboratory) and Michael Norman of the University of California at San Diego collaborated on this research. Credit: National Center for Supercomputing Applications

How unique is the Milky Way? To find out, a group of researchers led by Stanford University astrophysicist Risa Wechsler compared the Milky Way to similar galaxies and found that just four percent are like the galaxy Earth calls home.

"We are interested in how the fits into the broader context of the ", said Wechsler. "This research helps us understand whether our galaxy is typical or not, and may provide clues to its formation history."

The research team compared the Milky Way to similar in terms of luminosity--a measure of how much light is emitted--and distance to other bright . They found galaxies that have two satellites that are as bright and close by as the Milky Way's two closest satellites, the Large and Small Magellanic Clouds, are rare.

Published in the May 20 issue of the , the findings are based on analyses of data collected from the Sloan (SDSS). The work is the first of three papers that study the properties of the Milky Way's two most massive satellites.

Supported in part by the National Science Foundation (NSF), the SDSS is the most extensive survey of the optical sky performed to date.

In more than eight years of operations, SDSS has obtained images covering more than a quarter of the sky, and created 3-dimensional maps containing more than 930,000 galaxies and 120,000 quasars. For this analysis, Wechsler's group studied more than 20,000 galaxies with properties similar to the Milky Way and investigated the galaxies surrounding these Milky Way "twins", to create a "census" of galaxies similar to the Milky Way in the universe.

The work represents one of the most extensive studies of this kind ever performed.

Scientists can also compare the SDSS data to galaxies simulated by a computer model. Since they are currently unable to see all the way back to the Big Bang, this is one way researchers are trying to understand how the universe as we see it today began.

In order to learn more about possible conditions in the early universe, the group performed computer simulations to recreate the universe from specific sets of starting conditions. Then they compared their simulations to the SDSS data set. In this way, the group was able to test different theories of galaxy formation to determine whether or not each would result in a universe that matches what we see today. The results of their simulation matched the result found in the SDSS data set: just four percent of the simulated galaxies had two satellites like the Magellanic Clouds.

"This is an excellent example of data-enabled science," said Nigel Sharp, of NSF's Division of Astronomical Sciences. "Comparing the 'fake' and 'real' Universes is how we discriminate between successful and unsuccessful theories. This work interconnects three of the four legs of science: theory, observation and simulation, for a powerful scientific result."

Their results also lend support to a leading theory of galaxy formation called the Cold Dark Matter (CDM) theory. This theory provides what many consider to be the simplest explanation for the arrangement of galaxies throughout the universe following the Big Bang. It assumes that most of the matter in the Universe consists of material that cannot be observed by its electromagnetic radiation (dark) and whose constituent particles move slowly (cold). Dark matter, an invisible and exotic material of unknown composition, is believed to influence the distribution of galaxies in space and the overall expansion of the universe. The rareness of this aspect of the Milky Way may provide clues to its formation history.

"Because the presence of two galaxies like the Magellanic Clouds is unusual, we can use them to learn more about our own galaxy," said Wechsler. Using their simulation, the team identified a sample of simulated galaxies that had satellites matching the Milky Way's in terms of their locations and speeds.

"The combination of large surveys of the sky like the SDSS and large samples of simulated galaxies provides a new opportunity to learn about the place of our galaxy in the Universe," said Wechsler. "Future surveys will allow us to extend this study to even dimmer satellite galaxies, to build a full picture of the formation of our galaxy."

The theoretical and numerical work that produced the simulations used as a comparison for the SDSS data were supported by an award funded under the American Recovery and Reinvestment Act of 2009.

Explore further: Is the Milky Way doomed to be destroyed by galactic bombardment? Probably not

More information: iopscience.iop.org/0004-637X/733/1/62

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2.5 / 5 (8) May 23, 2011
I suppose one will have to read the paper to find out just what their assumptions and starting conditions were as it's clearly not revealed here. Makes the article almost useless.
4 / 5 (1) May 23, 2011
So what is the significance of this?
If any at all.
Rather than search for galaxies that have near by neighbours it would be more significant to determine how many galaxies were similar to our galaxy.
Not that this would necessarily be any more significant than their present research.
4 / 5 (2) May 23, 2011
That means we are pretty unique! If there are 500 billion galaxies in the visible universe, it means there are ONLY 20 billion galaxies with luminous satellite galaxies like our own!

20 billion is a pretty small number!
1.5 / 5 (4) May 23, 2011
Seems that satellite galaxies are formed from ejections from the parent galaxy, each growing over time from within. Likely this condition would be somewhat rare. If instead it were the reverse with satellites condensing and colliding to form the larger galaxies, would not we see a far greater population of satellites? That is what this means, I think.
1 / 5 (1) May 23, 2011
This is one time I agree.

Is it a statistical model or is it a dynamical model? Are the initial conditions determined randomly, or by some pre-conceived notion?

Are the results an average of dozens or hundreds of full runs, with slightly different variables and slightly differen initial conditions, or is this the result of a single run of a single model?

A model could agree with an isolated subset of observations by pure coincidence or some sort of mathematical artifact.

How do you simulate a galaxy with 400 billion stars on a super computer that only has at most a few terabytes of RAM. That's like 4 bytes of data per star. That isn't even enough to store basic physical data, like mass, velocity, and spin, in units relevant to galactic scale...what about planets, dust, comets, etc?

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