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Milky Way is much bigger than we thought

February 2008
SYDNEY: By doing simple calculations on data freely available online, Australian astrophysicists have discovered that our Milky Way galaxy may be twice as thick as previously estimated.

Bryan Gaensler of the University of Sydney led the team who now estimate the disc of our galaxy to be 12,000 rather than 6,000 light years thick. Proving not all science requires big, expensive apparatus, the researchers downloaded data from the Internet and analysed it in a spreadsheet.

"We were tossing around ideas about the size of the Galaxy, and thought we had better check the standard numbers that everyone uses," said Gaensler. "It took us just a few hours to calculate this for ourselves. We thought we had to be wrong, so we checked and rechecked and couldn't find any mistakes."

Pulsating stars

Their results were presented at a meeting of the American Astronomical Society in Austin, Texas, last month, and are now slated for publication in an academic journal. The new estimate differs from previous calculations because Gaensler's team was more selective about the pulsars they used as a data source.

Short for 'pulsating stars', pulsars are rotating neutron stars - the remnants of massive suns that have collapsed into extremely dense objects. Rather like the beam of a lighthouse, they emit electromagnetic radiation in the form of radio waves as they rotate.

"As light from these pulsars travels to us, it interacts with electrons scattered between the stars (the Warm Ionised Medium, or WIM), which slows the light down," said Gaensler.

In particular, the longer (redder) wavelengths of these pulses slow down more than the shorter (bluer) wavelengths, so by seeing how far the red lags behind the blue we can calculate how much WIM the pulse has travelled through, he said. The concept is similar to counting the time between a crash of thunder and a flash of lightning to measure how far away it is.

By using the light from pulsars to measure where the WIM ends, we can estimate the edge of the galaxy, said Gaensler. "If you know the distance to the pulsar accurately, then you can work out how dense the WIM is and where it stops – in other words where the galaxy's edge is."

Less, but more

Of the thousands of pulsars known in and around our galaxy, only about 60 have well known distances. But to measure the thickness of the Milky Way, Gaensler's team focussed only on those sitting directly above or below us. Previous estimates measured the distance to pulsars diagonally above and below us in the direction of the upper and lower edge of the galaxy. But this yields a less accurate estimate of the thickness of the galactic disc, said the researchers.

The gradual accumulation of new data has yielded a larger number of pulsars directly above or below us, so Gaensler's team was able to focus on 20 to 30 of these. "We used less data, but it's much more reliable."

Coincidentally, a recent study from the Max Planck Institute for Radio Astronomy in Bonn, Germany, suggested that there is around five times as much magnetism in the Milky Way as would be predicted for a galaxy of our size. Without any prior knowledge of his work, Gaensler said that the study proposed that a Milky Way twice as thick as we thought might be one possible way to explain the discrepancy.

He added that it's not just magnetism, but also heat, pressure and many other factors abut our understanding of the Milky Way that could be knocked out of kilter if the new theory proves to be correct. And by extension, this could effect our understanding of all galaxies, which itself is built on knowledge of the Milky Way.

Rate of star formation

"The study is intriguing and it could be an important result," commented Quentin Parker of the Anglo-Australian Observatory at Macquarie University in Sydney.

He said the findings could mean that either the galaxy itself is wider than we thought, or the ionised gas of the WIM simply goes much further beyond the stars that make up the edge of the galaxy than we thought. "It's all a matter of semantics. And depends whether you're talking about the size of the galaxy in terms of dust, gas or stars."

However, said Parker, even if the results indicate that the ionised gas goes out further than we thought, this would mean that the star forming region of the galaxy is much bigger than we thought. "If we've underestimated star formation in our own galaxy, we've underestimated it in other galaxies too… and this could affect our understanding of the rate of star formation in the history of the universe."

"Some colleagues have come up to me and have said 'That wrecks everything!'" concluded Gaensler. "And others have said 'Ah! Now everything fits together!'"

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