Milky Way is slimmer than previously believed

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Washington, May 28 : New measurements by an international team of scientists have revealed that the Milky Way galaxy is slimmer than previously believed.

The discovery is based on data from SEGUE, an enormous survey of stars in the Milky Way - one of the three programs that comprise the Sloan Digital Sky Survey (SDSS-II).

Using SEGUE measurements of stellar velocities in the outer Milky Way, a region known as the stellar halo, the researchers determined the mass of the Galaxy by inferring the amount of gravity required to keep the stars in orbit.

Some of that gravity comes from the Milky Way stars themselves, but most of it comes from an extended distribution of invisible dark matter, whose nature is still not fully understood.

To trace the mass distribution of the Galaxy, the SEGUE team used a carefully constructed sample of 2,400 "blue horizontal branch" stars, whose distances can be determined from their measured brightness.

According to Xiangxiang Xue of the National Astronomical Observatories of China, blue horizontal branch stars can be seen to large distances, enabling the team to measure velocities of stars all the way out to distances of 180,000 light years from the sun.

The most recent previous studies of the mass of the Milky Way used mixed samples of 50 to 500 objects. They implied masses up to two trillion times the mass of the sun for the total mass of the Galaxy.

By contrast, when the SDSS-II measurement within 180,000 light years is corrected to a total mass measurement, it yields a value slightly under one trillion times the mass of the sun.

The new mass determination is based on the measured motions of 2,400 blue horizontal branch stars in the extended stellar halo that surrounds the disk. These measurements reach distances of nearly 200,000 light years from the Galactic center, roughly the edge of the region.

The visible, stellar part of our Milky Way in the middle is embedded into its much more massive and more extended dark matter halo, indicated in dim red.

The blue horizontal branch stars that were found and measured in the SDSS-II study, are orbiting our Milky Way at large distances.

From their speeds, the researchers were able to estimate much better the mass of the Milky Way's dark matter halo, and found it to be much 'slimmer' than thought before.

"The enormous size of SEGUE gives us a huge statistical advantage," said team member Hans-Walter Rix. "We can select a uniform set of tracers, and the large sample of stars allows us to calibrate our method against realistic computer simulations of the Galaxy," he added.


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