"Halo" around the Milky Way mapped

[n2doc]

The halo of dark matter surrounding our Milky Way Galaxy is shaped something like a gigantic, flattened cosmic beachball, astronomers announced today. The report is being presented to the meeting of the American Astronomical Society in Washington, DC, by Dr. David R. Law (Hubble Fellow at UCLA) and co-authors Drs. Steven Majewski (University of Virginia) and Kathryn Johnston (Columbia University). This result is important because it is the first time that the three-dimensional shape of an individual dark-matter halo has been conclusively measured.

Dark-matter haloes account for over 70% of the mass in galaxies such as the Milky Way, but this dark matter is invisible; all we see when we look up in the sky is the small amount of stars and gas sitting in the centers of these haloes. So how do astronomers 'see' invisible dark matter? It might not be possible to detect it through normal means, but dark matter obeys the laws of gravity and tugs on small dwarf galaxies as they orbit around the Milky Way. By observing the orbits that these dwarf galaxies follow, astronomers can deduce where the dark matter must be using Newton's law of gravity.

While it would take roughly a billion years to watch a typical dwarf galaxy orbit just once around our home galaxy, dwarf galaxies get shredded by tidal forces as they orbit the much more massive Milky Way and leave stars like breadcrumbs along their path. These stellar breadcrumbs can be traced in huge astronomical surveys such as the Two-Micron All Sky Survey and the Sloan Digital Sky Survey and can therefore be used to infer the orbits of their parent dwarf galaxies.

Using observations of such tidal debris from a dwarf known as the Sagittarius Dwarf Galaxy, astronomers have been able to reconstruct the orbit of Sagittarius and derive models for the Milky Way and its dark-matter halo. These models had met an impasse, however: Different parts of the orbit suggested wildly different solutions. "Until recently," says Law, "we simply didn't understand why the Sagittarius stream of stars behaves as it does."

more:

http://www.astro.ucla.edu/~drlaw/Sgr/

[Polly Hennessey]

1. Universe

Thank you for all the information. It is fascinating.

[shireen]

2. this is so cool!

Thanks! I needed to read something wonderful and amazing like this! The news these past 2 days has been so devastating.

[redqueen]

3. "...and leave stars like breadcrumbs along their path."

Love that.

[n2doc]

4. Another Galaxy, maybe what we look like from far away...

http://www.cosmotography.com/images/small_ring_ngc5907.html