Wednesday, 16 January 2013

Milky Way galaxy only half as massive as thought

The Milky Way galaxy, home toour solar system, may actually be only half as massive as currently believed,scientists say. 

Stars in the far outer reaches of the Milky Way, between 260,000 and 490,000 light-years from the galactic centre, are cruising around surprisingly slowly, researchers found. 

Galactic mass and star velocities are linked, so the results could have big implications, reported. 

"Because these velocities are so low, the mass of our galaxy may be much lower than we once thought," lead author Alis Deason, of the University of California, Santa Cruz said at the 221st meeting of the American Astronomical Society in Long Beach, California.
"If we infer the properties of the stars that we think are reasonable, then we find the mass of the Milky Way could be half as massive as we currently believe," added Deason, who performed the research while at the University of Cambridge in England. 

The Milky Way is composed ofthree main parts: a central bulge, a relatively flat disk and a roughlyspherical surrounding halo.
Deason and colleagues looked far out into the Milky Way's halo, which extends far beyond the 100,000-light-year-wide disk.
They measured the radial velocities of a sample of distant halo stars using two different instruments: the European Southern Observatory's 8.2-meter telescope in Chile and the 4.2-meter William Herschel Observatory in Spain.
They found that the dispersion, or spread, of halo-star velocities was about half that seen for stars closer to the galactic centre.
Using this information, the team calculated that the total mass of the Milky Way out to such extreme distances may be between 500 billion and 1 trillion times that of our Sun - substantially lower than the current leading estimate, Deason said.

But the new study is not necessarily the final word on the Milky Way's mass, which is not well understood.
"The problem is, we are really in unknown territory," Deason said.
"We are assuming properties of these stars that are the same in the inner parts of the galaxy. And this is something that really needs to be verified, what we're assuming, in terms of their density profile and also what their orbits are like," she said.
Future work along these lines could help astronomers map the distribution of mass throughout the Milky Way, Deason said, potentially shedding light on the mysterious dark matter. 

"I think we will be able to use measurements like this to not only say what the total mass is, but also if the dark matter distribution is what we expect. At the moment, we just don't know," she said.

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