This image shows the formation of the Milky Way galaxy at 16 million to 13.7 billion years old.
Credit: National Centre for Supercomputing Applications
This image from a supercomputer simulation shows the density of dark matter in our Milky Way galaxy which is known to contain an ancient thin disk of stars. Brightness (blue-to-violet-to-red-to-yellow) corresponds to increasing concentration of dark matter. The bright central region corresponds roughly to the Milky Way's luminous matter of gas and stars and the bright clumps indicate dark-matter satellites orbiting our Milky Way galaxy, which are known as 'substructure'.
Credit: Stelios Kazantzidis, Ohio State University
CALIFORNIA: By comparing the Milky Way to similar galaxies, researchers have found that just 4% of the billions of galaxies that populate the cosmos are like the one we call home.
The researchers compared the galaxies in terms of luminosity - a measure of how much light is emitted - and distance to other bright galaxies. 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 very rare.
"We are interested in how the Milky Way fits into the broader context of the universe", said the lead author and astrophysicist Risa Wechsler from Stanford University in California. "This research helps us understand whether our galaxy is typical or not, and may provide clues to its formation history."
A census of galaxies
Published in the Astrophysical Journal, the findings are based on analyses of data collected from the Sloan Digital Sky Survey (SDSS), major multi-filter imaging and spectroscopic redshift survey involving a 2.5-m wide-angle optical telescope located at Apache Point Observatory in New Mexico that provides the 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.
The rare Magellanic Clouds
The scientists could 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 that 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 4% of the simulated galaxies had two satellites like the Magellanic Clouds.
